The Thermocouple Gauge

The thermocouple gauge is more straightforward than the Pirani gauge and less complicated electronically. The thermocouple gauge has a thermocouple attached to a filament under constant electrical load, and it measures the temperature at all times. If the filament becomes hotter, it means that there is less air/gas available to conduct heat away from the wire, and therefore there is greater vacuum within the system. 

There are two different types of thermocouple gauges One has three wires and the other has four. Both have a dc meter (or voltmeter) that reads the voltage from the thermocouple. The three-wire unit uses ac to heat the filament wire, whereas the four-wire unit may use ac or dc. Although there are essentially no differences in performance between the two, they will likely require different controllers (or different settings) for use. 

The advantages of the thermocouple gauge are fairly consistent with the four stated for the Pirani gauge with a few exceptions  

Thermocouple gauges can be made smaller and are more rugged than 

Although the thermocouple gauge is subject to the same variations in 

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The disadvantages of the thermocouple gauge are somewhat different from those of the Pirani gauge ... 

The thermocouple gauge scale is nonlinear, but the readings can be accurately interpreted by the controller. 

A thermocouple gauge should not be placed on any system with mercury unless there is strict controls set to trap and prevent the mercury from reaching the gauge. The reason for this is that the mercury can contaminate the wires of the thermocouple gauge and create false readings of a (virtual) leak (see Sec. 7.6.4). 

A precision aneroid manometer is used for measurements in the 760— 1 torr range. Thermocouple gauges are used in the 1 — 1 x 10 3 range. A cold cathode ionization gauge is used in the high vacuum range down to 10-6 torr. 

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. 

Fig. 7.9. Sensing element for the thermocouple vacuum gauge. The thermocouple is in contact with a heated filament and measures its temperature. A variant involves a thermistor which serves both as a heating and a sensing element.

Conventional high pressure NICI spectra were obtained using a Hewlett-Packard 5985B quadrupole GC/MS, as described previously (1). Methane was used as the Cl reagent gas and was maintained in the source at 0.2-0.4 torr as measured through the direct inlet with a thermocouple gauge. A 200 eV electron beam was used to ionize the Cl gas, and the entire source was maintained at a temperature of 200° C. Samples were introduced into the spectrometer via the gas chromatograph which was equipped with a 25 meter fused silica capillary column directly interfaced with the ion source. For all experiments, a column coated with bonded 5% methyl phenyl silicon stationary phase, (Quadrex, Inc.) was used and helium was employed as the carrier gas at a head pressure of 20 lbs. Molecular sieve/silica gel traps were used to remove water and impurities from the carrier gas. 

Fig. 10.50 Location of the pressure gauge (P) and the thermocouples (7) at the five axial barrel positions. The three cross sections A-A, B-B and C-C are used for contour plots of the numerical results. [Reprinted by permission from T. Ishikawa, S. Kihara, K. Funatsu, T. Amaiwa, and K. Yano, Numerical Simulation and Experimental Verification of Nonisothermal Flow in Counterrotating Nonintermeshing Continuous Mixers, Polym. Eng. Sci., 40, 365 (2000).]...

Incidentally, thermocouples can be affected by CFCs and display a reading three to five times greater than the real pressure. The gauge itself is not affected because the difference is due to the greater thermal conductivity of a CFCs to air. The user needs to divide the thermocouple reading by a factor of three to five to obtain actual pressure. 

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). 

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

The thermocouple pressure gauge is a bimetallic pressure gauge (range 10 mbar to 10 3 mbar), invented by Voege in 1906115 it measures the temperature between the "hot" junction and a reference cold junction, as affected by collisions of gas molecules and concomitant heat loss from the wire. 

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