Full Text
IMPROVING THE SENSITIVITY OF A THERMOCOUPLE PRESSURE GAUGE
A thermocouple pressure gauge of the usual design[^1] is a bulb connected to a vacuum system, in which a very thin metallic wire, heated by an electric current, is placed. When the pressure in the system changes, the temperature of the wire changes as a result of the change in the amount of heat removed from the wire through the gas. This change in temperature is detected by means of a thermocouple attached to the heater. In the course of measurements the heater current is kept constant.
The author of the note being reviewed[^2] proposed modifying the design of the thermocouple gauge so that a change in pressure would automatically cause a change in the heater current, so that the change in its temperature due to the change in current would occur in the same direction as that due to the change in pressure. In this case, naturally, the sensitivity of the gauge, \(\Delta t/\Delta p\), should increase, where \(t\) is the temperature of the heater and \(p\) is the pressure. The design of the modified thermocouple gauge and its electrical supply circuit are shown in Fig. 1.
In a glass bulb 6 cm in diameter two wires of “Evrika” alloy and nichrome (diameters 0.1 and 0.078 mm, respectively), welded to one another at the middle, are mounted. Two arms of this “cross” serve as a thermocouple, and the other two as the heater. A molybdenum wire 0.1 mm in diameter is connected in parallel with the heater inside the gauge.
During measurements the current \(I\) in the external circuit (the total current of the heater and the shunt) is kept constant. The thermoelectromotive force of the thermocouple, which depends on the temperature of the heater, is measured with an instrument \(\Pi\).
When the pressure in the gauge increases, the temperature of both the heater and the shunt decreases. The resistance of the heater in this case remains practically unchanged, since the “Evrika” alloy and nichrome have a temperature coefficient of resistance close to zero. The resistance of the shunt, made of molybdenum, which has a considerable tempera-
temperature coefficient of resistance decreases. This causes a redistribution of the current between the shunt and the heater: the shunt current increases, while the heater current falls, which causes an additional decrease in its temperature. The opposite picture is observed when the pressure is lowered.
Figure 2 shows the calibration curves of the manometer described, in the range of air pressures from 1 to 100 microns of mercury. Curve A was taken without a shunt; curve B with a shunt. This graph shows that the sensitivity of the manometer with a shunt is considerably higher than without a shunt.
Fig. 2.
The resistances of the heater and shunt used by the author at room temperature were, respectively, equal to 12 ohms and 4 ohms.
The chief merit of the manometer described is the possibility of obtaining comparatively high sensitivity and accuracy in measuring low pressures (on the order of \(10^{-3}\) mm Hg and below).
L. Kh.
References
- S. Dushman, Scientific Foundations of Vacuum Technique, IL, 1950.
- N. A. Florescu, Journ. Sci. Instr., 29, 298 (1952).