Electronic thermocouple calibration

Errors in the Electronic Thermocouple Calibration The thermocouple interfaces with the computer via an analog-to-digital conversion board. Since the thermocouple s output is in millivolts, the interface board will linearize and set the limits of the signal. The instrument usually also will... 

Thrner gauges may be used to determine scale thickness in situ. These are Wheatstone bridge circuit devices that have proved very useful for 40 years or so. As with chloral thermocouples, calibration may be difficult, and the level of magnetic iron content (magnetite) in the deposit may affect the readings. More modem electronic versions, similar to paint thickness testers, are now available. 

Temperature was measured using a Pt-PtlORh thermocouple, calibrated to the melting point of silver. The hot junction was placed about 3 mm below the bottom of the cell. Temperature was kept constant within 0.5 K by means of an electronic controller connected to a separate thermocouple. 

Schooley, J. F., Ed., 1982, Temperature—Its Measurement and Control in Science und Industry, Vol. 5, American Institute of Physics, New York. An excellent source of state-of-the-art thermometry comprised of papers from the Sixth International Temperature Symposium. Topics which are covered include temperature scales and fixed points, radiation, resistance, thermocouple, and electronic thermometry, temperature control, and calibration techniques. Preceding volumes in the series date back to 1939. 

Ambient temperature variations will affect the accuracy and reliability of temperature detection instrumentation. Variations in ambient temperature can directly affect the resistance of components in a bridge circuit and the resistance of the reference junction for a thermocouple. In addition, ambient temperature variations can affect the calibration of electric/electronic equipment. The effects of temperature variations are reduced by the design of the circuitry and by maintaining the temperature detection instrumentation in the proper environment. 

Figure 5. Experimental apparatus 1 - AG cylinder, 2 - electronic balance, 3 - insulated chamber, 4 - heat exchanger, 5 - thermostat, 6 - computer with software, 7, 10 - pressure sensors, 8, 16 — 19 — valves, 9 - calibrated volume, 11 - vacuum pump, 12 - hydrogen vessel, 13 - helium vessel, 14,15 - reducer, 20 - flow meter, 21,23 - thermocouples, 22 - Ians, 24 - sorbent bed, 25 - heat exchanger.

When two different metal surfaces are brought into contact, the surface space charges that were present at their interfaces with a vacuum will be modified. The electrons from the metal of lower work function will flow into the other metal until an interface potential develops that opposes further electron flow. This is called the contact potential and is related to the work-function difference of the two metals. The contact potential depends not only on the materials that make up the solid-solid interface but also on the temperature. This temperature dependence is used in thermocouple applications, where the reference junction is held at one temperature while the other Junction is in contact with the sample. The temperature difference induces a potential (called the Seebeck effect), because of electron flow from the hot to the cold Junction, that can be calibrated to measure the temperature. Conversely, the application of an external potential between the two Junctions can give rise to a temperature difference (Peltier effect) that can be used for heat removal (refrigeration). 

Measuring temperature requires a thermometer or a temperature probe. A thermometer consists of a narrow tube that contains a liquid. The height of the liquid indicates the temperature. A change in temperature causes a change in the volume of the liquid, which results in a change in the height of the liquid in the tube. Electronic temperature probes make use of thermocouples. A thermocouple produces an electric current that can be calibrated to indicate temperature. 

Qualitative observations of the TLC color play are possible with the naked eye. But for precise temperature field analysis, an optical-electronic calibration process is required where the TLC color play has to be analyzed at a series of different temperatures. This procedure should be carried out by slow heating of the system, to ensure a homogeneous temperature distribution inside the FOV. At various temperature levels single pictures are recorded and saved in coimection with the actual temperature recorded by thermocouples, for example. 

Differential calorimeters consisting of two graphite discs (with separate thermocouples each) have been designed and used mainly for cahbration of routine dosimeters (Janovsky 1985). During irradiation, only one of the two graphite discs is irradiated, while the other one is used as reference. The temperature difference measured between the two discs serves to calculate the dose. Radak et al. (1973) designed similar instrument for calibration at low electron energies (400 keV). 

The conditions for the measurements with the LCT model were as follows micro sample holder with a PtRhlO-Pt thermocouple and quartz sheath normal sample mass 2-3 mg heating rate 10°Cmin DTA sensitivity 25 gV (total deflection) TG range 10 mg (total deflection) temperature measurement range 10 mV paper speed of recorder 4 mm min platinum crucible (diameter 5 mm, height 3 mm, open type) reference crucible empty and temperature of the free end of thermocouples for temperature measurement was 0 °C. A TWT-464 electronic potentio-metric recorder made by the Shanghai Second Automatic Instrument and Meter Factory was used for recording data. KNO, In, KCIO4 etc., were used to calibrate the temperatures, and a... 

When certain materials are bonded together, electrons tend to transfer from one to the other. This is called the Volta effect. If two such materials are joined together with two junctions at the same temperature, the plus Volta emf at one junction will be balanced by a minus emf at the other and no current will flow. However, if the two junctions have different temperatures, a current will flow from one junction to the other. This is called the Seebeck effect, and is the basis of the thermocouple. Figure 11.1 (a) shows a thermocouple with a small voltmeter in series with two thermocouple wires (iron and 60 Cu-40 Ni constantan). The emf will be proportional to (T2 - Tj). Figure 11.1 (6) gives the calibration curve for an iron-constantan couple. Thermocouples are used to measure very high (furnace) temperatures, and when the upper range of an iron-constantan thermocouple is reached, a platinum-rhodiiun couple [also shown in Fig. 11.1(6)] may be employed. 

Calibration of a DTA involves adjustment of instrumental electronics, handling and manipulation of the data in order to ensure the accuracy of the measured quantities temperature, heat capacity and enthalpy [614,615,621]. Temperature sensors such as thermocouples, resistivity thermometers or thermistors may experience drifts that affect the mathematical relationship between the voltage or resistance and the absolute temperature. Also, significant differences between the true internal temperature of a sample with poor thermal conductivity and the temperature recorded by a probe in contact with the sample cup can develop when the sample is subjected to faster temperature scans. The important quantity measured in DTA experiments is the AT output from which enthalpy or heat capacity information is extracted. The proportionality constant must thus be determined using a known enthalpy or heat capacity - the power-compensated DSC requires lower attentiveness as it works already in units of power. The factors such as mass of the specimen, its form and placement, interfaces and surface within the sample and at its contact to holder, atmosphere... 

Calibrate the gas chromatograph column oven temperature sensors using an independent, electronic temperature measuring device such as a thermocouple or platinum resistance temperature detector. 

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