Professional Article. Fire and Ice

Transcription

1 Professional Article Fire and Ice

2 Professional Article Spectrumanalysis Fire and Ice Temperature measurement is important in the industry. In the chemical industry as well as for environmental protection very many temperature sensors are employed for control. Due to their low cost and their simple handling thermocouples are often preferred. Typical questions posed in this context are: How does it work? Why does an analog voltmeter show zero? Which function have the compensation cables and the cold-junction compensation box? Is it possible to connect a thermocouple directly to a standard digital multimeter? Which mistakes are to be prevented? The following article s purpose is to answer these questions. Historical review Le Chatelier was the first in 1887 to use a thermocouple for temperature measurement; it was a Pt PtRh alloy and quite apt to measure precisely from 0 to 1300 degrees C. Thermoelectric effect: an explanation using physics In each metal there are electrons which can move quite freely within the metal as their binding forces to the nucleus are low. Metals known as good conductors provide one electron on an average for transporting electrical charge. This means that the number of electrons available for charge transport is identical to the number of atoms on an average base. The density of atoms per unit of volume is different with every metal. Hence the concentration of free electrons must differ accordingly. In a metal in thermal equilibrium, i.e. the temperature of which is identical everywhere, the negative charges, the electrons, are distributed evenly. Is there a temperature difference T1 T2 across the metal the electrons will diffuse from the warmer to the colder part. Their behaviour is similar to that of gas, hence the expression electron gas is used. A gas will expand if heated and contract if cooled. (Picture 1). Like a gas the electrons will fill the available space evenly. T 1 T 2 T 1 T 2 T 1 = T 2 T 1 > T 2 Picture 1 Distribution of electrons depending on temperature If two metals of different carrier concentration are brought into contact like silver (high concentration) and copper (low concentration) electrons will diffuse from the metal with the higher concentration to the other until an even distribution is achieved in both. (Picture 2). As a consequence a diffusion voltage or contact voltage will develop because the metal with the lower electron count will charge up negatively with respect to the other metal. Picture 2 Equalization of carriers This diffusion of electrons may be explained by their kinetic energy. In thermal equilibrium there are no pressure differences between the two electron gases and also the electric forces causes by the field of the contact voltage cancel. 2

3 Theoretically it should be possible to measure this contact voltage. However, as soon as a voltmeter is connected there will arise further contact voltages at all joints of dissimilar metals. These compensate exactly the contact voltage of the thermocouple. This is proven in the circuit version 1 shown later. This rule holds: The algebraic sum of all thermally generated voltages within a closed circuit is zero as long as all parts are on the same temperature. A thermally generated voltage requires a temperature difference. Temperature ranges of thermocouples Thermocouples allow the measurement of all practically relevant temperatures. The range available extends from 270 (AuFe NiCr) to degrees C (WRe Re3 WRe 25) Thermocouples fabricated from non-metallic materials allow still higher temperatures. Niobcarbide circoniumcarbide will take degrees C. The HM 8112 digital multimeter measures DC and AC, frequency, temperature (using thermocouples, Pt sensors and thermistors) The thermoelectric voltage scale The thermoelectric voltage scale allows to determine which contact voltages are created by the various combinations. In order to identify an unknown material, usually, the first part of the thermocouple is made of Pt and the second of the one to be tested. Holding the junction at 100 degrees C and the cold junction at 0, the thermoelectric voltage measured will be typical of the material under test. The values Kxp given in the thermoelectric scale are those for the materials named in combination with platinum in mv/100 K in the temperature range of interest. For cost reasons Pt in seldomly used in practice. The value of the voltage resulting from a combination of material A with a material B, KAB, is given by: KAB = KAP - KBP. Material X Konstantan (Cu-Ni) 3.47 Nickel 1.94 Palladium 0.28 Platinum 0.00 Tungsten Copper Manganin Iron Proportionality constant K xp in mv/100 K Nickelchromium Silicon Table 1 Thermoelectric scale given for a temperature difference between the joints of 0 and 100 degrees C Example: Fe-Konst: KKonst - KFE = = 5.34 mv/100 K. (According to DIN standard ist should be 5.5 mv/100 K.) The thermal voltage VT is derived like this: VT = KT (TM -TC), where KT = Thermoelectric constant depending on the metal TM = Temperature of measurement spot TC = Temperature of reference junction Example: A copper konstantan thermocouple with a sensitivity of 0.05 mv/k deleivers 16 mv. What is the temperature of the measurement spot if the reference junction is held to 0 degrees by a bottle of ice? TM - TC = VT/KT TM - TC = 16mV/0.05mV/K = 320K, with TC = 0 degrees it follows TM = 320 degrees C. Types of thermocouples and how to differentiate between them In order to differentiate each pair was given an identification character. There are "type J", "type K" thermocouples etc. In table 2 only those listed in DIN IEC 584 are shown. In addition there are the older DIN 437/- 10/13/14, the British BS 4937 and French NF C standards. Each standard has 3

