Pirani or Thermocouple Gauges

As mentioned in Sec. 7.5.14, many gauges read an inferred pressure, not real pressure. Some vacuum gauges use the thermal conductivity of gases present in the system to infer the pressure of the system. These gauges are based on the concept that less gas will conduct less heat. Because different gases have different thermal conductivities,83 the user needs to make allowances if the gas in a system has a different thermal conductivity than the particular gas a gauge has been calibrated to use. 

One particular advantage with gas leak detectors is the constant distance from the leak to the thermal conductivity sensing device. This constant distance makes the time lag consistent wherever you are sniffing with the probe. When relying on installed thermocouples (on the vacuum system), a leak located right near the thermocouple will provide an immediate and full-strength response, whereas a leak some distance from the thermocouple will have a more subtle effect and will require some time before this effect is observed. 

Do not breathe too close to the sniffing probe or its reference chamber because the C02 from your breath could cause a deflection of the needle. If you are going to use a helium test gas after doing a bubble test, you must wait until the system is completely dry. The water will cause a negative deflection and the helium will cause a positive deflection, providing a combined weaker deflection. If, coincidentally, the water and helium are perfectly balanced, there will be no deflection. 

Once a leak region is indicated, move the detector away from the system being tested and allow the needle to come to a normal position. Then you can go back in and zero in on the specific location. 

---

Never turn a gauge on until the pressure is below 1 pm or less. You must have a second gauge for higher pressures (for example, a Pirani or thermocouple gauge). 

The apparatus is set up as shown in Figure E9-1, p. 529. An all-glass oil diffusion pump should be used so the student can observe how it operates. If either a Pirani or thermocouple gauge is available, it should be connected at I of Figure E9-1. The molecular still used here is a very simple design, is inexpensive, and can be made by almost anyone. Mineral oil should not be used for the oil bath. Its flash point is too close to the distillation temperature to be safe. (An oil bath is used rather than a circular heating mantle so the student can watch the entire operation). It is suggested that two to three spare fuses be taped to the top of the Variacs. 

Let the system pump down to whatever is its natural lowest pressure. If a Pirani or thermocouple gauge is attached to the system, measure this pressure every few minutes to determine how fast a system of this size can be pumped out. When the oil temperature reaches 200 C, continue the distillation for another 30 minutes. 

Thermocouple gauges work on a similar principle but have a thermocouple as sensor connected to a heated platinum filament. The e.m.f. of the thermocouple is measured with a galvanometer or potentiometer. Such gauges have a normal working range from 10 to 10" Torr, but otherwise have characteristics similar to Pirani gauges. 

A thermocouple gauge, Pirani gauge, or manometer must have access to the section being tested. 

Gauges. Because there is no way to measure and/or distinguish molecular vacuum environment except in terms of its use, readings related to gas-phase concentration are provided by diaphragm, McCleod, thermocouple, Pirani gauges, and hot and cold cathode ionization gauges (manometers). 

The use of helium as a probe (or tracer) gas in leak detection is not new. Before the mass spectrometer, it was used with thermocouple and/or Pirani gauges because of the greater thermal conductivity of helium than air. If you list all the attributes of a perfect probe gas, helium obviously does the job ... 

---