Gauge, pressure, manometers

Figure 1. Principal circuit of high temperature cell for voltammetry investigations under excess gas pressure 1 - high-temperature stainless steel box 2 -quartz box 3 - crucible and country electrode 4 — indicated electrode 5 - reference electrode 6 - thermocouple 7 - Pt lead wire for crucible 8 - water cooling for cell cover 9 -valve of pressure release in cell 10 - hose coupling 11 - gas control valves 12 - intermediate gas container (filling volume -2 liters) 13 - gauge-pressure manometer.

AP can be measured by placing a sensitive pressure gauge or manometer at the inlet to the powder bed while venting to atmospheric pressure. 

Suitable instrumentation, such as a pressure gauge or manometer, shall be connected to the vessel to indicate the pressure acting on the... 

Notice that a positive AP means the system has a pressure higher than atmospheric pressure, whereas a negative AP means the object has a pressure lower than atmospheric pressure. Manometers become inconvenient for measuring gauge pressures of greater than about 1 atm because of the large column of mercury that must be contained. Less dense liquids, such as water, can be used in a manometer to measure smaller pressure differences. 

The establishment of the International Temperature Scale has been accomplished largely with the aid of measurements made with the helium gas thermometer. The most precise gas thermometry method is the constant-volume method, in which a definite quantity of the gas is confined in a bulb of constant volume Eat the temperature T to be determined and the pressure p of the gas is measured. A problem is encountered however in measuring the pressure a way must be found to communicate between the bulb and the pressure gauge. This is usually accomplished by connecting the bulb to the room-temperature part of the system by a slender tube and allowing a portion of the gas to occupy a relatively small, constant dead-space volume at room temperature. Thus, it is important that the gas volume in the pressure manometer be as small as possible. 

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. 

If P2 is atmospheric pressure, then the gauge pressure at point 1 is simply the difference in the levels of the manometer fluid. 

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 pressure of a gas in a pipeline is measured with an open-end mercury manometer. The mercury level in the arm connected to the line is 14 mm higher than the level in the open arm. What is the gauge pressure of the gas in the line ... 

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. 

Example 2.14. Figure 2.14 shows a tank of gas connected to a manometer. The manometer is a U-shaped glass tube open to the atmosphere at one end and containing water. From the elevations shown, calculate the gauge pressure in the vessel. ... 

We want to know the pressure at D. The simple yay to work manometer problems is to start with some pressure we know and to work step by step to the pressure we want to find. In this case, we know that the gauge pressure at A is zero, because it is open to the atmosphere. Thje water is practically a constant-density fluid therefore, we can use Eq. 2.11 to find the pressure at B ... 

Manometers which are open to the atmosphere are gaiuge-pressure devices and should be calculated in gauge pressure. 

After completion of the fluorination, the liquid CIF3 is poured from the Dry Ice-cooled trap into a steel cylinder again cooled with Dry Ice (without acetone ) (use a good hood, protective goggles and rubber gloves) and a threaded shut-off valve is immediately screwed on. When the steel cylinder has warmed up to room temperature, an iron manometer and an additional valve are screwed on and the contents of the cylinder are allowed to escape imtil a gauge pressure of 1.2 atm. is reached. The GIF, Gig and Fg are thereby removed. 

Under most conditions when measuring the pressure drop of a gas, its density may be ignored, as pm > /on- When the second leg is open to atmosphere, the manometer reading represents the gauge pressure—the absolute pressure will equal the pressure differential plus the barometric pressure. 

Pressure Manometer Differential pressure cell (mechanical or semi-conductor) Bourdon gauge... 

Sometimes diaphragm manometers for gauge pressure have an absolute pressure scale. Here one has to keep in mind that the measurement result is dependent on changes in atmospheric pressure. This effect can be partially compensated by rotating the scale or resetting the needle. For that purpose the instrument has to be evacuated down to a low pressure (lower than the lower limit of indication) and then to be adjusted to show zero. 

Outside the laboratory, mechanical gauges are used to measure gas pressure (Fig. 4.8). A typical tire gauge is probably the most familiar. Tire gauges show the pressure above atmospheric pressure, rather than the absolute pressure measured by a manometer. Even a flat tire contains air that exerts pressure. If it did not, the entire tire would collapse, not just the bottom. The pressure of gas remaining in a flat tire is equal to atmospheric pressure. If a tire gauge shows 25 psi, that is the gauge pressure of the gas (air) in the tire. The absolute pressure is nearly 40 psi—the 25 psi shown by the gauge plus about 15 psi (1 atm = 14.69 psi) from the atmosphere. 

The apparatus for vapor pressure determinations is relatively simple. There are many variations of vapor pressure thermometers, but one described by Scott, shown in Fig. 8.24, will suffice as a common example. Any type of accurate dial gauge or manometer can be used, and in the proper range great precision is obtained. For example, a mercury manometer that can be read to 0.1 mm will yield measurements that may be accurate to 0.0001 degree at... 

It has been pointed out that the use of manometers or recessed transducers to measure pressures in the steady, laminar flow of viscoelastic fluids can give erroneous results ( hole-pressure errors") because the tap hole distorts the streamlines in its immediate vicinity. The streamlines sort of dip into the hole as they go by. With a polymer fluid, do you think that the gauge pressure would be greater than, less than, or the same as that measured with a flush transducer (with no distortion of the streamlines) Why (A sketch is worth at least a dozen words here.) This situation is analyzed in Refs. 7 (p. 150), 10 (Chapter 11), and 11 (p. 75), but don t cheat and look there first... 

Vacuum gauge, capacitance manometer gauge A vacuum gauge that uses the deflection of a diaphragm, as measured by the changing capacitance (distance) between surfaces, as an indicator of the pressure differential across the diaphragm, the pressure on one side being a known value. 

In the U-tube manometer the pressure on one leg balances the pressure on the other leg. By performing a fluid balance and knowing the density of all fluids and their height, one can calculate the pounds per square inch difference between the two. Often the second leg is open to atmospheric pressure so that the pressure difference represents gauge pressure and must be added to barometric pressure to... 

Industrial and Control Instruments. Mercury is used in many industrial and medical instmments to measure or control reactions and equipment functions, including thermometers, manometers (flow meters), barometers and other pressure-sensing devices, gauges, valves, seals, and navigational devices (see Pressure measurements Process control Temperature measurement). Whereas mercury fever thermometers are being replaced by... 

Pressure reducing valves should be of steel constmction, designed for minimum and maximum operation conditions. Pressure gauges should be of ak-kon constmction. Pressure rehef valves should be of the spring-loaded type. Rupture disks may be used only as auxkiary equipment. Differential pressure measurements using mercury manometers should be avoided in ammonia service. 

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