PHOENIX MARS LANDER 2007 THERMAL AND EVOLVED GAS ANALYZER CALIBRATION REPORT. Version /22/2009

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1 PHOENIX MARS LANDER 2007 Prepared By: TEGA TEAM - -

2 Table of Contents Background TEGA Engineering Subsystem AD590 Temperature Sensors PRT Temperature Sensors Engineering Voltages and Currents Status Channels TA_MANIFOLD_PRES (0), TA_OUTLET_PRES (2) Oven and Shield Error Signals TA_T_HEATER_TEMP (23) Oven and Shield Voltage and Current Pulse Width Monitors TA_FULL_DETECT (42), TA_FULL_DETECT_RAW (43) Calculating the Calibration Curves for the Oven Pt Sense Winding TA0: TA: TA3: TA5: TA6: Further Discussion Converting TEGA DSC telemetry to true power Pressure Transducer Calibration EGA Mass to Voltage Conversions Theory Practice Calibration (coefficient derivation) In-Situ Calibration Ground Data Reduction References

3 Background Phoenix TEGA is part of the instrument package onboard the 2007 Phoenix Mars Lander. The Phoenix mission, the first chosen for NASA's Scout program, began exploration of the Northern plains of Mars during TEGA stands for Thermal and Evolved Gas Analyzer and is directed toward sampling and understanding the chemistry and mineralogical components of the soil through thermal (heat capacity) and gas analysis of soil samples delivered by the Robotic Arm from various sampling locations beneath and beside the Lander. An additional capability is using the Evolved Gas Analyzer to measure trace components of the Martian Atmosphere. A detailed description is contained in the TEGA Instrument Paper (Boynton et al 2008) but a brief description is presented below. TEGA has three maor components: The Control Electronics (CE); the Thermal Analyzer (TA); and the Evolved Gas Analyzer (EGA). The control electronics (CE) is enclosed in a payload package beneath the Lander deck and contains a microcontroller and power conditioning and control system as well as a housekeeping component. The housekeeping component measures currents, voltages, pressures, and temperatures of the various sensors scattered over the instrument. The Thermal Analyzer sitting above on the deck receives the sample from the Robotic Arm seals it into an oven and runs a calorimetry scan under control of the CE. The TA is also supplied with two gas supplies and valves to route the gas from the supply tanks and eight ovens to the EGA. The Evolved Gas Analyzer is a 4 channel magnetic sector mass analyzer that measures the concentrations of gasses either from the ovens or the atmosphere. The primary purpose of this document is to provide the critical information required to convert the raw data from the instrument into standard units corrected to the degree possible for the understood effects of environmental and instrument behavior. Some of these conversions were calculated in-situ in real-time with the maority taking place as a post process to the receipt of the data on the ground

4 2 TEGA Engineering Subsystem The TEGA Engineering Subsystem is based on a radiation hardened Analog Devices AD7872 A/D converter. This is a signed, 4-bit converter with an input range of -3V to +3V. A summer at the input to the ADC is used for level shifting, allowing the system to process input signals of 0 to +6V, -3 to +3V, and 0 to -6V (Figure ). Channel Selector + - G= ADC +3V 0V -3V Polarity Selector Figure : Analog Signal Processing Figure 2 shows the transfer function of the ADC. The TEGA flight software converts the ADC output to a signed, 6-bit integer using the rules in Table. Figure 2: ADC Transfer Function Input Range Conversion 0 to +6V Invert bit 3-3 to +3V Sign Extend by copying bit 3 into bits 4 and 5-4 -

5 0 to -6V Invert bit 3 then subtract 0x3FFF Table : Converting ADC output to signed, 6-bit integer 2. AD590 Temperature Sensors Most of the temperatures in TEGA were measured with Analog Devices AD590 temperature sensors. These sensors pass a current that is proportional to the temperature, with a nominal slope of µa/ºk and absolute accuracy of ±.7ºK from 28 to 423ºK (-55 to +50ºC). All of the flight sensors were tested for linearity down to -00ºC. The excitation voltage for the sensors was +2V, and the output current was read out with a 5.4K, 0.% resistor, resulting in a nominal voltage to the ADC of 5.4mV/ºK (42.08dN/ºK). Each readout channel was characterized before launch with a precision current source, and the results are shown in Table 2. Name (Channel #) Location A B TA_MANIFOLD_TEMP (3) Valve manifold in the TA TA_PLUMBING TEMP (6) Plumbing between cal tank and valve manifold TA_PLUMBING_2_TEMP (7) Plumbing between valve manifold and transfer tube unction TA_CAL_TANK_TEMP (9) Cal gas tank near heater TA_CAL_TANK_COLD_TEMP (46) Cal gas tank at coldest spot TA_TRANS_TUBE_TEMP (24) Transfer tube between TA and EGA TA_EGA_MAN_TEMP (8) Valve Manifold in the EGA TA_EGA_PLUMB_TEMP (4) EGA plumbing between transfer tube and ion source TA_CPU_TEMP (0) FPGA-A, the hottest component on the CPU board TA_PWR_SPLY TEMP () Thermal plate near shield power converters TA_PWR_SPLY_2_TEMP (2) Filter board near linear optocouplers TA_PWR_CNTL TEMP (3) Center of thermal plate TA_PWR_CNTL_2_TEMP (4) Power control board near oven/shield power monitor circuit TA_A2D_TEMP (5) On the A/D converter on the analog board TA_EGA_ELECT_BOX_TEMP (22) EGA electronics frame EGA_PROC_TEMP (87) EGA CPU Chip lid Table 2: AD590 conversions. Temperature (C) = raw / A + B. This assumes a perfect, µa/ºk sensor. 2.2 PRT Temperature Sensors For locations where the temperatures to be measured were likely to fall outside of the -55 to +50ºC range of an AD590 sensor, PRT sensors were used. These sensors change their - 5 -

