Thermal probe temperature, calibration

Figure 7.11. Wollaston thermal probe temperature calibration curves. The soUd line was constructed by melting organic crystals of known T. The broken Une was constructed by holding the probe just out of contact with the variable temperature stage at various temperatures.

A calibration facility must produce the desired velocity range for the meter to be calibrated. The air temperature should be kept constant over the test to ensure constant density. For thermal anemometers, velocity calibration only is not sufficient. They should also be checked for temperature compensation. In the case of omnidirectional probes, sensitivity to flow direction should be tested. In the case of low-speed (thermal) anemometers, their self-convection error should be measured, and, for instruments measuring flow fluctuation (turbulence), dynamic characteristics testing should be carried out as well. ... 

A temperature accuracy test of the column oven measured with a calibrated thermal probe is used. An acceptance criterion of 35 2°C is adopted. 

A good reference material has a number of desirable properties including a well-documented value, avmlability in a suitable form for analysis, homogeneity, stability, low toxicity, and traceability to a national reference laboratory (NRL). In traditional DSC and TMA, metals like indium, tin, and zinc meet these criteria. These metals are not suitable for temperature calibration for localised thermal analysis, however, as they may contaminate the probe tip thus changing its resistance and defeating the object of calibration.. 

A temperature calibration procedure for TMA has been proposed (53-55) and subsequently included as an ASTM method (Test Method for Temperature Calibration of Thermomechanical Analyzers, E1363-90). It uses a penetration probe and the melting temperature of one or more standard materials. Pure metals with sharp melting points are the standards often used. An open DSC pan may be used to contain the calibrant material. Another potential material would be the selected shape memory alloy, reported to be reproducible to 1°C (56). Several reviews on temperature calibration for TMA have been published based on ASTM efforts in this area (54,55). Sircar (26) suggests that, when used for elastomer evaluation, temperature calibration for TMA should be conducted with low melting liquids as in DSC. For calibration of the expansion, one manufacturer s manual (TA Instruments) recommends aluminum for calibrating the linear expansion parameter. Other calibration standards suggested for the linear coefficient of thermal expansion (CTE) are lead (57) and copper (58). 

In real life, the line source takes the form of a hypodermic sensor probe of finite length and diameter [2]. Typical probes are 50 mm (2 in.) long and about 1.5 mm (1/16 in.) in diameter. They contain a heater element that runs the whole length of the hypodermic. A thermocouple sensor is also located halfway down the length of the probe, to measure the temperature rise associated with the transient. These and other nonlinearities require that the probes be calibrated against a reference material. The resultant probe constant is the ratio of the actual thermal conductivity of the reference material to that measured by the instrument. Silicone fluids have been used for the purpose. 

Contact temperature sensors represent a class of temperature probes that are used to determine the temperature of a medium through the thermal equilibrium attained between the sensor and the medium when in contact. The change in the medium temperature is inferred from a corresponding variation in the sensor resistance or output voltage, which can be used to obtain the temperature difference via an a priori calibration of the sensor output with temperature. The commonly implemented contact temperature sensors include thermocouples, resistance temperature detectors (RTDs), and thermistors. 

It has to be noted that the calibration of dissolved oxygen sensors of the Clark type requires homogeneous temperatures from the cathode to the seawater environment. Whenever the probe-body temperature differs from the environment, the diffusive fluxes are not defined and the calibration is not valid. Most sensors require thermal equilibration times of about 20 s or more due to their considerable heat capacities, which is considerably longer than the response times. 

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