Gauge, pressure, manometers mechanical

G. Bourdon Gauges. As an alternative to mercury manometers there is a variety of gauges based on mechanical or electrical pressure transducers. This section presents a description of purely mechanical gauges which still find use in this electronic age.4 The metal Bourdon gauge (Fig. 7.5) is fashioned around a semicircular thin-walled metal tube with mechanical linkage to a pointer. Fused-quartz spiral gauges are also available. In this case, a thin spiral is sensitive to a pressure differential, and the deflection is balanced with air pressure in the surrounding envelope. The air pressure is then measured with a manometer. 

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

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. 

If the pump is a filter pump off a high-pressure water supply, its performance will be limited by the temperature of the water because the vapour pressure of water at 10°, 15°, 20° and 25° is 9.2, 12.8, 17.5 and 23.8 mm Hg respectively. The pressure can be measured with an ordinary manometer. For vacuums in the range lO" mm Hg to 10 mm Hg, rotary mechanical pumps (oil pumps) are used and the pressure can be measured with a Vacustat McLeod type gauge. If still higher vacuums are required, for example for high vacuum sublimations, a mercury diffusion pump is suitable. Such a pump can provide a vacuum up to 10" mm Hg. For better efficiencies, the pump can be backed up by a mechanical pump. In all cases, the mercury pump is connected to the distillation apparatus through several traps to remove mercury vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used. 

If vapour pressure measurements are to be an essential part of the work to be undertaken, a cold cathode manometer is probably the best choice, despite the fact that it needs to be calibrated for each molecular species, and its use with mixtures of gases containing two or more species is correspondingly more difficult. If such mixtures are to be investigated, or if the chemicals concerned are corrosive, it is probably most efficient to use a mechanical gauge as a null-point instrument and to measure the pressure by means of a McLeod gauge. 

Spring or Mechanical Gauges (British), which include Petavel Manometer (See above) and High-Pressure Spring Gauge (p C338)... 

Electronic manometers provide a convenient method of pressure measurement in a tensimeter, and the general arrangement may be very simple (Fig. 9.3). The one problem which must be anticipated is long-term zero pressure drift, which can be encountered with an electronic pressure gauge. Drift is minimized by maintaining a constant temperature on the pressure transducer and by avoiding mechanical vibration at the transducer. 

Cold traps must be used if mercury is used in your system (such as manometers, diffusion pumps, bubblers, or McLeod gauges) and if your mechanical pump has cast aluminum parts. Mercury will amalgamate with aluminum and destroy a pump. Even if your mechanical pump does not have aluminum parts, the mercury may form a reservoir in the bottom of the mechanical pump, which may cause a noticeable decrease in pumping speed and effectiveness. Aside from a cold trap between the McLeod gauge and the system, place a film of low vapor pressure oil in the McLeod gauge storage bulb. This oil will limit the amount of mercury vapor entering the system that makes its way to the mechanical pump. In addition, an oil layer should be placed on the mercury surface in bubblers and other mercury-filled components. 

The mechanical phenomena gauges measure the actual force exerted by the gas. They include a U-tube, a capsule dial, a strain, a capacitance manometer, a McLeod gauge, etc. Vacuum is measured according to the displacement of an elastic material or by measuring the force required to compensate its displacement. The measurement ranges from atmospheric pressure to 102 Pa in rough vacuum conditions. 

The international unit of pressure, the Pascal, Pa, is seldom used in this country at this time, mainly because it has inconvenient numbers that are not related to the common measuring devices. A person can visualize 10 mm of mercury, but not 1330 Pascals. Normally, mm Hg is used with coarse gauges, such as the U-tube manometer. For pressures obtained with mechanical pumps, the term torr is common (1 torr = 1 mm Hg), and the term micron x 10 atmosphere) is used with diffusion pumps. We must eventually switch to Pascals, so the pressure in Pascals or kilo Pascals (kPa) will be included, where convenient, in parentheses. A person s lungs produce a vacuum of about 300 torr, and the tentacles of an octopus can attain 100 torr. 

Several terms for pressure measuring devices are used interchangeably including transmitters, transducers, gauges, sensors, and manometers. More precisely, a gauge is a self-contained device that converts a force from the process to a mechanical motion of needle or other type of pointer. A manometer is a term reserved for an instrument that measures the hydrostatic head of a liquid and generally operates near atmospheric pressure. A transducer or transmitter combines the sensor with a power supply and a converter— generally mechanical-to-electrical or mechanical-to-pneumatic. The sensor... 

Pressure can be measured by means of manometers which show the pressure in terms of the different levels of a liquid in a U-tube, by mechanical pressure gauges which record the differential effect of pressure forces on the inside and outside surfaces of a coiled tube, and electronic devices which measure the change of electrical characteristic of an element with pressure. 

Sample Preparation. Gas phase samples have routinely been prepared in cylindrical Klmax KG-55 ampoules (23) using a pressure-drop procedure to determine mixture compositions (2 -26. J+6). Pressure measurements for H2/C3F6 and CH4/C3Fe samples were carried out to an absolute accuracy of O.O5 Torr by means of a Wallace and Tlernan 0-200 Torr model FA-1 15 mechanical gauge. More recent experiments have utilized a Barocell model U7l(-capacitance manometer provided with a model 5TO-D-1OO0T-1B2-H5 temperature stabilized 0-1000 Torr sensor. Individual component pressures measured with the calibrated Barocell had an absolute accuracy of 0.005 Torr. Unless noted otherwise the total sample pressure corresponding to the irradiation temperature was always 1000 10 Torr. Gravimetric checks (g5. Ut) of the total pressures were routinely carried out. 

For many applications, gas-phase samples can be prepared using a routine vacuum line consisting of mechanical and diffusion pumps, a cold trap, thermocouple and capacitance manometer gauges and a manifold with attachments for sample tubes and gas inlets. NMR sample tubes are either sealed with a glass torch or constructed with self-sealing top assemblies. Preparation of samples at elevated pressures requires quantitative transfer operations. With few exceptions, gas-phase NMR samples used to study conformational processes contained a volatile bath gas in addition to the sample molecule, which usually has a low vapour pressure. Molecules with low vapour pressures tend to behave nonideally even at very low pressures and this factor must be taken into consideration when converting measured gas pressures into densities. 

---