6 resistance proportionally to the temperature, with a slight nonlinearity. The conversion for these sensors is done in two parts: ) Calculate the resistance of the sensor from the raw ADC data R = raw * M + N 2) Use a polynomial to convert the resistance to temperature T = A + B * (R/R0) + C * (R/R0)^2 + D * (R/R0)^3 + E * (R/R0)^4 Table 3 gives the coefficients for the 6 PRT sensors, and Table 4 gives their locations in the instrument. Channel R0 M N A B C D E TA_OVEN_TEMP (20) NOTE E E NOTE 2 TA_SHLD_TEMP (2) NOTE E E NOTE 2 TA_EGA_BAKEOUT_TEMP (5) E E TA_EGA_GEC_TEMP (25) E E TA_COVER TEMP (6) E E TA_COVER_2_TEMP (47) E E TA_INPUT_FUNNEL LO_TEMP (7) E E TA_INPUT_FUNNEL_2_LO_TEMP (9) E E EGA_MAGNET_TEMP_ (85) 00 Note Note EGA_MAGNET_TEMP_2 (86) 00 Note Note Table 3: PRT conversion coefficients Channel TA_OVEN_TEMP (20) TA_SHLD_TEMP (2) TA_EGA_BAKEOUT_TEMP (5) TA_EGA_GEC_TEMP (25) TA_COVER TEMP (6) TA_COVER_2_TEMP (47) TA_INPUT_FUNNEL LO_TEMP (7) TA_INPUT_FUNNEL_2_LO_TEMP (9) EGA_MAGNET_TEMP_ (85) EGA_MAGNET_TEMP_2 (86) Table 4: PRT Sensor Locations Location TEGA Sample Oven Heated Shield around the Sample Oven EGA Ion Source Housing Inside of the EGA GEC Getter mounting post On the body of the North side cover actuator On the body of the South side cover actuator Backside of the TA0 funnel Backside of the TA5 funnel Main EGA magnet, Ion Source end Main EGA magnet, Ion exit end Note : The two EGA magnet PRT sensors use a significantly different readout circuit. For these V R + F V R two sensors, RG R = R S, V R + F V R RG where V=raw/3276.6, R S =3240, V R =5, R F =0000, and R G =

7 Combining and simplifying, step becomes R = raw 8023 NOTE 2: The sense windings in the ovens and shields are 99.99% pure Platinum wire. The datasheet lists the α for this wire at , making it distinct from the standard Pt curves ( , , and ). NOTE 3: Each oven and shield has its own R0. These values are given in Table 5. Cell # Oven R 0 Shield R Table 5: Oven and Shield R 0 Values 2.3 Engineering Voltages and Currents There are many voltages and currents that are monitored primarily to assess instrument health and safety. All of these monitors use a linear, result = A + raw * B conversion. A few of the monitors require some temperature compensation. Channel A B Unit Description Note TA_BUS_A_VOLT (26) E E- Spacecraft bus voltage, measured AFTER the input filters, V 03 i.e. as close to the actuators as possible TA_BUS_A_CUR (34).3978E E- 04 A Bus A (RPC06) current draw NOTE 4 TA_BUS_B_CUR (35) -.090E E- 04 A Bus B (RPC03 + RPC09) current draw NOTE 5 TA_EGA_CUR (36) -.236E E- 04 A EGA PECM current draw NOTE 6 TA_CPU_PLUS_5_VOLT (29) E E- 04 V TEGA CPU board +5V rail voltage monitor TA_CPU_PLUS_5_CUR (37) E E- 05 A TEGA CPU board +5V rail current monitor TA_ANLG_PLUS_2_VOLT (30) E-4 V TEGA Analog board +2V rail voltage monitor TA_ANLG_PLUS_2_CUR (38) E-5 A TEGA Analog board +2V rail current monitor TA_ANLG_MINUS_2_VOLT (3) E-4 V TEGA Analog board 2V rail voltage monitor TA_ANLG_MINUS_2_CUR (39) E-5 A TEGA Analog board 2V rail current monitor TA_PLUS_5_VREF () E-4 V Direct readout of a precision +5V reference TA_PRES_SENSE_FD_BK (8) E-4 V +0V reference to pressure sensors TA_AGD_0_3 (27) E-4 V Unused MUX input, tied to analog ground TA_AGD_3_ (28) E-4 V Unused MUX input, tied to analog ground EGA_ION_PUMP_VOLT (8) E- V EGA Ion Pump High Voltage monitor EGA_ION_PUMP_CUR (82) E-2 µa EGA Ion Pump Current monitor EGA_SWEEP_VOLTAGE (83) E- V EGA Sweep Voltage monitor - 7 -

