416 Vacuum Systems

7.5.11 Condensable Vapors and the McLeod Gauge

Condensable vapors are gases near their condensation temperatures or pressures. In the low pressures of a vacuum system they are gases. Examples of fluids that form condensable vapors at STP are water, alcohol, and mercury.

The McLeod gauge works on the ideal gas law principle that the pressure of a gas increases proportionally as its volume decreases. However, condensable vapors do not obey the ideal gas law and therefore, under standard use, the McLeod gauge cannot accurately measure them. Obviously, condensable gases with low vapor pressures at STP do not affect the readings of McLeod gauges significantly enough to alter the validity of the readings.

When a two-way stopcock is opened, all the gases (condensable and non-con- densable) within the vacuum system can pass into the McLeod gauge, the vacuum bulb, and Capillary B. When the three-way stopcock is rotated, the gases that were in vacuum are brought to atmospheric pressures. Any condensable gases that "condense" at higher pressures will occupy less space. This "less space" will show up as a better-quality vacuum than really exists within your system. The McLeod gauge will indicate a lower reading equal to the vapor pressures of the condensable gases at their ambient temperatures.

To determine whether your readings from a McLeod gauge are being affected by condensable vapors, take measurements of hl and h2 for three to four levels (see Fig. 7.45). If the product of (h2 - hx)h2 is not the same but (h2-hx) = P (a constant) seems to hold, then you have at least one condensable vapor present.* With accurate measurements, Wear found it possible to identify the condensable vapors, estimate their concentrations, and determine the vacuum system pressure and partial pressure due to noncondensable gases. To avoid condensable vapor problems in McLeod gauges, limit the amount of condensable vapors that enter the McLeod gauge by using chilled or chemical vapor traps.

7.5.12 Mercury Contamination from McLeod Gauges

There are two mechanisms for contamination from McLeod gauges: (1) contamination which is backstreamed into the system from the McLeod gauge and (2) contamination which comes from the McLeod gauge storage bulb during the evacuation process.

Research by Carstens, Hord, and Martin52 displayed a relatively high level of mercury being pumped out of a McLeod gauge during the evacuation process.

*To make these measurements, a special vacuum system is required. A complete derivation of the formulas and procedures can be found in the book The Design of High Vacuum Systems and the Application of Kinney High Vacuum Pumps by CM. Van Atta, © 1955 by Kinney Manufacturing Division, New York Air Brake Company.

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Fig. 7.45 By making linear measurements as the mercury rises within a McLeod gauge, it is possible to determine whether there are condensable vapors within the McLeod gauge.

However, once a layer of low-vapor-pressure oil was added to cover the mercury in the storage bulb, the readings went down by a decade:

The research authors pointed out that because the levels were erratic and low, the actual health dangers were also rather low.

One should also remember that the oil within the mechanical pump also becomes a trap for emitted mercury vapors. However, the mechanical pump becomes only a temporary trap for the vapors. Once in the pump, the mercury collects in little pools and is slowly emitted into the atmosphere. It has been observed that pump performance is impaired by mercury pools (found within the mechanical pump) by the author's personal observation.

Stopping mercury migration into a system can be achieved by placing a cold trap between the gauge and system. Stopping mercury contamination from a McLeod gauge's storage bulb can be achieved with a thin layer of low-vapor-pres- sure oil over the surface of the mercury. This oil will not affect the gauge's performance in any way.

7.5.13 Cleaning a McLeod Gauge

Invariably, the mercury within a McLeod gauge will get dirty. The telltale evidence is when the mercury does not cleanly run down the glass tubing and/or you see a film on the surface of the mercury. Traps are the best way to avoid this problem.

