McLeod gauge reading

A liquid trap can be placed between the McLeod gauge and the rest of the system to prevent mercury from accidentally spraying throughout your system. If you do not want condensable vapors affecting the McLeod gauge readings or do not want mercury vapors to enter your system, a cold trap can be placed between the liquid trap (shown in Fig. 7.41) and the main vacuum line. 

Langmuir made measurements on the amounts of gas adsorbed at very low pressures by means of direct readings with a McLeod gauge, and came to the conclusion that saturation was reached with the completion of a unimole-cular layer. 

In 1851, Newman developed a mechanical pump that achieved a vacuum of 30.06 in. of mercury on a day that the barometer was reading 30.08 in. This pump was very impressive for the time. Vacuum technology was further enhanced by the invention of the Toepler pump in 1862, the Sprengel pump in 1865, and the McLeod gauge in 1874. 

Be aware that the old oil from a pump (and any solvent used to clean out the old oil) more than likely contains any toxic materials that may have come from the vacuum system. For example, if the system had a McLeod gauge, it is likely that the old oil is contaminated with mercury. The amount of contamination concentration determines how the oil or solvent can be disposed of. Unfortunately, because of the possibility that specific EPA-established concentration levels will change before you read this book, no disposal procedures are provided. Therefore, contact the EPA, or local regulatory agencies, to verify the various toxicity levels and the proper disposal procedures for materials of those levels. 

There are two different methods of reading a McLeod gauge One uses a linear scale, whereas the other uses a square scale. Because the square scale is primarily used, I will limit my explanation to its use. Refer to Fig. 7.42. 

After reading the pressure, rotate the plug of the three-way stopcock 180° to the 2 position to draw the mercury back into the storage bulb. Once the mercury is back in the storage bulb, turn the three-way stopcock 90° to a closed position. Once a vacuum reading has been made, turn the two-way stopcock connecting the McLeod gauge and the vacuum system 90° to a closed position. 

Research by Carstens, Hord, and Martin displayed a relatively high level of mercury being pumped out of a McLeod gauge during the evacuation process. Typical readings (in mg/m3) were... 

The McLeod gauge will measure the pressure of dry gas to an accuracy of about 1 % and is used to calibrate other types of vacuum gauges. It is slow in operation and does not give continuous readings. It does not measure the pressure accurately if condensable vapours are present, and is limited to 10 torr if no refrigerated trap is used. 

These manometers are not satisfactory below 5 to 10 mm, because of the relatively large error in reading the pressure. It is, therefore, desirable to have available a tilting McLeod gauge (Fig- 1 22), which will read up to 5 or 10 mm. The mercury is normally contained in the large bulb A, and when the pressure in the system is to be... 

Because it operates with a Hg column, the McLeod gauge does not read the partial pressure of mercury (ca. 10-3 torr) which continually fills those portions exposed to liquid Hg. 

Figure 6-10. Tilting McLeod gauge. (A) Starting position. ( B) Reading position. (Courtesy - Ace Glass Co., Vineland, NJ, Photograph)...

Most McLeod gauges are calibrated with a quadratic scale. Tilt the gauge until the level of the Hg (A in B above) so it is just to the top of arm C (in B above) and read the pressure at B. This is a quadratic calibration and follows equation 6-1. 

A student made a McLeod gauge in glass blowing class. The volume was measured to be 6.6 cm. The diameter of the capilbo, was 2 mm. What pressure would read if the Hg was 9.2 cm Item the top of the capillary ... 

A McLeod gauge is to be designed so that its highest pressure to be read is 8 torr. If a 2 mm capillary is used and the length from the top of the bulb to the top of the capillary is 10 mm, what must the volume of the system be ... 

Read the pressure by using the McLeod gauge. Refer to Figure 6-10, p. 64, to see how this is done. 

One optional, but highly recommended accessory to a double manifold is an active pressure gauge that monitors the pressure in the vacuum line. Since leaks from the line can ruin experiments with air-sensitive compounds, it is critical to be aware of the status of the vacuum at all times. A traditional mercury McLeod gauge can provide accurate readings of pressure, but thermocouple and Pirani gauges can more quickly and... 

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