Pressure Difference Measurement

12.3.7.1 Introduction

The absolute, barometric pressure is not normally required in ventilation measurements. The air density determination is based on barometric pressure, but other applications are sufficiently rare. On the other hand, the measurement of pressure difference is a frequent requirement, as so many other quantities are based on pressure difference. In mass flow or volume flow measurement using orifice, nozzle, and venturi, the measured quantity is the pressure difference. Also, velocity measurement with the Pitot-static tube is basically a pressure dif­ference measurement. Other applications for pressure difference measurement are the determination of the performance of fans and air and gas supply and ex­haust devices, the measurement of ductwork tightness or building envelope leakage rate, as well as different types of ventilation control applications.

The measured pressure differences in ventilation applications are low or very low. The measurement range varies from a few pascals to several thousand pascals. At the lower end are typically building leakage and air movement-re­lated measurements, where only a few pascals can cause a remarkably large air­flow. The largest pressure differences probably occur in fan performance determination and similar applications. This wide range requires special de­mands on the measuring equipment and selection of the correct instrument for each application (Fig. 12.15).

12.3.7.2 Mechanical Manometers4’18’29

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.

Fluid Manometers

Fluid manometers are devices where the readout of the pressure differ­ential is the length of a liquid column. The most fundamental implementa­tion of this principle is the IJ-tube manometer. This is simply a tube of U shape filled with manometer fluid, as shown in Fig. 12.16. The pressure dif­ferential is applied at both ends of the tube, making the manometer fluid move downward in one limb and upward in the other, until the forces acting on the fluid are in balance.

In ventilation applications, where the density of the manometer fluid is much higher than the density of air, the pressure difference AP can be ex­pressed using the equation

AP = pgb, (12.24)

Where P is the density of the manometer fluid, G is the acceleration due to gravity, and H is the height between the two columns of the manometer fluid. Because the density of the manometer fluids commonly used is quite high (800-1000 kg nr3), the sensitivity of the U-tube manometer is low.

Pressure Difference Measurement

Pressure Difference Measurement

Pressure Difference Measurement

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The influence of rhe absorption of water vapor from the atmosphere into the fluid

The specific gravities of oils and alcohols are about 0.8, of water 1.0, and of mercury 13.6. Alcohol has a low surface tension; however, it tends to absorb water and evaporate, and its density varies considerably with temperature.

A very obvious way to change the measurement range and sensitivity of a fluid manometer is by using fluids of different densities. There are only a few suitable liquids with specific gravity between that of water and mercury. Ethylene bromide has a specific gravity of 2.2 and acetylene tetrabromide 3.0., but they are corrosive.

Other Mechanical Pressure Transducers

The most frequently applied mechanical manometers in ventilation appli­cations are fluid manometers, but the following types are also used. The Bour­don tube is a small-volume tube with an elliptic cross-section bent to the shape of a circular arc, the C-type. One end is open to the applied pressure while the other end is closed. The pressure inside the tube causes an elastic deformation of the tube and displaces the closed end, which is then converted, by means of a linkage mechanism, into the movement of a pointer. The Bourdon tube may be of a spiral or helical design as well.

The metallic bellows is a series of circular parts, resembling the folds in an accordion. It is joined together in such a manner that it can freely expand or contract axially by changes in pressure. The metal used must be thin enough to be flexible, ductile enough for ease of fabrication, and have a high resis­tance to fatigue failure. This pressure is mechanically amplified and converted to the movement of a pointer. The bellows is the most sensitive of the nonfluid transducers and most suitable for small pressure differences.

A diaphragm is a flexible membrane used for pressure measurement, usually made of metal. A capsule consists of two diaphragms attached at their perimeters

To form a closed volume. A pressure difference applied over the membrane de­forms the structure. This deformation can be converted into the movement of a pointer by means of attached linkages. The diaphragm surface can be of different shapes: dished, flat, or corrugated. The choice depends on the force and deflection needed. The diaphragm can be utilized as a pressure transducer. A capsule made of diaphragms offers a larger movement. The sensitivity can further be increased by attaching several capsules in series to form a stack.

12.3.7.3 Electrical Manometers4,28,29

Electrical manometers have developed during the last 30 years. Modern electrical manometers are well suited for ventilation applications, both in the laboratory and in the field. The advantage of this type of instrument is that they are sensitive enough to measure small pressure differences with electrical output, enabling monitoring. A convenient feature, especially in the field is that the instrument is hand-held and there is no need for leveling on a bench, as for fluid manometers. The conversion of the pressure difference into an electrical signal can be based on several different phenomena.

