Gauge, vacuum thermocouple

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.

Figure 4.4.5. DifFerential vapor-pressure apparatus. 100 ml Pyrex flasks connected (a) to a differential pressure transducer (c) with digital readout (d) and (b) to vacuum pump (e) and absolute pressure vacuum thermocouple gauge (f). The constant temperature in the water bath is maintained by a temperature controller (g). The transducer and cormecting glassware are housed in an insulated box (i) and kept at constant temperature shghtly above the measuring temperature by controller (j). Polymer solution and pure solvent (here water) are stirred by underwater magnetic stirrers (h). [Reprinted with permission from Ref. 66, Copyright 1989, American Chemical Society].

Answer by Author Yes, we have subjected some of them to perhaps a dozen cycles between room temperature, liquid nitrogen, and liquid hydrogen. This has been to a thermocouple gauge vacuum only. 

Thermocouple gauge (vacuum technology) A pressure gauge that measures gas density by the cooling effect on a thermocouple junction. See also Vacuum gauge. 

Type J thermocouples (Table 11.58) are one of the most common types of industrial thermocouples because of the relatively high Seebeck coefficient and low cost. They are recommended for use in the temperature range from 0 to 760°C (but never above 760°C due to an abrupt magnetic transformation that can cause decalibration even when returned to lower temperatures). Use is permitted in vacuum and in oxidizing, reducing, or inert atmospheres, with the exception of sulfurous atmospheres above 500°C. For extended use above 500°C, heavy-gauge wires are recommended. They are not recommended for subzero temperatures. These thermocouples are subject to poor conformance characteristics because of impurities in the iron. 

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 ate provided by diaphragm, McCleod, thermocouple, Pitani gauges, and hot and cold cathode ionization gauges (manometers). 

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 vxuum gauge Bimetallic vacuum gauge Thermistor vacuum gauge Cold-cathode ionization vacuum gauge... 

These measure the change in thermal conductivity of a gas due to variations in pressure—usually in the range 0.75 torr (100 N/m2) to 7.5 x 10"4 torr (0.1 N/m2). At low pressures the relation between pressure and thermal conductivity of a gas is linear and can be predicted from the kinetic theory of gases. A coiled wire filament is heated by a current and forms one arm of a Wheatstone bridge network (Fig. 6.21). Any increase in vacuum will reduce the conduction of heat away from the filament and thus the temperature of the filament will rise so altering its electrical resistance. Temperature variations in the filament are monitored by means of a thermocouple placed at the centre of the coil. A similar filament which is maintained at standard conditions is inserted in another arm of the bridge as a reference. This type of sensor is often termed a Pirani gauge. 

There are two basic ways for a vacuum gauge to read a vacuum direct and indirect. For example, say that on one side of a wall you have a known pressure, and on the other side of the wall you have an unknown pressure. If you know that a certain amount of deflection implies a specific level of vacuum, and you can measure the current wall deflection, you can then determine the pressure directly. This process is used with mechanical or liquid types of vacuum gauges. On the other hand, if you know that a given gas will display certain physical characteristics due to external stimuli at various pressures, and you have the equipment to record and interpret those characteristics, you can infer the pressure from these indirect measurements. This indirect method is how thermocouple and ion gauges operate. 

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. 

Oils in a vacuum system can negatively interact with vacuum gauges (mercury can destroy thermocouple gauges and hydrocarbons on thori-ated iridium filaments of hot-ion gauges require constant recalibration). 

Figure 2. Experimental setup of trickle-bed reactor. Key a, reactant vessel b, gas rotameter r, trickle-bed reactor c, shut off valve v, regulating valve d, regulator e, sample valve m, pressure gauge f, liquid rotameter g, temperature recorder h, thermostat i, tubing insulation 1, gas outlet bubbler q, level control n, cryostat p, vacuum pump t, thermocouple s, gas preheater w, purge.

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