Absolute manometer

A final example of the application of effusive flow is found in the so-called absolute manometer designed by Knudsen for the measurement of very low pressures. If a disk is suspended near a heated surface at a distance which is small compared to the mean free path of gas molecules, then a molecular effusion will take place between the gas molecules in the space between the disk and surface and the rest of the gas (Fig. VII.8). The rate at which molecules enter this space will be proportional to PgTg, where To refers to the temperature of the gas and Po refers to the pressure. The rate at which molecules leave will be proportional to PsTr, where 5T, refers to the mean temperature of the space between disk and surface and... 

The sensitivity gp is determined empirically, by calibration against an absolute manometer such as a McLeod. Sensitivities for the ionization... 

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. 

A quantity of hydrogen is confined in a platinum chamber of constant volume. When the chamber is immersed in a bath of melting ice, the pressure of the gas is 1000 torr. (a) What is the Celsius temperature when the pressure manometer indicates an absolute pressure of 100 torr (b) What pressure will be indicated when the chamber is brought... 

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 specific surface area of eement is eommonly determined directly by air. permeability methods. In the Lea and Nurse method (LI 5). a bed of cement / of porosity 0.475 is eontained in a cell through which a stream of air is f passed, and steady flow established. The specific surface area is caleulated ( from the density of the eement, the porosity and dimensions of the bed of j powder, the pressure differenee aeross the bed, and the rate of flow and ] kinematie viscosity of the air. In the Blaine method (B36), a fixed volume of I air passes through the bed at a steadily deereasing rate, whieh is controlled / and measured by the movement of oil in a manometer, the time required i being measured. The apparatus is ealibrated empirically, most obviously / using a cement that has also been examined by the Lea and Nurse method. The two methods gave elosely similar results. The Blaine method, though not absolute, is simpler to operate and automated variants of it have been devised. 

Manometer h. The pressure manometer is connected to the stainless-steel capillary by a capillary T-tube containing a capillary stopcock, X, which can be opened to admit or remove gas from the bulb. During measurements this stopcock is kept closed and the quantity of gas in the bulb and dead space is thereby fixed. The pressure manometer should have a small internal volume and be capable of measuring absolute pressures with a resolution of 1 Torr or better. As discussed in Chapter XVIII, either capacitance or strain-gauge manometers can be used. The latter are generally less expensive, and high-precision versions of these gauges are available. 

The pressure gauge used in this experiment should be a direct-reading gauge with a relatively small and constant internal volume. Reproducibility is more important than absolute accuracy since relative measurements are made on air (the standard gas used for calibration) and the other gases. The critical feature is that the same initial P and final P inlet pressures are used in all runs. Thus, one can use capacitance, reluctance, or strain-gauge manometers (see p. 596-597). The latter manometers are the least expensive and are adequate if models with the best resolution are chosen. 

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) ... 

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 diOerence of pressure in the layer was measured by the manometer, the absolute error being 0.5 mm of water column. 

Figure 1,7 (a) Open-end manometer showing a pressure above atmospheric pressure, (b) Absolute pressure manometer. 

Equation (1.25) can be used only with consistent units. Note that you must add the atmospheric pressure (i.e., the barometric pressure) to the gauge, or relative pressure (or manometer reading if open on one end), in order to get the absolute pressure. 

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... 

---