4 Professional Article Spectrumanalysis different colours; a K type thermocouple according to old DIN is red/green, it is brown/blue in the British standard and yellow/purple in the French standard. This makes identification of unknown thermocouples in installations difficult, especially if a defective one has to be replaced. Also, the colour alone often is not sufficient. In practice it is tested whether the thermocouple is magnetic, as Fe, Ni without chromium and Nisil are ferromagnetic. This excludes a number of possible types. As mentioned it may be difficult to determine the exact type of thermocouple in practice. Many parameters have to be considered when selecting a thermocouple (temperature of measurement spot, ambient temperature, humidity, electromagnetic fields etc.) The company Condustrie MET AG in CH 8260 Wetzikon specializes in design and manufacturing of temperature sensors. Compensation cables Compensation cables are less expensive than thermocouples (especially with Pt and Rh). They are flexible and have a low resistance due to their large area. They are preferred for covering long distances. Picture 3 Therocouple type K with connectors and compensation cables Two types of plugs are common (picture 3): The green plug with female counterpart is the standard plug for type R and S thermocouples, mostly used in fixed installations. Handheld instruments use the yellow plug with female counterpart shown which is destined for type K thermocouples. The metallic rod shown is is a thermocouple in a hermetic metal housing made from stainless steel. The thermocouple wires reside in magnesium oxide. Bending and stressing can thus not cause shorts. These rods are mainly used in industry when the measuring spots are difficult to reach, here bending may help, or when there are vibrations to be encountered. The rods may be connected with special pressfit terminals or they can be connected using the compensation cables shown. Standard circuits, reference spot and compensation box Picture 4 Temperature measurement circuit using thermocouple There are two standard circuits. Picture 4 shows the practical application. In the version of picture 4 the reference spot are points 2 and 3. This is very important for absolute measurements. It must be kept always on a constant known temperature (e.g. ice bottle). For longterm measurements an ice bottle is less than desirable. It is then replaced by a socalled compensation box.. This box holds the reference junction and a NTC for the compensation of influences of the ambient temperature on the reference. The reference is mostly constructed a an isothermal block which is as well an electric isolator as a good heat conductor. The HAMEG HM8112 digital multimeter is designed to operate with thermocouples, Pt sensors and thermistors. The second option of connecting a thermocouple is shown in picture 5. This version can often be found in literature. Here the reference is not any more the point of change to a copper wire, but point 2. For absolu- 4

