1.2) Considering the substrate to be a semi-infinite solid maintained at an initial temperature T i and is
4.4 Calibration of Thermal Sensors
4.4.1 Constant source of temperature
Before getting the actual result, a sample test run is taken to get the XRD of the phenolic resin, so that the resin effect is nullified while taking the actual sample test. Figures 4.4(a-c) show the XRD pattern of Chromel wire, Constantan wire and E-type CSJT respectively. The selection rule for checking the ‘hkl’ value has been taken from the literature [Fultz and Howe, 2008].
Clearly, it can be observed from the graph that there is a certain change in the peak value as captured in case of sensor sample, when compared with individual thermocouple material. Figure 4.5 shows the comparative plot of the sample, which successfully justifies the plastic deformation that is taking place due to abrasion of one material over the other.
thermocouple is the most important parameter responsible to link the voltage variation with the temperature change of the medium. The study of the variation in the output with respect to the inputs is known as the Sensitivity. It depends on materials temperature and crystal structure.
In general, the typical sensitivity and the significance of different types of conventional thermocouples have been listed as follows:
a) Chromel- Constantan (E-type): It has a sensitivity of about 68 µV/°C. It is mostly suited for cryogenic uses and it is non-magnetic with a temperature range of -50 °C to 740 °C.
b) Chromel-Alumel (K-type): It is a general-purpose sensor, which has a sensitivity of about 41 µV/°C. It operates in the very wide range of temperature -200 °C to 1350 °C.
c) Copper-Constantan (T-type): It is best suited for working in the temperature range of - 200 °C to 350 °C. It has a sensitivity of around 43 µV/°C.
d) Iron-Constantan (J-type): It operates in the temperature range of -40 °C to 750 °C. It has a sensitivity of about 50 µV/°C. The restricted range in temperature can be associated with the Curie point of iron, which is 780 °C.
e) Nicrosil-Nisil (N-type): It has a sensitivity of about 39 µV/°C. It has an operating range from -270 °C to 1300 °C. Due to its stability and oxidation resistance, it is suitable for wide range of operation. It is suited for application such as a nuclear reactor.
If ∆𝑇 is the difference in temperature and ∆𝑉 is the thermoelectric voltage between the two ends of the material, then the sensitivity of the material can be calculated as,
, V
Sensitivity S T
(4.1) The calibration of the thermocouple is performed by an “oil-bath based experimental technique”
that provides gradual step rise in temperature [Kumar and Sahoo, 2013]. The calibration rather the experiment is mainly performed to check the linearity between the change in voltage signals with the corresponding changes in temperature across the sensing the material during up-scaling and down-scaling process i.e., during heating and cooling process. Before the sensor is fabricated, the bare wire (locally purchased) must be calibrated, to ascertain the sensitivity of the sensor to be
fabricated. The calibration of the sensor prior to fabrication is essential as the sensitivity of the thermocouple depends on the material property, not on the formation of the surface junction. If we consider the calibration of the E-type thermocouple, bare chromel wire of 3.25 mm and constantan wire of 0.813 mm, having 1 m in length is taken. A small hole is drilled at the top of the chromel wire and the constantan is press-fitted inside the drilled chromel wire so that the connectivity is maintained. The setup consists of a heater, an oil-bath, a scientific thermometer, two beakers and Data Acquisition System (DAS). One end of the thermocouple wire is inserted in the beaker and the other end is kept inside the ice-bath so as to maintain the reference junction temperature in this case as 0 °C, and further, it is connected to the DAS, for acquiring the change in voltage. In this method, hot air is produced in a beaker (where the thermocouple is placed), by heating oil kept in another container as shown in Fig 4.6. The thermocouple experiences convective heating through the hot air inside the beaker placed in the oil bath. The oil is heated using a constant temperature water bath. Further, a scientific thermometer is mounted in the beaker along with the thermocouple to manually record the temperature of the air during heating and natural cooling. Sufficient care is taken to minimize the heat losses from various other sources. DAS (Agilent 34970A), having a sampling frequency of 2 GHz, is used to monitor the change in voltage across the sensor for corresponding changes in temperature. The air is heated from 30 °C to 75 °C with a step change of 5 °C and the temperature, as well as the voltage change, are recorded. The process is repeated during cooling with the same value of step change in temperature. Before noting the final readings, the bath temperatures are monitored at several locations to ensure the uniform temperature gradient in the entire region of hot air. In the present experiment, the entire procedure was repeated three times in order to check the repeatability of the thermocouple.
The calibration result consists of three set of readings, taken both for heating and cooling process and the average value is plotted thereafter as shown in Fig 4.7, which is for E, T, J, and K-types thermocouples respectively. From the graph, it is quite evident that the calibrated thermocouple wires for different types have good repeatability. The graph provides a satisfactory linear variation of voltage with the change in temperature. The calibrated thermocouples namely, E, T, J and K-types respectively as observed have a Sensitivity of 58.96 µV/°C, 28.47 µV/°C, 43.82 µV/°C and 36.02 µV/°C respectively. The specifications of the instruments used for the calibration process is elaborated in Appendix C.
Fig. 4.6: Schematic of oil-bath based calibration set-up with known temperature
(a) (b)
(c) (d)
(e)
Fig. 4.7: Calibration graph showing variation of voltage with temperature of (a) E-type, (b) J-type, (c) T- type, and (d) K-Type CSJTs; (e) the bar chart showing the comparison of sensitivity value
Table 4.1: Comparison of sensitivity between in-house developed coaxial surface junction thermocouple and the conventional thermocouple
Sl.
No.
Types of Thermocouple
Experimental Sensitivity of CSJT (µV/°C)
Ideal Sensitivity of thermocouple (µV/°C) [ASTM, 1993]
1. E-type 58.96 68
2. T-type 28.47 43
3. J-type 43.82 50
4. K-type 36.02 41