8 EGA_MULTIPLIER_VOLT (80) E- V EGA Channel Electron Multiplier high voltage monitor EGA_EMISSION_CUR (72) E- µa EGA Ion Source Emission Current monitor NOTE 7 EGA_TRAP_CUR (7) E- µa EGA Ion Source Trap Current monitor NOTE 7 EGA_FILAMENT_CUR_ (78) E-4 A EGA Filament # drive current monitor EGA_FILAMENT_CUR_2 (79) E-4 A EGA Filament #2 drive current monitor EGA_PLUS_5_VOLT (75) E-4 V EGA CPU board +5V rail monitor EGA_PLUS_2_VOLT (76) E-3 V EGA Analog +2V rail monitor EGA_MINUS_2_VOLT (77) 0-8.9E-4 V EGA Analog -2V rail monitor EGA_GEC_CUR (84) E-4 A EGA Getter current monitor Table 6: Voltage and Current Monitors NOTE 4: This sensor has a significant sensitivity to both voltage and temperature. The initial coefficients get us to +57/-99mA. Adding temperature compensation gets us to +75/-67mA: result = result +.08E E-03 * TA_PWR_SPLY_2_TEMP (temperature in ºC) Adding voltage compensation improves this to ±0mA: result = result E E-04 * RAW_TA_BUS_A_VOLT E-08 * RAW_TA_BUS_A_VOLT^ E-3 * RAW_TA_BUS_A_VOLT^3 (note that this uses the raw ADC output for the bus A voltage) NOTE 5: This sensor is sensitive to temperature. The initial coefficients get us to ±90mA. Applying temperature compensation gets us to ±20mA: result = result E E-03 * TA_PWR_SPLY_2_TEMP (temperature in ºC) NOTE 6: Initial coefficients get us to ±25mA. Temperature compensation improves this to ±0mA. result = result E E-04 * TA_PWR_SPLY_2_TEMP (temperature in ºC) NOTE 7: Not valid after the filament short on SOL Status Channels There are several channels that are used to report digital status. EGA_STATUS_BITS (70) This is a 6 bit status word that shows the internal status of the EGA. Bit Function 0 Emission ON Filament selected 2 Filament 2 selected 3 High Voltage Enabled 4 CEM High Voltage Enabled 5 Sweep High Voltage Enabled 6 Currently Sweeping 7 Emission Current Setting (0 = low, = high) 00-23V (0V) 9,8 Emission Energy (NOTE 8) 0-27V (-3V) 0-37V (-3V) -90V (-66V) 0 Emission Fault Magnet Sensor Fault 2 Magnet Sensor 2 Fault 3-5 Unused Table 7: EGA Status Bits - 8 -

9 NOTE 8: The filament short on sol 4 altered the emission energy control circuit. After sol 4 the emission energy was 24V less. The post-short values are in parentheses. EGA_FILAMENT SEL (73), EGA_FILAMENT_2_SEL (74) Prior to the filament short on Sol 4, these two channels provided positive feedback that the intended filament was truly selected. The selected filament should read V, and the other filament should read.6666v. The filament short altered the circuit configuration such that these readings are not meaningful after Sol 4. EGA_AVG_IDLE_CALLS (88), EGA_MIN_IDLE_CALLS (89) These two channels track the load on the EGA CPU. The Idle function is called repeatedly whenever the CPU is not busy. This function simply accumulates statistics on how often it is called. The collected statistics are read out, and the accumulations reset every time an EGA engineering packet is created. EGA_AVG_IDLE_CALLS is the average number of times the idle function was called per second since the last engineering packet. EGA_MIN_IDLE_CALLS is the minimum number of idle calls in any second since the last engineering packet. Together, these channels give an indication of both the average and the peak load on the CPU. 2.4 TA_MANIFOLD_PRES (0), TA_OUTLET_PRES (2) There are two pressure sensors in the TEGA plumbing. The Manifold Pressure Sensor is located in the plenum between the carrier and calibration valves and the TA inlet valves. This sensor is used to regulate the gas flow through the ovens. The Outlet Pressure Sensor monitors the pressure at the TA inlet to the EGA. Both sensors exhibit significant nonlinearity at very low pressures, but become quite linear as the pressure increases (see section 5). As a result, there are two equations for converting the raw value to pressure for each sensor. Channel Threshold A B C Constraint TA_MANIFOLD_PRES raw < E-08 raw E-05 0 NOTE 9 raw < E-07 TA_OUTLET_PRES NOTE 0 raw E-05 0 Table 8: Pressure Calibration Parameters Pressure = A + B*raw_pressure + C*raw_pressure^2 NOTE 9: TA Manifold at +65. There were occasions where the manifold was not run at +65C. See section 5 for temperature corrections. NOTE 0: EGA Manifold at Oven and Shield Error Signals TA_OVEN_ERR (44), MEM_OVEN_ERR (57), TA_SHLD_ERR (45), and MEM_SHLD_ERR (58) The oven and shield control systems use a high-gain difference amplifier to subtract the commanded temperature from the measured temperature. This difference is read out every 3.3mS and used by the software to calculate the proper width for the next pulse