5 te measurements point 2 must be kept at a constant known temerature or a correction will be necessary. The points of change to copper wires must remain at the same temperature in order for their thermal voltages to cancel. Advantages of thermocouples Affordable Longterm stability Small und low thermal capacitance Fast Active sensor Large temperature range (0 to 2800 degr.c) Robust. Disadvantages of thermocouples As each pair of dissimilar metals constitutes a thermocouple (i.e. also solder to copper, appr. 3 µv/degr. C) ) it is mandatory to watch out for undesired therocouples in the complete circuit. The voltages generated are extremely small. In order to realize resolutions of 0.1 degree very low-drift amplifiers are requeired. Depending on the type medium to gross nonlinearities must be compensated for. The signals are very low, e.g. 7 to 75 µv/degr. The reference junction must be kept on a constant known temperature or its output corrected. The mistakes most frequently encountered in practice A thermocouple is manufactured by soldering or welding two wires of dissimilar metals together. A bad solder or welding joint will lead to measurement errors. Sharp bends may lead to breaking the joint or to shorts. Another mistake is wrong polarity. Detection and repairing of faults If the measurement junction is heated and the voltmeter still shows zero an open within the circuit is the probable cause. Bad or missing solder joints may be found with an ohmmeter. A resistance in excess of 1 K is a clear indication of a fault in the system. A short anywhere in the circuit does not mean that the output will be zero. Instead, a new thermocouple will be created on the spot of the short which will cause erroneous results. This can be identified by heating the measurement junction: if the voltage does not change or not appreciably it is highly likely that a short exists somewhere. If a thermocouple is connected with the wrong polarity this will not cause e.g. 150 degr. instead of degr.! Depending on the type of measuring instrument any value in-between may be indicated. The test is the same: heating up the measurement junction. If the voltage drops with increasing temperature the polarity is wrong. A socalled diffusion error is cause by particles from the surrounding atmosphere diffusing into the junction. This is mostly provoked by strong mechanical stress on the thermocouple (such as by bending or vibration) or at high temperatures ( 1000 degr. C). Such defective thermocouples are called poisened. This error is hard to detect as the temperature will still be indicated almost correctly. The diffusion error shows up by a slow drift of the measurement value. When replacing a poisened thermocouple it is recommended to also replace all compensation cables, connectors etc. as they may also be poisened. Type Material Temperature- Accuracy Color DIN IEC 584 Thermosp. (+)-wire (-)-wire Range Probe (+)-wire (-)-wire in mv Category 1 (A) bei 100 C T Cu Cu-Ni -40 to +350 C 0.5 C brown white J Fe Cu-Ni -40 to +750 C 1.5 C black white E Ni-Cr Cu-Ni -40 to +800 C 1.5 C red white K Ni-Cr Ni -40 to C 1.5 C green white N Ni-Cr-Si Ni-Si -40 to C 1.5 C red white R Pt-Rh (13%) Pt 0 to C 1 C yellow white S Pt-Rh (10%) Pt 0 to C 1 C yellow white B Pt-Rh (30%) Pt-Rh (6%) +600 to C 1.5 C n.s. n.s Cu-Ni = Konstantan; Ni-Cr-Si = Nickel-Chrome-Silicon = Nicrosil; Ni-Si = Nickel-Silicon = Nisil Table 2 Comparison of different Thermocouple-Types 5

6 Professional Article Spectrumanalysis Proof of statement that the sum of all thermal voltages within a closed circuit is zero It is further stated that this also holds for a circuit consisting of any number of dissimilar metals. A thermal voltage can only be generated when there is a temperature difference. Proof: The sum of all voltages in the mesh M is: -V 1 + V 2 + V 3 = 0, hence: V 1 = V 2 + V 3 V 1 : (Iron-konstantan): KFe-CuNi = KFe - KCuNi: (-3.47) = 5.34mV/100 K V 2 : (Copper konstantan): KCu-CuNi = KCu - KCuNi: (-3.47) = 4.19mV/100 K V 3 : (Iron-Copper): KFe-Cu = KFe - KCu: (0.72) = 1.15mV/100 K As V 1 = V 2 + V 3: 5,34mV/100 K = 4.19mV/100 K mV/100 K Picture 5 Thermocouple with reference junction This calculation proves that zero voltage will be measured if a voltmeter is directly connected to a thermocouple and all parts of the circuit are on the same temperature. A thermal voltage will only be created if there is a temperature difference between measurement and reference junctions.. 6