1. Operation characteristics of the thermocouple vacuum gauge
The thermocouple vacuum gauge is a relative vacuum gauge, and the corresponding value of the pressure - thermocouple potential is difficult to be calculated accurately by the calculation method. Therefore, it is often calibrated with the absolute vacuum gauge or calibration system under the standard environment conditions. Under certain heating current conditions, the calibration curves of the dry air with DL-3 type thermocouple vacuum gauge are shown in figures 4-9 and 4-10.
Fig. 4-9 DL-3 type thermocouple vacuum gauge calibration curves (10 ~ 10-1Pa)
Fig. 4-10 DL-3 type thermocouple vacuum gauge calibration curves (102 ~ 10Pa)
It is not difficult to see from figure 4-10 that when the pressure begins to gradually decrease from 0.1MPa (not drawn in the picture), the thermocouple electric potential E is always near the zero value. The reason is that there are many gas molecules and the heat of the conduction is much, so the temperature of the hot wire is low and the thermocouple electric potential E is low, and the heat conduction is independent of the pressure at this time (λ≤r1), so the potential E does not change (a small amount of change is caused by gas convection). When the pressure drops to 102Pa, and the thermocouple potential E begins to increase. Because the decrease of the heat conduction of the gas and the increase of the temperature of the hot wire. As the pressure P continues to decrease, the thermocouple potential continues to increase, but it becomes more and more slowly and finally tends to a certain value. After that, p is reduced again, and the E no longer changes, that is, the thermocouple vacuum gauge has reached its limit of measurement, the limit of which is about 10-1Pa.
2. Influence of gas type
The thermalcouple vacuum gauge has different results for different gases. This is due to the different thermal conductivity of different gas molecules. However, the shape of the p-E calibration curves of various gases is the same. Therefore, when measuring the pressure of different gases, the pressure readings of the dry air (or nitrogen) scale and the relative sensitivity of the measured gas can be multiplied, and the actual pressure of the gas can be obtained:
The relative sensitivity of dry air (or N2) is usually 1, and the relative sensitivities of some other commonly used gases and vapors are shown in Table 4-2. The relative sensitivity shows the nature of gas heat conduction. According to the form, the relative sensitivity Sr of gas or vapor with the same number of atoms number in gas molecules increases with the molecular weight increases.
Table 4-2 The relative sensitivities of some other gases and vapors
3. Determination of heating current of thermocouple tube
Due to the materials and processes used to manufacture thermocouple regulators, the heating current of each regulator is not the same; the heating current is also increased due to the regulation of “ageing” during use; in addition, heating of different measurement ranges There is also a difference in current.
(1) 10 ~ 10-1Pa range: Regulators should be vertically invertedand the heating current is fixed at a vacuum better than 10-2Pa (at this time the thermal potential is 10mV, full-scale), and the heating current can be adjusted from 95 to 150mA.
(2) 102 ~ 10Pa range: Regulators should be vertically invertedand heating current is adjusted at 0.1MPa, the heating current can be adjusted to the range of 175 ~ 300mA or more.
The structure of the thermocouple gauge is more complex than the resistance gauge, but the p-E calibration curve is less affected by the outside temperature.