10 The exact conversion from the error signal to degrees is different for each TA and also varies with the temperature, but a reasonable estimate is 63 dn / degree for the oven error and 763 dn / degree for the shield error. This measurement is reported two ways. The TA_OVEN_ERR and TA_SHLD_ERR channels take an extra reading of these channels, while the MEM_OVEN_ERR and MEM_SHLD_ERR report the last scheduled (3.3mS) reading. 2.6 TA_T_HEATER_TEMP (23) The T heater is a heater / temperature sensor assembly on a plumbing unction between each oven and the manifold. There is one T heater for each oven. The assembly consists of a coil of heater wire wound on the plumbing unction wired in parallel with a 00Ω PRT. The PRT is mounted on the metal support for the unction. This assembly is driven by a PWM controller that measures the parallel resistance in between the pulses to determine the temperature. As usual, there is a twist: The current source used to measure the resistance of the assembly has a significant temperature coefficient. Calculating the temperature requires several steps: ) Calculate the sense current (I S ) based on the smoothed TA_PWR_CNTL_2_TEMP engineering channel TA_PWR_CNTL_2_TEMP I S = 000 2) Calculate the resistance of the assembly, R T. 4 2 raw RT = 9.27 I S 3) Use R T and the known resistance of the heater winding (R H ) to calculate the resistance of the PRT, R S. RH RT RS = RH RT 4) Calculate the temperature of the PRT (T S ) from R S using our standard PRT equation R S RS RS RS T S = A + B C D E R0 R0 R0 R0 Where R 0 = 00, A = , B = 236.9, C = , D = , and E = ) And finally, calculate the temperature of the actual plumbing (T T ) from T S. T T T = S

11 Table 9 gives the R H value for each of the cells. Cell # R H Table 9: T Heater Winding Resistances 2.7 Oven and Shield Voltage and Current MEM_OVEN_VOLT (52), TA_OVEN_PLUS_5_VOLT (32) MEM_OVEN_CUR (53), TA_OVEN_PLUS_5_CUR (40) MEM_SHLD_VOLT (54), TA_SHIELD_PLUS_30_VOLT (33) MEM_SHLD_CUR (55), TA_SHIELD_PLUS_30_CUR (4) In order to accurately calculate the power going to the oven and shield, we need to measure the voltage and current during the power pulse. Since the A/D converter takes roughly 6µS to do a conversion, it would not be practical to measure all four of these during a single pulse. Instead, the readings are taken in a round-robin fashion with one reading taken on each pulse. The reading is taken approximately 5µS after the start of the pulse to allow the pulse to reach full voltage and the readout circuits to stabilize. These readings are not accurate until the pulse width is greater than about 8µS (pulse_width > 60). The MEM_ channels report the most recent of these timed readings. The TA_ channels are read out at some random time with respect to the power pulse. They should not be used. The conversions for these channels are linear, with a linear temperature correction for the TA_PWR_CNTL_2_TEMP sensor, which is located in the middle of the circuit with the temperature sensitivity. Table 0 gives the conversion coefficients for these 4 channels. Channel A B C Accuracy MEM_OVEN_VOLT E E E-04 ± mv MEM_OVEN_CUR E E E-04 ± 4mA MEM_SHLD_VOLT E E E+00 ± 0mV MEM_SHLD_CUR E E E-04 ± 7mA Result = A + B * raw + C * TA_PWR_CNTL_2_TEMP (TA_PWR_CNTL_2_TEMP in ºC) Table 0: Conversion coefficients for Oven and Shield Voltage and Current 2.8 Pulse Width Monitors MEM_OVEN_WIDTH (60) MEM_SHLD_WIDTH (6) MEM_T_WIDTH (59) - -

12 These channels report the width of the most recent pulse from the respective PWM controller. The oven and shield controllers are 6 bits with a resolution of 50nS, and the T heater controller is 0 bits with µs resolution. While calorimetric calculations are possible using these channels, the preferred method is to use the data in the DSC science data packets. 2.9 TA_FULL_DETECT (42), TA_FULL_DETECT_RAW (43) The flow of soil into the oven is monitored by an optical sensor. As soil particles fall into the oven, they modulate the light falling on a photodetector. The TA_FULL_DETECT_RAW channel is used immediately before a soil loading operation to get light and dark readings that are used to set the thresholds for soil detection. There is no calibration per se for this channel, but lower values indicate brighter light. The TA_FULL_DETECT channel is the output of an analog integrator that sums the light to dark transitions on the detector. During a soil loading operation, high time resolution readings are collected on these channels. These readings are sent in the LED science data packets

13 3 Calculating the Calibration Curves for the Oven Pt Sense Winding The coefficients used to convert the resistance of the oven and shield sense windings to temperature were determined for TEGA- and carried over to TEGA-2 without any further validation. Additionally, the room temperature resistance of the windings were measured twice during TEGA-2 assembly, and those measurements came out slightly different. The Callendar van Dusen method is applied to the TEGA-2 flight data to check the validity of the coefficients and to select which set of resistance measurements lead to believable temperatures. The Callendar van Dusen method uses four calibration points to fully define the R vs T curve for a given alloy of Pt. The calibration points are: R 0 at T=0ºC R 00 at T=00ºC R H at some T H >> 00ºC R L at some T L << 0ºC The Callendar van Dusen coefficients are R00 R0 α = (eq. ) 00 R0 RH R0 TH R0 α δ = (eq. 2) TH TH R R T T L T + δ 00 β (eq. 3) TL TL L 0 L L 0 00 R α = 3 Given only α, one can get a first order approximation of R for a given T with = R0 ( + α T ) (eq. 4). Given α and δ, one can calculate R T very accurately for R T T T temperatures 0ºC using R T = R0 + R0α T δ (eq. 5). To get an accurate R T for temperatures below 0ºC we also need the β coefficient, and the equation 3 T T T T is R T = R0 + R0α T δ β (eq. 6)

