Vacuum gauges thermocouple gauge

Vacuum gauge, thermocouple gauge A vacuum gauge that uses the cooling of a heated thermocouple junction as an indicator of the gas pressme (density). 

Freeze-dryer controls have experienced the most dramatic developments in recent years. The original mercury gauge and thermostatically controlled heaters were initially replaced with analogue vacuum gauges, thermocouples and controls. Eventually, the latter two were superseded by digital devices. 

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... 

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.

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). 

Fig. I. Standard Pyrex glass vacuum line a, thermocouple gauge [Fredericks] b, Heise-Bourdon tube gauge [Dresser] c, reservoir, 500 mL d, removable cold trap e, removable U-trap /, 18/9 ball-and-socket joint g, 2-mm Pyrex high-vacuum stopcock [Kontes] h, three-way high-vacuum stopcock i, Pyrex glass manifold, 10-12-mm diam j, Pyrex glass traps to fit into a standard Dewar flask k, 10/30 outer joint.

Freshly distilled sulfur dichloride (0.612 g, 6 mmol) and trimethylsilyl cyanide [Aldrich] (1.19 g, 12 mmol) are condensed at - 196° into a 50-mL Pyrex round-bottomed flask that is equipped with a Kontes Teflon stopcock to which is attached a 10/30 inner standard taper joint. A Teflon-coated stirring bar is placed in the vessel. The reaction vessel is connected to the vacuum line by a 10/30 outer Joint. The vacuum system is equipped with a Heise Bourdon tube gauge [Dresser Industries] and a Televac thermocouple gauge [Fredericks]. The quantities of SCI2 and (CH3)3SiCN are measured in the vacuum line by means of PVT techniques, assuming ideal gas behavior. 

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].

The apparatus used to determine the pure component isotherms is illustrated in Fig. 1. This system consists simply of the adsorption cell, a constant temperature bath, a source of adsorbate (i.e., CH4), and a high-vacuum oil manometer to measure the adsorption pressure, TTie reference pressure of the manometer is monitored with a thermocouple vacuum gauge. Adsorption temperatures of 76 and 88.5 K were obtained with atmospheric baths of liquid N2 and respectively the other temperatures as measured with a copper-con-... 

The sample is 100 cubic feet (STP) of air compressed into a bottle containing 100 cc STP of active krypton and 75 cc STP of active xenon. Average yields are about 90% for krypton and 95% for xenon. The time required for the separation is about 8 hours of which 3 hours are spent in bleeding the sample into the system. Before the run all charcoal traps have been heated to 350 C and pumped on until the thermocouple vacuum gauge in the manifold... 

With C still in liquid nitrogen, pump until the pressure on the thermocouple vacuum gauge in the manifold is less than one micron. This should require less than five minutes and results in the removal of helium from C with ho loss of krypton. If the trap does not pump down to one micron in less than five minutes, discontinue pumping and proceed to the next step. 

In operation, the pressure was held constant at the desired value by using the signal from either an ion, thermocouple, or mercury manometer vacuum gauge to admit air through a controlled leak. Readings were then taken after thermal equi-... 

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. 

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