A McLeod gauge must be removed from a system when it is being cleaned. This removal probably will involve the talents of a glassblower. The mercury should be carefully poured out of the gauge and sent to a mercury distiller for cleaning. The grease should be removed by an appropriate solvent. Silicon grease should never

418 Vacuum Systems

be used on a McLeod gauge because it requires far too much maintenance and replacement.*

Dissolve any mercury remaining in the gauge with nitric acid. Heating the nitric acid will facilitate the cleaning^ Use gloves and work in a fume hood! After draining and thorough rinsing with tap water, there should be a final rinse with distilled water. If you choose to extend the process one more step with a methanol rinse to facilitate drying, be sure that all the nitric acid has been removed because the acid is not compatible with organic materials.

It can be very difficult to get liquid into various parts of a McLeod gauge because of the closed capillary tubing. One solution to this problem is to use a vacuum cleaning setup as shown in Fig. 7.46. A Teflon stopcock is used because it requires no grease.

When the stopcock shown in Fig. 7.46 is turned to one position, a vacuum will be created in the item to be cleaned. Rotating the stopcock 180° will allow the cleaning or rinsing solution to be drawn into the piece. Then by rotating the stopcock one more time while holding the item in a draining position, the liquid can be removed into a filter flask.

Do not break apart a McLeod gauge for easier cleaning. The calibration of the various parts is extremely accurate. Some McLeod gauges have sections that are intended to be dismantled and have ground glass joints between sections. Some tipping McLeod gauges have plastic end caps (on the closed tubes) to facilitate cleaning.

three-way Teflon stopcock

item cleaned

Filter

flask

Heavy walled tubing

Fig. 7.46 Vacuum apparatus for cleaning hard-to-clean apparatus.

*Silicone stopcock grease requires cleaning and replacement every month or two whether the stopcock is used or not (see page 198).

+ Nitric acid containing mercury is a toxic waste. It must be saved and disposed of under proper conditions and with licensed firms.

7.5.14 Thermocouple and Pirani Gauges

Thermocouple and Pirani gauges both use the physical characteristic of heat (within a vacuum) to infer the amount of vacuum within a system. If an object is hot, the only way it can cool down is to transfer its heat to the surrounding area by conduction or radiation. The standard way this transfer happens is that the object conducts its heat through the air (and anything else it is touching) and radiates its heat with IR radiation to other surfaces. In a vacuum, there is less air through which a hot object can conduct its heat, and therefore it can only lose heat through conduction of the remaining air and IR radiation. Both gauges have fast response times and are excellent for determining pressures between 1 and 10"3 torn They also can be used in leak detection (see Sec. 7.6.9).

Thermocouple and Pirani gauges both have filaments within them that are exposed to the vacuum of the system. These filaments are always under small, constant electric loads and, because of their resistance, they get hot. At higher pressures, the air/gas in the system conducts all the heat from the wire. As the vacuum increases, less heat can be lost through (air/gas) conduction, and more heat is then maintained by the wire. Once the vacuum is high enough, heat is lost primarily by conductance from the wires holding the filament and IR radiation, thus creating the lower limits of the gauge.

Both gauges are dependent on the thermal conductivities of the gases that surround them. Because different gases have different thermal conductivity, different gases will indicate different values for the same pressure. It is possible to adjust a gauge reading to a "calibrated accurate" pressure if you know the constant with which to alter the gauge reading. The real vacuum can be determined with Eq. (7.14) and the appropriate sensitivity constant. For a list of sensitivity constants, see Table 7.12. It is impossible to achieve either gauge's potential accuracy when measuring a vacuum system filled with unknown gases.

Pressure = Gauge reading

Sensitivity constant

Electrical Warning: These types of gauges generally do not use high voltage. They do, however, use 110- tol20-V current, which means that common sense should be observed. For instance, do not pull on a cord when unplugging, but instead pull on the cord outlet. Avoid spilling conducting liquids around the gauges. All pieces should be grounded. If the unit has a three-pronged outlet, do not cut the ground off or bypass the ground by using a two-pronged extension cord. Unplug the unit if repair needs to be done. Replace worn and/or frayed cords immediately. Cover all bare (exposed) electrical leads with tape, tubing, shrink tubing, or plastic screw caps. In addition, keep flammable liquids or gases away from electrical devices in case of any sparks and/or electrical arcs.