14 0 fort 0 α δ α δ By substituting A = α +, B =, and C = 2 α β (eq. 7, 8, and 9), fort < R T = R + AT + BT + CT T 00. [ ] we can rewrite eq. 6 in the more familiar form ( ) 0 These four data points are not available in the flight data. The data points that can be teased from the flight data are: R R at T = room temperature, measured during assembly for each oven T R, a good measurement of room temperature R H at T=359ºC for each oven. This point is from the mystery peak in the high temp day 2 data, which we believe is the Curie transition in the Ni oven body. Since the Curie point for Ni is sometimes given in the literature as 357ºC and sometimes as 36ºC, the average of the two is used for these calculations. α, the normalized slope of R vs T from 0-00ºC, is given as in the California Fine Wire datasheet. This is the same α as in eq. above. Since room temperature is not far from 0ºC, eq. can be used to calculate R 0 from R R RR and T R. Solving eq. for R 0 gives R0 =. For oven 4, this yields α TR + = R = Ω The value used for the TA4 R 0 was 33.68Ω..5x0-3 Sol 25 "Mystery Peak" Oven Duty Cycle Oven Sense Resistance Figure 3: Sol 25 Mystery Peak for TA4 90 Figure 3 shows the mystery peak in the high temp day 2 run for TA4 on sol 25. After some discussion, the science team concluded that the Curie point is at the peak of the transition. This peak is at R = Ω. Plugging this into eq. 2 yields δ = = There are no good candidates for R L and T L, so we can not calculate β. However, since there are no transitions of interest below 0ºC, this should not be an issue

15 Plugging these values for α and δ into equations 7 and 8 gives: A = = and B = = The numbers used during surface operations were A=3.9080x0-3 and B= x0-7. Using the new A and B values to recalculate the temperatures for the TA4 run shows that we were 0.4 degrees hotter at the top of the ramp (Figure 4). 000 old_ta_oven_temp new_ta_oven_temp 800 Oven Temperature TEGA Time Figure 4: Comparison of old and new conversions for TA4 0x0 3 Applying the same procedure to the rest of the surface runs yields the following: 3. TA0: R R = and T R = = R = Ω x0-3 Sol 70 "Mystery Peak" Oven Duty Cycle Oven Sense Resistance Figure 5: Sol 70 TA0 Mystery Peak. Peak at ohms - 5 -

16 From these numbers, we get δ = = TA: R R = and T R = = R = Ω (was ).5.0 x Figure 6: Sol 34 TA Mystery Peak. Peak at ohms δ = = TA3: R R = and T R = 24.8 = R = Ω (was ) x Figure 7: Sol 5 TA3 Mystery Peak. Peak at ohms - 6 -

17 δ = = TA5: R R = and T R = = R = Ω (was ) x Figure 8: Sol 77 TA5 Mystery Peak. Peak at ohms δ = = TA6: R R = and T R = = R = Ω (was ) x Figure 9: Sol 47 TA6 Mystery Peak. Peak at ohms - 7 -

18 δ = = The δ values are summarized in Table, and shown graphically in Figure 0, along with the average value

19 TA # δ No data No data Table : Summary of δ Values δ Mean δ =.77 δ TA # Figure 0: δ Values and Averages 6 7 This spread is also far too wide for these values to be useful, especially considering that δ/00 2 is the T 2 coefficient. Also, since δ describes a fundamental property of the Pt wire, this is enough of a spread to warrant further examination. As a quick check, the δ value for each TA was used to calculate a new temperature scale, then each mystery peak was plotted against this new temperature scale. Oven Duty Cycle 2.0x Average δ Res_ov_duty_0 Res_ov_duty_ Res_ov_duty_3 Res_ov_duty_4 Res_ov_duty_5 Res_ov_duty_ Calculated Oven Temperature Figure : Mystery peaks plotted each with its own temperature scale - 9 -

20 Figure shows these results. The markers show the location chosen as the peak. Since all of the markers lie exactly at 359 C, we can conclude that the δ values were calculated correctly for the given input data. To see ust how harmful this spread really is, the temperature scale for each TA was recalculated using the average δ of.7703 for all of the TAs. The results are shown in Figure 2. The markers now lie at ± 4.45 C. This isn t great, but it isn t horrible, either. Oven Duty Cycle 2.0x Average δ Res_ov_duty_0 Res_ov_duty_ Res_ov_duty_3 Res_ov_duty_4 Res_ov_duty_5 Res_ov_duty_ Calculated Oven Temperature Figure 2: Mystery Peaks Using a Common δ of.77 and the earlier R 0 measurements But how bad is this spread at 000 C? To get a rough idea, the TA4 max resistance was converted to temperature using δ =.346 and δ = (the two end members). This yields max temperatures of C and C, or 06.7 ± 60.2 degrees. This is really pretty horrible, but will have to do for this release of the RDR data. 3.6 Further Discussion As it turns out, there were two measurements of the R R and T R taken for each oven: One right after they finished cooling from the annealing bake, and a later one after the ovens were integrated into the TA assemblies. The earlier measurement was probably more accurate since it was done under very controlled conditions with a 6-digit, 4-wire ohmmeter. Unfortunately, the derived R 0 values do not agree between the two measurements, and consequently, neither do the derived δ values. All of the above calculations are based on the earlier measurement. TA # First measurement Second measurement δ based on first measurement δ based on second measurement