Positlon-to-Electrical Transducers

Position-to-electrical transducers cover all those mechanisms where the pressure difference is converted into mechanical movement, and this is further converted into an electrical signal. A mechanical structure can serve a coil, a diaphragm, a capsule, a stack of capsules, or a Bourdon tube. The coil move­ment is linked to an electrical device, such as a potentiometer, a capacitor, or a magnetic-coupled linear-variable differential transformer. Due to friction, po­tentiometers are not very sensitive devices. Usually they also need a large me­chanical travel, which creates limitations. The advantages are simplicity and low cost. Magnetic transformers provide a friction-free, sensitive, linear out­put signal. They are widely used in instruments for midrange pressure differ­ences. The capacitive transducer is the most sensitive and for this reason is the most used in low’- and very-low-pressure differential instruments.

Capacitive Transducers30’31

Capacitive transducers measure the deflection of an elastic diaphragm. The simplest capacitive construction is made of two adjacent metallic plates w’ith a dielectric material between them. When a pressure difference over one of the plates causes the plate to deflect, the capacitance changes as a function of the pressure difference. This relationship is nonlinear. To overcome the non­linearity characteristic, a three-plate differential capacitive sensor can be used. When an alternating voltage and its antiphase voltage are applied to the two outer plates, the voltage amplitude of the center plate (diaphragm) is a linear function of the deflection. Using signal conditioning, the AC voltage ampli­tude is converted to a DC output signal. Modern manometers, based on a ca­pacitive transducer, are able to measure from a few pascals up to many kilopascals, thus providing good sensitivity over a wide range.

Strain Gauge Transducers

Strain gauges operate on the resistance change of the gauge material with ap­plied strain. The slight molecular structure deformation of a metallic wire causes a

Change in resistance of a metallic strain gauge. The concentration and mobility changes of minority charge carriers influence the resistance of a semiconductor strain gauge. The strain gauge can be either bonded to a diaphragm that is ex­posed to the pressure difference, or the strain can be transferred to the gauge by mechanical linkage. The pressure difference-induced strain is measured using a Wheatstone bridge circuit. Strain gauges are widely applied in pressure measure­ment. The sensitivity of certain types is suitable also for ventilation applications.

12.3.7.4 Measurement Errors

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.

It is essential to ensure that the following criteria are met; otherwise errors will result. First, the mouth of the hole inside the duct must be smooth and flush with the duct inner surface. No burrs or other irregularities must be on the surface in the vicinity of the hole. Second, the hole must be perpendicular to the tube axis. The size of the hole has an effect on the measured pressure as well. A general rule is, the smaller the hole the better. Very small holes do, however, slow down the re­sponse of the instrument. Usually the hole diameter is a few millimeters. Note also that the smaller the hole, the greater the risk of blockage. Further information on the effect of the hole size can be found, e. g., in Ower and Pankhurst.’2

When working with fluid manometers, the following factors should be re­membered to reduce errors. Manometer fluid may enter the connecting rubber hose, resulting in a faulty reading. Air bubbles in the manometer fluid must be removed. Dirt in the measuring tube or impure manometer fluid will influence the meniscus formation and thus the reading. In general, for all pressure dif­ference measurement, the hoses transferring the pressure to the manometer should be absolutely tight, clean, and free of any internal obstructions.

12.3.7.5 Calibration

From the calibration point of view, manometers can be divided into two groups. The first, fluid manometers, are fundamental instruments, where the in­dication of the measured quantity is based on a simple physical factor: the hy­drostatic pressure of a fluid column. In principle, such instruments do not require calibration. In practice they do, due to contamination of the manometer itself or the manometer fluid and different modifications from the basic princi­ple, like the tilting of the manometer tube, which cause errors in the measure­ment result. The stability of high-quality fluid manometers is very good, and they tend to maintain their metrological properties for a long period.

The second, mechanical and electrical manometers, require more frequent calibration. Changes in the elastic properties of the pressure transducer, wear­ing in mechanical parts, and electronic circuitry drift influence the properties of the instruments, giving rise to repeated calibration.

The principle of calibration is to compare the measurement result of the manometer to be calibrated to that of the measurement result of the reference

Instrument when both manometers are connected to the same reference pres­sure. The problem is providing a stable reference pressure, that is not sensitive to environmental temperature variations. Stabilizing the environment temper­ature is a complex matter and adds to the cost of the calibration equipment.

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