Temperature Measurement

Concepts Concept from Claudius Galenus 8 Mixtures of Ice/boiling water Latin temperatura (blending/mixing) At left Galileo s Florentine Thermograph

Florentine Thermometer

Fahrenheit ~1650 Florentine style thermometers developed Started playing about 1707 Previous Florentine thermometers had arbitrary scaling Fahrenheit used fixed 2-3 point references to establish scale. Ice/Salt (0) Body temperature (96)

Fahrenheit In 1714 Gabriel Fahrenheit stunned the world by producing two thermometers that produced the same readings!! Recognized the need for non-arbitrary temperature scales. Unlike Newton put that into practice.

Florentine Thermometer

Celsius Astronomer Anders Celsius (1701-1744) Chose Ice/Boiling water for scale with 100 divisions During 16th Century about 20 different scales developed.

Kelvin William Thompson (1824-1907) Ideal Gases Extrapolated gas behavior and recognized zero point Same scaling as Celsius

Kelvin

Laboratory Temperature Sensing Many types of sensors available Wide variety of Sensitivity Range Stability Calibrations Proper selection often based on several characteristics and/or limitations

Basic Sensor types Glass bulb Still in wide use Stable Scales from 0.1 -> 5 degrees Available calibrated, good lab reference Easy to read, long time constant Mercury must be routed to HAZMAT

Thermocouple Thomas Seebeck (1821) discovered Seebeck effect. Dissimilar Alloys produce thermoelectric circuit

Thermocouples When left open a potential develops based on junction temperature. Practical use not simple. Electric connection of device causes other TC junctions

TC Compensation Connecting eventually to copper forms two more junctions. Use of isothermal block for connection ensures formation of reference junction

TC Compensation Must track changes in compensation junction T Often use other types of sensors for measurement of ref T

Thermocouples Commercial products available with readouts Connectorized or bare junctions available based on needs Selection guides helpful

RTDs 1824 again, Sir Humphrey Davy discovered metal resistances show pronounced temperature dependence.

RTDs Made of either wire or metal thin films. Wire is more stable and robust Thin films have higher R Platinum most commonly used

RTDs NOT self powered Often used in Wheatstone bridge configuration for sensing Good choice for cryogenics

Non-Contact Sensing Thermal IR sensors Often have TC outputs Case T signal problematic Selectable FOV

Semiconductor Sensors/IC s Wide variety of devices Most based on Semiconductor properties Many have linear output w/ T

Semiconductors Semiconductor P-N junctions formed by bringing different materials together Formation of depletion region at junction

PN Junctions When not biased, depletion region forms barrier to current flow Application of external bias modifies barrier

PN Junction Diode Reverse biasing raises potentials and blocks current flow Forward biasing reduces barrier and promotes flow Forward bias condition of interest here

Diode Current At some positive bias, current begins to flow Diode Drop measurable with DVM Voltage is characteristics of material

Ideal Diode V-I Ideal diode current relationship Current controlled by Boltzmann distribution Available energy to overcome internal barrier ev/kt must be constant for constant current Diode Drop i nversely proportional to junction T Separation of Boltzmann tail from bandgap with decreasing T DVM will read ~.6V at 290K ~1.0V at 90K I =I 0 e ev 0 /kt 1

Other IC devices Wide variety of sensors Transducers Convert characteristic value to signal Most common are T -> V and T -> I Simple to use, inexpensive Often limited T range

AD590

DS1820

Sensor Temperature Errors

Energy Flow Errors

Measurement Errors Basic T error. Sensor isolated from object thermally and at equilibrium T out of spec for measurement Measurement error Signal degradation before measurement (R,V) Additional signal (e.g. TC connection junctions) Inadequate calibration Improperly spec d measurement components (wrong tolerance, wrong T coeff.)

Self Heating Thermal impedance of mechanical structure of sensor controls energy flow Effect can be several degrees depending on contact and wattage

Calibration Responses often non-linear Polynomial or piecewise interpolation required ITS90 Standard tables available for most TC s, RTD s

IC Calibration IC types often have second order effects expressed as nonlinearity Removal can provide order of magnitude improvement in accuracy

Choosing Wisely

Really Chilly

Temperatures in Astronomy Telescopes General Environment for records Environment for Control Structure temperature (focus) Mirror T (match to air T) Measure ut for seeing estimation & control Process control (lubricant T, dry air T, dewpoint) IR Sensing of clouds

Temperatures in Instrumentation Simple Monitoring for logging and trending Dewar T, hold times Electronics temperatures Detector/Optics temperatures Thermal control of critical items Detector T. Control behavior etc Dark I Instrument monitoring/control Monitor dt in instrument, control gradients, control heating rates Protection. Heaters/Ventilation to keep components within T limits