21 Table 2: Comparison of first and second oven measurements Table 2 shows both sets of R 0 and δ values. Using the later set of R 0 measurements, the spread in the δ values is somewhat larger than the spread using the later set (.0 vs 0.85), and TA0 becomes a serious outlier. Leaving out TA0 and repeating the analysis of the spread in temperatures using the mean δ value (in this case.65), shows the mystery peaks at ± 2.5 degrees (Figure 3). Calculating the spread at the max temperature using the TA3 data and δ =.369 and δ =.858 (the two end members ignoring TA0) yields temperatures of C and C, or ± 3.4 degrees. This is better than the previous spread, but it leaves out poor TA0. Oven Duty Cycle 2.0x Average δ Res_ov_duty_0 Res_ov_duty_ Res_ov_duty_3 Res_ov_duty_4 Res_ov_duty_5 Res_ov_duty_ Calculated Oven Temperature Figure 3: Mystery Peaks Using a Common δ of This situation begs further analysis. Since δ is a fundamental property of the Pt wire, it should be constant for all ovens. This analysis shows that there is some second-order effect that we do not understand. One possibility is that Pt wire changes its resistance in response to strain as well as temperature. Since each oven is built by hand, there could be considerable variation in the tightness of the wind and the subsequent strain placed on the wire by thermal expansion of the oven. Further analyses that could be done include putting an oven and a bare loop of the Pt wire in a controlled environment to map out the curve, but considering the variability we have seen it is not clear if one more data point would help or hurt. If time and money permit, we will revisit this problem and release updated RDR data

22 4 Converting TEGA DSC telemetry to true power The TEGA PWM controller sends one power pulse to the oven and shield every mS. For each pulse, TEGA reads out the voltage (V), current (I), and pulse width W (W). The power in the pulse is V I. These powers are then summed over some number of pulses (N) for downlink to bring the data rate down to a reasonable value. TEGA reads the voltage and current as 4-bit integers in arbitrary units. It does not convert the voltage and current readings into true volts and amps prior to doing the multiplication in order to save time in the DSC interrupt routine. It is the responsibility of the ground data system to do this conversion after the fact. The true voltage is V B D, and the true current is I B 2 D2, where B, B 2, D, and D 2 are determined during instrument characterization. In order to calculate the true power in N pulses, we need the sum N W ( V B D ) ( I B D ) 2 2 = power = (Equation ) N. Since TEGA has no knowledge of B, B 2, D, or D 2, we need to develop a method where those numbers can be applied on the ground. We can write Equation as: B B N 2 = W V I B D N 2 = W V B + D D power = (Equation 2) N With this form, TEGA can calculate the 4 individual sums for downlink, and the gains and offsets can be applied on the ground. These 4 sums are in the DSC telemetry packet as pulse ( ). N W = ), power ( W V I N = 2 D N = W ), current ( W I These are assembled from the EDR data as follows: Pulse = N W = Power = W V I = Current = W I N N = = OVEN_PULSE I N = = OVEN_PWR_HI * OVEN_PWR_LO = OVEN_CUR_HI * OVEN_CUR_LO N 2 = W ), and voltage ( W V N =

23 N Voltage = W V = OVEN_VOLT_HI * OVEN_VOLT_LO = N = SUM_COUNT From the instrument characterization we know that D and D 2 are a dependent on the temperature of the readout circuit, so we substitute D = A C TA_PWR_CNTL_2_TEMP and D = A C TA_PWR_CNT L_2_TEMP Table 3 lists the A N, B N, and C N coefficients for the flight unit. Note that these are the same coefficients used to convert the MEM_OVEN_VOLT, MEM_OVEN_CUR, MEM_SHLD_VOLT, and MEM_SHLD_CUR engineering channels. (See Table 0 in TEGA Engineering Explained) A B C A 2 B 2 C 2 Oven E E E E E E-04 Shield E E E E E-04 Table 3: Oven and Shield Power Coefficients

