Pressure measurement manometers

Pressure in the volumetric apparatus was measured by a McLeod gage and by a wide-bore (30-mm.) mercury manometer. Pressures measured with the McLeod gage were corrected for capillary depression of the mercury meniscus. Pressure in the gravimetric apparatus was controlled by regulation of the tempera-... 

In order to minimize the temperature variation effects on the differential manometer pressure measurements, the whole apparatus is immersed in an oil bath maintained in constant temperature, by means of a PID temperature controller. The bath temperature variations are minimized by insulating the space over the bath and keeping its temperature constant to 0.1 °C by means of a PD temperature controller and an air circulation fan. In addition, the room temperature over the insulating cover is kept constant to 1 °C. 

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. 

Pressure measurement deviees sueh as a manometer are used without disturbing the system being monitored. Another type of reaeting system that ean be monitored involves one of the produets being quantitatively removed by a solid or liquid reagent that does not affeet the reaetion. An example is the removal of an aeid formed by reaetions in the gas phase using hydroxide solutions. From the reaetion stoiehiometry and measurements of the total pressure as a funetion of time, it is possible to determine the extent of the reaetion and the partial pressure or eoneentrations of the reaetant and produet speeies at the time of measurement. 

Mechanical manometers are the oldest, simplest, and most reliable pressure measurement instruments. They have some disadvantages, which is one reason the use of electrical manometers is expanding. Their simplicity and fundamental nature can, however, be an advantage. 

As well as measurement errors due to the pressure measurement instrument itself, other errors related to pressure measurements must be considered. In ventilation applications a frequently measured quantity is the duct static pressure. This is determined by drilling in the duct a hole or holes in which a metal tube is secured. The rubber tube of the manometer is attached to the metal tube, and the pressure difference between the hole and the environment or some other pressure is measured. 

One of the best and most convenient methods of measuring the flow in the terminal is to use the terminal characteristic pressure difference. This requires that the manufacturer of the terminal provide calibration curves, where the flow rate is expressed as the function of the characteristic pressure difference. Some devices have integrated pressure measurement tappings, and the user has only to attach a manometer to measure the pressure difference. 

Manometer pressure The pressure recorded on a manometer, measured in Pa or mm water gauge. 

Fig. 4-2. Pressure measurement. A. Barometer pressure — 755 mm. B. Closed-end manometer pressure = 105 mm. C. Open-end manometer pressure = 755 — 650 - 105 mm.

The usual laboratory gas flow meter is essentially a manometer which measures the pressure difference across a capillary tube through which the gas flows, as in Figure 74,1, showing a design described by W. Barkas (1939). This is intended to prevent a sudden change in pressure from forcing the liquid in the manometer into the rest of the... 

Differential Pressure Measurement Differential manometers are instruments that measure the difference in pressure between two adjacent rooms. Cleaner environments must have a higher pressure than adjacent, less clean environments to prevent flow of relatively dirty air into the cleaner environment. This differential pressure must be monitored and controlled. 

The pitot tube is a device for measuring v(r), the local velocity at a given position in the conduit, as illustrated in Fig. 10-1. The measured velocity is then used in Eq. (10-2) to determine the flow rate. It consists of a differential pressure measuring device (e.g., a manometer, transducer, or DP cell) that measures the pressure difference between two tubes. One tube is attached to a hollow probe that can be positioned at any radial location in the conduit, and the other is attached to the wall of the conduit in the same axial plane as the end of the probe. The local velocity of the streamline that impinges on the end of the probe is v(r). The fluid element that impacts the open end of the probe must come to rest at that point, because there is no flow through the probe or the DP cell this is known as the stagnation point. The Bernoulli equation can be applied to the fluid streamline that impacts the probe tip ... 

A detailed discussion of the problems encountered in vapour pressure measurements at low temperature is given in ref. [46,47], where also the use of an in situ manometer is described. Vapour pressure gas thermometry with other liquids besides He is discussed in ref. [43, p. 49],... 

Balson, E. W. (1947) Studies in vapour pressure measurement. Part in. An effusion manometer sensitive to 5 x 10 6 millimetres of mercury vapour pressure of D.D.T. and other slightly volatile substances. Trans. Farad. Soc. 43, 54—60. 

There are two types of U-tube manometer the open-ended manometer is obviously inconvenient for low pressure measurements if an organic liquid of low vapour pressure is used instead of mercury. The small U-tube with one sealed end is the most common and convenient form. The sealed end is closed whilst the tube is under a good vacuum (p < 10 Torr) and, provided that precautions are taken to preserve the vacuum, the pressure in this reference space (the Torricelli vacuum) can be neglected when calculating the measured pressure. 

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. 

