Absolute mercury manometers

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... 

For pressures between 0.03 Torr and about 10 Torr, oil manometers are more accurate than mercury manometers, since oil has a much lower density. For example, dibutyl phthalate, which is often used, has a density of 1.046 g cm at 20°C. The absolute pressure is given by... 

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. 

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 ... 

Two mercury manometers, one open-end and the other sealed-end, are attached to an air duct. The reading on the open-end manometer is 25 mm and that on the sealed-end manometer is 800 mm. Determine the absolute pressure in the duct, the gauge pressure in the duct, and the atmospheric pressure, all in mm 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. 

When di.stillation is carried out at pressures up to 2 atm absolute — cj. section 5.4.5 — mercury manometers of 1 m length are suitable for measuring the pressure above atmospheric (Fig. 369). 

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 ... 

Since the cell temperature is measured with a copper-constantan thermocouple, the absolute accuracy is dependent on the accuracy of the thermocouple calibration. This calibration was done in place with reference to the triple point and boiling point of nitrogen and the triple point of argon. The resultant accuracy was estimated to be 0.1°K, while resolution of the potentiometer (L N type K-3) was 0.03 K. In regard to pressure, the accuracy was 0.025 or 0.1 psi, depending on whether a mercury manometer or Bourdon-type gauge was being used. 

Here, = tp-is the difference between the retention time of the solute, tp and of the marker, t . Air is usually used as a marker, when the Thermal Conductivity (TC) detector is used, to account for the dead volume in the chromatographic column. The retention time of the marker has to be subtracted from the solute retention time to reflect the absolute value of the solute retention time as tp. F is the flow rate of the carrier gas measured at room temperature 7V, w is the mass of the stationary phase, and J is a pressure correction factor which depends on /, and P , the inlet and outlet pressures respectively. Pi and P are measured using electronic transducers which are interfaced at the inlet and outlet of the column. These transducers are usually calibrated using a mercury manometer. To calculate the interaction parameter, xn, of the polymer-solute system, Vg from Eqn 1 is utilized as follows ... 

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 closed-tube manometer, Fig. 5-1(a), is originally filled all the way to the closed end so that the difference in mercury levels is the absolute value of the pressure of the gas. An open-tube manometer shows the difference between the pressure of the gas and the barometric pressure—Fig. 5-l( ) shows less than barometric pressure and Fig. 5-l(c) shows a pressure higher than barometric pressure. 

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. 

Most commonly, one arm is evacuated and the manometer indicates the total pressure directly. A temperature correction is necessary to obtain the absolute pressure. This correction allows one to convert the observed reading p(mm) in millimeters of mercury to Torr (1 Torr = 1/760 atm = 1 mm Hg at 0°C and standard gravity) ... 

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... 

The use of open manometers for pressure measurement has the advantage that the whole range from atmospheric pressure down to a few millimetres can be covered. For greater precision the mercurj levels should be determined by cathetometer. An arrangement for reading and recording the mercury levels automatically has been described by Farquharson and Kermicle [33]. At a height of 80 cm the repeatability of operation is better than +0.05 mm and the absolute accuracy better than +0.11 mm. 

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