Simple mechanical pressure gauges don’t usually work at pressures below 10 Torr, so we need a different way of measuring pressure in our vacuum systems. We use the most inexpensive pressure sensor that works at these levels, which is the T/C gauge. It operates by measuring the thermal conductivity of the gas inside the chamber. As shown in Figure 36, the T/C tube contains a filament heated with an AC constant current and a T/C in contact with the filament. As the pressure decreases, the filament becomes hotter, because the number of gas molecules hitting the wire and conducting heat away from the wire decreases. As temperature rises, the T/C voltage increases and is measured by a sensitive meter that has been adjusted according to the T/C tube’s calibration curve.
Figure 36 A T/C vacuum gauge is able to measure pressures in the range of 1 to 10−3 Torr. It operates by measuring the thermal conductivity of the gas inside the chamber. As the pressure drops, the filament becomes hotter, causing the T/C voltage to increase.
Thermocouple gauge readout units (sometimes called T/C gauge controllers) are easy to come by on the surplus market. Make sure that the one you buy is compatible with the T/C gauge tube you intend to use. Alternatively, you may build your own gauge readout, as shown in the schematic diagram of Figure 37. This circuit is designed for the Teledyne Hastings type DV-6M T/C gauge. We use the model that is terminated with a KF-16 flange. The resistance of the heater filament is 18 Ω (don’t test this with an ohmmeter because you will burn the filament). It requires 0.38 VAC to operate at a current of 21 mA. The T/C’s output is 10 mV at high vacuum, and drops as pressure increases (see Figure 38). The DV-6M is useful in the range of 1 mTorr to 1 Torr, but is most sensitive between 10 and 200 mTorr. The filament is connected between pins 3 and 5, but the T/C and heater functions are combined in this tube (Figure 36), so only one connection (pin 7) carries the T/C’s output signal.
Figure 37 This is a simple readout circuit that you can build to measure pressure with the DV-6M T/C vacuum gauge tube. Before connecting the T/C tube, make sure that the transformer’s output across an 18 Ω, resistor is 0.38 VRMS.
Figure 38 The output of the T/C output of the DV-6M T/C vacuum gauge varies as a function of pressure. These values are valid when the gas in the vacuum chamber is air, and the current into the filament is calibrated to produce 10 mV at 0.01 mTorr. You can use these scales to calibrate your homemade T/C gauge readout unit.
In the readout circuit of Figure 37, a 555 timer IC controls transistor Q1, which drives the primary of a small audio transformer T1. The output of this transformer is used to heat the DV-6M gauge tube’s filament. An op-amp (U3) is used to amplify the signal from the T/C so that it can be measured with a voltmeter. Before connecting the gauge tube, connect an 18 Ω resistor at the transformer’s output and adjust the voltage across this resistor to exactly 0.38 VRMS.
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