Determination of Detonation Velocity (71-4) Sensitivity to Impact (Sensibilite/ au choc) (74-5) Sensitivity to Friction (75-6) Sensitivity to Initiation (Sensibilite a 1 amorce) (76) Pressure Measurements by Manometric Bomb, by Crusher Test and by Piezoelectric Manometer (79 97) Density Determination (99-100) Chronographs of Schulze and of Le Boulange (101) Tests for Stability by Methods of Abel, Spica, Vieille at 110°C, German at 135° Bergmann-Junk, Su, Hansen-Grotannelli, Silvered Vessel and Taliani (107-09) Explosion Test (109-10)... 

The float is effectively a two-dimensional manometer, and, like its open-ended counterpart, it measures the film pressure difference between the two sides of the float. This is another reason why it is imperative that no leakage occur past the float assembly Leakage would increase the pressure on the reference side of the float. For the same reason, the side of the float opposite the monolayer must be carefully checked for any possible source of contamination, not just misplaced surfactant. One way of doing this is to slide a barrier toward the float from that side to verify that no displacement of the float occurs. In all aspects of film pressure measurement, the torque must be measured with sufficient sensitivity to yield meaningful results. 

Stock solution. The concentration of the stock solution was determined by the sodium iodate-thiosulfate titration method. For each determination, a 100.0-ml solution was prepared and placed in a vessel connected to a manometer for measuring the pressure. The vessel was sealed after insertion of a measured piece of catalase-immobilized CoFoam. The reaction of catalase with peroxide produces O2, and an increase in pressure indicates a degradation of the peroxide. Thus, a change in pressure in the vessel is a measure of the reaction rate. Since it is sufficient to show differences in test samples, the ideal gas law was used to convert the pressure into mass. The barometer was calibrated with a gauge traceable to National Institutes of Standards and Technology (NIST) standards. 

During an adsorption measurement a certain quantity of gas passes through 7 into the part of the apparatus separated by 3, pump 1 being cold. The decrease of pressure in 2 is determined. Part of the entering gas is adsorbed by the thin metal film in C at a pressure measured with the manometer 4 and at a constant temperature. The quantity of gas adsorbed at a given pressure is calculated from the difference between the actual pressure measured with 4 and the pressure computed under the assumption that no adsorption had occurred. After the gas in the separated part of the apparatus had been pumped back into bulb 2 by means... 

The solubility experiments were carried out in a stainless steel system. It had a manifold with ports for adding hydrogen, for evacuating, and for attaching metal chambers of a known volume (a few hundred cubic centimeters). Also connected to the ports were Ashcroft bourdon gages for measuring pressures of 1-100 atm and a mercury manometer for measuring pressures below 1 atm. The ports were equipped with valves, and the volumes of all spaces accessible to the manifold were known accurately. 

Pressure-Measuring Device. Accurate pressure measurements are made with a manometer if needed. In most instruments the carrier gas, hydrogen, and air (for flame ionization detector) are provided with separate gauges. 

It is suggested that the usual test pressure is between 0.3 and 0.5 MPa and the cavity should be at least 25 ml volume to minimise pressure loss during the course of the test. The low pressure cavity should be of as small a volume as possible and this requirement is helped by the use of rigid porous packing to support the test piece against the pressure of the test gas. The low pressure side is connected to a pressure measuring device. Traditionally (as described in the standard) this is a capillary U-tube manometer which has an adjustable height reservoir and a bypass valve. 

Zanoni, B., Peri, C. Giovanelli, G., and Pagliarini, E. 1999. Design and setting up of a water vapor pressure capacitance manometer for measurement of water activity. J. Food Sci. 38 407-423. 

Sood, V.C. and Heldman, D.R. 1974. Analysis of a vapor pressure manometer for measurement of water activity in nonfat dry milk.. /. Food Sci. 39 1011-1013. 

Pressure is measured extensively in the chemical processing industries and a wide variety of pressure measuring methods has been developed. Some of these have already been discussed in Volume 1, Section 6.2.2, viz. the manometer (which is an example of a gravity-balance type of meter), the Bourdon gauge (an example of an elastic transducer) and mention is made of the common first element in most pressure signal transmission systems—the differential pressure (DP) cell (Volume 1, Section 6.2.3). The latter also frequently forms part of a pneumatic transmission system and further discussion of this can be found in Section 6.3.4. 

C. Flow Measurement by Pressure Drop across an Orifice. Another common scheme for the measurement of flow is based on the determination of the pressure drop on either side of a constriction, such as an orifice or venturi. Either a liquid-filled differential manometer or a pressure transducer with associated digital readout may be used for this pressure measurement. The flow rates determined by these meters are in units such as cm3/s, and it is necessary to make a correction for total pressure to convert these to standard cm3/s or mol/s. 

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. 

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