Absolute vacuum mercury manometer

Vacuum Absolute Water Pump Mercury Manometer Manometer... 

The pressure of gas being pulled though a line by a vacuum pump is measured with an open-end mercury manometer. A reading of -2 in. is obtained. TOat is the gas gauge pressure in inches of mercury What is the absolute pressure if Paim = 30 in. Hg ... 

The first range can be adequately handled with a simple mercury manometer. ( 1 ) However, absolute pressure transducers, such as the Baratron 200 Series manufactured by MKS Instruments, offer much greater accuracy over a wider pressure range.(8) If accurate and precise pressure measurements are commonly conducted on the vacuum line, the transducers are the sensors of choice. If not, mercury manometers are the economical alternative. 

Evacuated samplers which are sealed with a valve or stopcock must be checked for leaks by evacuating to an absolute pressure of 25 mm Hg, or less. The valves are closed and the vacuum checked after one hour or more. Prior to sampling, the containers should be prepared in an uncontaminated atmosphere, and the evacuated pressure obtained should be measured with a mercury manometer. The volume of collected sample at barometric pressure is calculated as follows ... 

For any vacuums better than 120 mm Hg (or 25 in Hg, at sea level), an ordinary vacuum pressure gauge will not be accurate enough for technical purposes. An absolute mercury manometer, as shown in... 

All these complications can be avoided when making field measurements by using the vacuum manometer shown in Fig. 6.6. The difference between the two mercury levels is the absolutely correct, inches of mercury absolute pressure, or millimeters of mercury (mm Hg). 

The pressure measured by this manometer system is the absolute pressure because the reference (pressure above the mercury) in the vertical tube is a vacuum. 

The pressure of the system was controlled to within 0.5 mm Hg for each series by a Cartesian diver manostat connected to a positive air leak, a vacuum pump, and a surge volume of 12 liters. An absolute mercury-in-glass manometer with a 0.1 mm sliding vernier was used to measure the pressure. Temperatures were monitored via two copper-constantan thermocouples one was located just above the liquid surface and the other just below it. Maximum differences of 0.4°C were detected, but an average of the two readings was reported to 0.2°C. Glacial acetic acid and acetone, both meeting ACS specifications, were used. 

We ignore any temperature corrections to convert the mercury density and also ignore the gas density above the manometer fluid. Then, since the vacuum reading on the tank is 64.5 cm Hg below atmospheric, the absolute pressure in the tank is... 

Because a manometer is a device for measuringj pressure differences, to use one to measure absolute pressure we must measure the difference between the pressure in question and a perfect vacuum. In principle this is impossible, because there is no such thing as a perfect vacuum,[but in practice we may produce vacuums of sufficient quality that the error introduced by calling them perfect is negligible. This idea is used in the mercury barometer shown in Fig. 2.16. This common device is found in most laboratories for measuring the pressure of the atmosphere. The pressure of the atmosphere acts on the mercury in the cup at the bottom and is opposed by the weight of the column of mercury. Calculating this, we find... 

---