24 5 Pressure Transducer Calibration Two pressure transducers are used to control and monitor the pressure and gas flow in the TEGA instrument. One Sensor is mounted on the inlet side of the Thermal Analyzer(TA) manifold and is used to provide feedback for the pressure regulation valves from the two gas supply tanks. The second sensor is mounted in the Evolved Gas Analyzer(EGA) manifold and is used to monitor the gas pressure adacent to the Mass Analyzer port and the pressure down-flow of the ovens. The transducer signals are amplified and conditioned by electronics located in the Lower Payload Electronics bay to give nominal signals in the pressure range from a few mb up to Bar(4.7 PSI absolute). The TA Manifold sensor is a 9C030A and the EGA sensor is a 9C05A transducer both originally from the Honeywell(Sensym) and repackaged to provide a light weight interface to the manifolds. Both transducers read pressure in the absolute sense in that they can read pressures down to a vacuum. Deficiencies recognized after the initial calibration required a recalibration done insitu on Mars to update the parameters used to transform raw counts to pressure and flow. The Sol 4 Checkout along with EGA runs through Sol 66 are used to provide the calibration constants used in the final processed data. Given an estimate for the absolute pressure at the landing site, the Mass Spectrometer was used to locate several additional points at higher pressures by using the initial Net Counts/mBar to extrapolate this measurement to higher count rates at higher pressures. A fit for of the pressure as given by the count rate in the Mass Analyzer on Sol 66 was then transferred to the Manifold pressure transducer using data derived from the Sol 4 Checkout. The Sol 4 Checkout consisted of several measurements made while both the manifolds were still capable of being held at a static and equalized pressure by a diaphragm allowing the transfer of the calibration from the TA_OUTLET_PRES to the TA_MANIFOLD_PRES. Later on Sol 4 a puncture mechanism breeched this diaphragm allowing flow to the Mass Analyzer within the EGA and exhausting to Mars. Cross checks for the converted pressure values was correlated against other information related to the volume and flow characteristics of the TEGA plumbing system. These checks demonstrated good agreement between calculated pressures and expected values based on measured values of the Flight Model and equivalent runs in the Engineering Qualification model. The calibration values for the TA_OUTLET_PRES and TA_MANIFOLD_PRES are two part fits shown in table that convert the raw signal to a final pressure in Bar. A second order polynomial is used below the indicated threshold and a linear fit above. The equation for Manifold Pressure is valid for times when the TA_MANIFOLD_TEMP is at or near +65 C and the Outlet Pressure conversion is valid when the EGA_MANIFOLD_TEMP is at or near +35 C. An additional temperature compensation equation is also shown in table 2 for the TA_MANIFOLD_PRES but is not applied in the exported data. Although the devices are temperature compensated it was found that the output signal is not correct when the device is changing temperature rapidly or has a persistent thermal gradient across the transducer. The compensation provided can be

25 applied only when the temperatures are stable and cannot be universally applied on a point by point basis during or within a few minutes after abrupt temperature changes. No temperature Compensation is available at the present time for the OUTLET Pressure Sensor. The non-temperature compensated conversions are relevant to the maority of the data taken during the mission. Channel Threshold A B C Constraint TA_MANIFOLD_PRES DN < E-08 Note TA_MANIFOLD_PRES DN E-05 Note TA_OUTLET_PRES DN < E-07 Note 2 TA_OUTLET_PRES DN E-05 Note 2 Pressure=A+B*raw_pressure+C*raw_pressure^2, C is zero if not shown Table. Pressure Calibration Parameters Note : TA Manifold at +65 Note 2: EGA Manifold at +35 Channel A B Constraint TA_MANIFOLD_PRES E-05 Note 3,4 TA_MANIFOLD_PRES(T arb )= TA_MANIFOLD_PRES+A+B*T Table 2. Temperature Compensation for Manifold Sensor Note 3: Manifold temperature stable and absolute manifold pressure less than 00mB Note 4: Temperature in Celsius

26 0.20 Sol 4 Checkout TA Manifold Pressure High Temp Estimated Manifold Pressure ( Bar) Coefficient values ± one standard deviation K0 =0.628 ± K = ±.4e-005 K2 =2.8325e-008 ± 2.3e-009 Above 3760 Coefficient values ± one standard deviation a = ± b =8.3527e-005 ± 4.4e-007 TA_OUTLET_PRES_SAM fit_ta_outlet_pres_sam_ht fit_ta_outlet_pres_sam_lin_ht TA Manifold (RAW DN) Figure. Example Conversion for the Manifold Pressure The Pressure Transducers provided as part of the TEGA package are primarily to control and monitor the gas flow through the system and as such are required to have an absolute accuracy no better than 0% and a resolution on the order of a tenth of a mb. The measurements made on Sol 66 used as the calibration basis have an estimate of 7.5mB for the ambient Mars atmosphere compared to a value of 7.85 mb from MET reported post mission (about 4.5% error) under steady state conditions

27 6 EGA Mass to Voltage Conversions 6. Theory The Evolved gas Analyzer (EGA) is a magnetic sector mass spectrometer. Ions generated in the ion source are accelerated by a sweep voltage, pass though a magnetic field generated by a permanent magnet where they are deflected and arrive at one of the four detectors. When a given mass is commanded to be scanned the instrument automatically determines the correct sweep voltage to apply. This section outlines the mass-to-voltage equations used in the instrument, the voltage-to-mass equations used in the ground data reduction as well as the in-situ fine tuning of the calibration parameters. The relationship between the mass-to-charge ratio, magnetic field strength, radius of 2 2 curvature to the detector, and accelerating voltage is expressed in m / z = B r / 2V (Equation 3). 2 2 m / z = B r / 2V (Equation 3 ) where: r = radius of arc of ions deflected in the magnetic field V = acceleration voltage applied to ions leaving the ion source (sweep voltage) B = magnetic field strength m/ z = mass to charge ratio of ion For this instrument all ions are assumed to have a single charge ( z = ), making the mass to charge ratio equal to mass ( m /) = m. In order to get a specific mass to reach a detector the sweep voltage, V, is selected via a 6 bit DAC over a range of 2000 to 0 volts. ( ) V = dn (Equation 4) where: V = Voltage dn = 6 bit DAC code Since the magnetic field strength B varies as a function of temperature it becomes necessary to adust the sweep voltage to compensate for the magnet temperature. During surface operations the main magnet experienced temperatures from -50 to 0 C. Over this range the function of the field strength versus temperature is mostly linear

