420 Vacuum Systems

7.5.15 The Pirani Gauge

The Pirani gauge uses the principle that (usually) the hotter a wire gets the greater its electrical resistance. Therefore, if the resistance of a wire is going up, it must be getting hotter. This relationship implies that less air/gas is available to conduct heat away from the wire, and therefore a higher vacuum is being achieved.

The accuracy of a Pirani gauge is typically ±20%, although an individual (clean) gauge properly used over a two-year period may show a sensitivity drift of only 2%.53

One immediate complication of the Pirani gauge is ambient temperature: As the room temperature gets hotter, the filament gets hotter, making the gauge read a false "better vacuum" To solve this problem, a dummy filament is included in the Pirani gauge. The dummy filament is evacuated and sealed off (at a lower vacuum than what is likely to be used with the Pirani gauge) and is used as a standard to help calibrate a zero point.

An electrical diagram for a Pirani gauge is shown in Fig. 7.47, where V and D comprise the Pirani tube. D is the dummy filament tube that is sealed off, and V is the tube that is exposed to the vacuum system. The filaments in the V tube are connected to a bridge circuit called a Wheatstone bridge with two resistance units called Rj and R2. Power from the power supply passes across the Wheatstone bridge and is adjusted to the proper setting by R3, whose value is read on the mil-

Table 7.12 Properties of Various Gases0

" Adapted from Spinks, Vacuum Technology,Franklin Publishing Co., p. 22, and from Guthrie, Vacuum Technology,John Wiley & Sons., p 504 (1963).

* Sensitivity constant: These constants are accurate ± 10% over any given range of pressures.

cViscosity: At 15°C, given in micropoises.

dUnits are 103AT, where K = thermal conductivity at 0°C, cal/cm/sec/°C.

liammeter M2. The current read on Mj is proportional to the vacuum. An ammeter will read the current, which is proportional to resistance in ohms.

To set a Pirani gauge, the vacuum on V is set to a pressure lower than what the gauge can normally read. Next Ml is set on its zero point by adjusting the resistance of R2. Thereafter Mj will give proper readings as the pressure is raised to the range of the gauge.

The advantages of a Pirani gauge are as follows:

1.It has a rapid response to changes in pressure.

2.The electrical circuitry in the gauge leads to easy adaptation to recording, automatic devices, and computer sensing.

3.Electrically it is very simple.

4.It measures the pressure of permanent gases as well as vapors.

The disadvantages of a Pirani gauge are as follows:

1.Because not all gases have the same thermal conductivity, different gases will provide different pressure readings for the same pressure.

2.It is limited to the pressure range of about 10"1 to about 10~4 torr.

3.If there is any change in the filament wire's surface condition within the Pirani gauge, there will be a change in the heat loss. This change will result in a change of the gauge calibration as well as a change in the zero.

7.5.16 Cleaning Pirani Gauges

If a Pirani gauge becomes contaminated with backstreamed oil, rinsing the gauge with a suitable solvent should be sufficient. Be sure to rinse with distilled water followed by a methanol rinse. Be gentle with the gauge so as not to break the internal wire, which is fragile. Cleaning is likely to change the calibration, so be prepared to recalibrate the gauge after cleaning.

D

Supply

Fig. 7.47 Pirani gauge schematic.

7.5.17 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:

5.Thermocouple gauges can be made smaller and are more rugged than Pirani gauges.

6.Although the thermocouple gauge is subject to the same variations in apparent readings from real pressure (because of variations in the thermal conductivity of different gases), the differences are less apparent than the Pirani gauge.

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

1.Because not all gases have the same thermal conductivity, you will get different pressure readings for the same pressure with different gases, although the differences for the thermocouple gauge are not as great as those for the Pirani gauge.

2.It is limited to the pressure range of about atmospheric to about 10"3 torn

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

4.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).

Physical abuse, improper cleaning, and age can all cause a thermocouple to break. The symptoms may either be no response to the controller or a jerky twitching of the controller's needle. In either case, the thermocouple is not likely repairable and a new one will be necessary.

7.5.18 Cleaning Thermocouple Gauges

The thermocouple gauge is more durable than the Pirani gauge, which means that after you pour in the appropriate solvents for cleaning, you can shake the gauge for cleaning agitation. This cleaning procedure should be followed by rinses of water, distilled water, and, finally, methanol. There is no viable cleaning technique to remove mercury from a thermocouple gauge.

7.5.19 The lonization Gauge Family

All previously mentioned gauges require a certain level of particle density for operation. Once the level of particle density has dropped below a certain level (approximately 1018 particles/m3), it is not possible to detect transfer of momentum forces either from gas to solid wall or from gas to gas. On the other hand, it is possible to ionize gas particles and then "count" the ionized molecules.

A molecule in a "normal" state has a neutral charge; there is an equal number of electrons and protons. If you subject the molecule to a high amount of energy and knock out one of the electrons, the molecule is now ionized with a positive charge. This charge allows you to force the molecule to travel, bend, be focused (if necessary), and be counted. The number of (positive) ions created is always directly proportional to the molecular number density. It is only proportional to the pressure if the temperature is known and kept constant, and the type of gas being analyzed has known calibration constant for the type of gauge you are using.

There are essentially two types of ionization gauges used in the laboratory: The hotand the cold-ion gauges. A third type, the radioactive ionization gauge, is so limited in both scope and use that it will not be discussed in this book.

The concept of the ionization (ion) gauge is quite simple. Under a given electrical load, the available gas within the vicinity of the vacuum gauge is ionized either by heat or by a high-field (electrical) emission. Then, the ionized gas is collected and counted. From this count you can interpret what you have read as a unit(s) of vacuum and thereby infer the vacuum within the system.

One ironic peculiarity of ion gauges is that the ions collected by the gauges for counting are not re-released to the vacuum system and therefore are bound up as in getter pumps. Therefore an ion gauge also acts as a pump. This feature itself sounds great: What vacuum system couldn't use a little extra pumping? However, this feature adds an accuracy problem because there is no way of knowing whether the vacuum within the confines of the gauge (where active pumping is going on) and the vacuum within the rest of the system is the same. Thus, to maintain accuracy between the pressure within the gauge and within the system, the gauge should not be left on for extended periods of time and the gauge should be connected to the system with large-diameter tubing. This setup decreases the opportunities for a pressure gradient to be established and facilitates equalization between the gauge and the system if a gradient condition occurs.

Hot-cathode gauges are considered fast pumps, but cold-cathode gauges pump 10 times faster. At 10"10 torr there are only 106 molecules per cubic centimeter. If