28 6.2 Practice There are two modes in which the EGA can sample masses, sweep mode and hop mode. In sweep mode the voltage is changed in steps in order to cover an entire mass range with even sized steps in mass space. In hop mode only the center of peaks are sampled with either 5 or 7 voltage settings evenly separated in mass space. (Figure 4) Figure 4: Sweep modes vs. Hop modes Sweep mode and hop mode have different requirements of the mass-to-voltage calculations with regards to precision. Small errors in the conversion only affect the start and end voltage for a sweep mode but can make a hop mode fall on the side of a peak or completely miss the peak. 6.3 Calibration (coefficient derivation) In order for hop modes to be precise it was necessary to understand the effect of the magnet temperature on the location of peaks. To determine the exact coefficients for the mass-to-voltage equations several masses on all four channels were continuously scanned and tracked in voltage space while the EGA was slowly ramped in the Mars simulation chamber from -50 to 20 C. Figure 5 shows an example of this data for mass 44 on channel

29 Figure 5: Sweep voltage vs. magnet temperature for mass 44 on channel 4 For each channel the mass, voltage, and magnet temperature data was fit to an equation with the form show in Equation 3 (see Figure 6). This equation is, in part, obtained from equation by setting the magnetic field strength B to a linear form ( B = gx + h), combining constants, and solving for V. The linear part of equation 3 ( mx + n) was added to correctly fit the data although the m term was typically fixed to zero. 2 ax + bx + c f ( M, x) = V = + mx + n (Equation 3) M where: V = Voltage in DAC steps (see equation 2) x = Magnet temperature (average of the two raw magnet temperature readings, see TEGA engineering explained) M = Mass in AMU a,b,c,m,n = Coefficients to be fit Table shows the actual results from these fits which were loaded into the EGA flight software. Channel Channel 3 a a b b c c m 0 m 0 n n Channel 2 Channel 4 a a b b c c m 0 m

30 N n Table : EGA flight software default mass-to-voltage coefficients. When the filaments in the ion source are run at high emission it is necessary to add 20 to the n term from their nominal low emission values. This result was determined experimentally by running the instrument at high and low emission and quantifying the shift in peak location. Subsequently, if the instrument is well calibrated at high emission is it necessary to remove 20 from the n term when switching to low emission. Figure 6: Results from fitting channel 4 data to equation 3. Mass and magnet temperature are displayed on the independent x and y axis, sweep voltage is on the dependent z axis. 6.4 In-Situ Calibration The mass-to-voltage equations described above provides an accurate (~ ± 0. AMU) mass-to-voltage conversion over a large temperature range. There are however many factors, such as filament warm up, that can effect the calibration in subtle ways. These errors are significant enough that tightly clustered mass hop points would not be centered on a peak simply using the default coefficients. To correct for this an in-situ calibration was performed several times during each run

31 To fine tune the calibration, for each channel, two well known masses (M, M 2 ) with large peaks are swept over with high resolution (Figure 7). The EGA flight software then determines the center of those peaks in voltage space (V, V 2 ) and updates the internal coefficients. Figure 7: Actual calibration sweeps for channel 3 using masses 8 and 44. The dotted black line shows where the instrument determined the peak centers to be. Once the voltage at the center of two peaks has been found the instrument solves equation 4 which is a simplified version of equation 3 where the magnet temperature dependent terms have been replaced with A and B. Once solved the c and n coefficients are update for each channel. V A = B (Equation 4) M n + n where: (From Equation 3) - 3 -

32 A = ax 2 B = mx + n M = Mass + bx + c V = Voltage x = Magnet temperature during the calibration sweeps Using M, M 2, V, V 2 from the calibration scan solve equation 4 as a system of equations using substitution. B = A = ( M V M 2 V2 ) ( M M 2 ) ( M V ) ( M B) (Equation 5) where: M =Mass of peak M 2 =Mass of peak 2 V =Voltage at center of peak V 2 = Voltage at center of peak 2 Finally equation 6 describes how the c and n coefficients are updated after a successful calibration. c = n = A 2 ( ax + bx) ( B mx) (Equation 6) Figure 8 shows the effect of a calibration scan on a tightly spaced hop sample on mass 44, channel 3. The black line shows where the peak center was calculated using equation 3 both before and after calibration. The green dots show where the hop points would have been sampled

33 Figure 8: The effect of an in-situ calibration on a tightly spaced hop mode. The left side shows where hop points (green) would have been placed using the default coefficients. The right side is after the coefficients were updated with a calibration scan. Before each calibration scan the instrument was typically commanded to reload the default coefficient values. If a calibration scan fails for some reason (not enough counts, could not find peak center) the default coefficients remained in place. 6.5 Ground Data Reduction When processing data returned from the EGA it is necessary to invert the processes described above. The EGA science data is in terms of counts at a certain voltage setting, so converting from voltage to mass is required. Starting with equation 3 and solving for M results in equation 7. M 2 ( ax + bx + c) = V ( mx + n) (Equation 7) The calibration sweeps are also returned so adustments to the coefficients over the course of a run can be replicated on the ground. 6.6 References Herbert, Christopher G., and Robert A.W. Johnstone. Mass Spectrometry Basics. Florida: CRC Press, 2003 Wright, Wilfred, and Malcolm McCaig. Permanent Magnets. Great Brittan: Oxford University Press,