Pressure measurement is the analysis of the force exerted by a fluid (liquid or gas) on a surface.
Different Types of Pressure Measurement Apparatus or Devices:
A. Simple Manometers
(ii) U-Tube Manometer
(iii) Single Column Manometer
B. Differential Manometers
(i) Two Piezometer Manometer
(ii) Inverted U-Tube Manometer
(iii) U-Tube Differential Manometer
3. Mechanical Gauges
(i) Bourdon Tube Pressure Gauge
(ii) Diaphragm Pressure Gauge
(iii) Bellows Pressure Gauge
(iv) Dead-Weight Pressure Gauge
- Atmospheric pressure is measured by a device called a barometer, thus, the atmospheric pressure is often referred to as barometric pressure.
- The barometer consists of an inverted mercury-filled tube with a mercury container that is open to the atmosphere.
- The pressure at point Bis is equal to the atmospheric pressure, and the pressure at C can be taken to be zero since there is only mercury vapour above point C and the pressure is very low compared to Patm and can be neglected.
- Writing a force balance in the vertical direction gives
- In barometer, Hg is used because of its two important properties:
(i) Hg is a high-density fluid.
(ii) Hg has very low vapour pressure.
- The atmospheric pressure at a location is the weight of the air above that point. So, as one goes up in the atmosphere, one feels a reduction in pressure as the air above that person continuously reduces.
- The barometer was invented by Torricelli. To honour him, pressure is represented in the unit of ‘torr’ where 1 torr = 1 mm Hg.
Manometers are classified as
1. Simple: which measure pressure at a point
2. Differential: which measures the pressure difference between any two points.
A. Simple Manometers:
- A piezometer is the simplest form of manometer which can be used for measuring moderate pressures of liquids.
- It consists of a glass tube inserted in the wall of a pipe or a vessel, containing a liquid whose pressure is to be measured. The tube extends vertically upward to such a height that liquid can freely rise in it without overflowing.
- The pressure at any point in the liquid is indicated by the height of the liquid in the tube above that point.
- The pressure measured corresponds to gauge pressure. To find absolute pressure at the point, atmospheric pressure is added to the gauge pressure.
- The location of the point of insertion of a piezometer makes no difference in reading. To avoid the effect of capillarity, the pipe-dia of the piezometric tube should be sufficiently large.
- Cannot be used when large pressure in lighter liquids is to be measured.
- Gas pressure can not be measured, because gas forms no free atmospheric surface.
(ii) U-tube Manometer:
- A U-tube manometer consists of a glass tube in a U-shape, one end of which is connected to the gauge point and the other end open to the atmosphere.
- The tube contains a liquid of specific gravity greater than that of the fluid of which the pressure is to be measured.
- Limitations imposed by piezometers are removed by the use of U-tube manometers.
- The choice of the manometric liquid depends on the range of pressure to be measured. for low-pressure range, liquids of lower specific gravity are used and for high range, generally, mercury is employed.
Consider a U-tube simple manometer is measuring the pressure of a fluid of specific gravity S1 To write an equation for the pressure of the fluid following points should be kept in mind.
- Start from one end of the gauge to another.
- Write the pressure at one end. Add the change in pressure while moving from one level to another.
- Use a positive sign if the next level of contact is lower than the first and a negative one if it is higher.
- This method requires reading of fluid level at two or more points since a change in pressure causes a rise of liquid in one limb of the manometer and a drop in the other.
(iii) Single Column Manometer:
- The limitation of the U-tube manometer is removed in a single-column manometer.
- It is a modified form of a U-tube manometer in which a shallow reservoir having a large cross-sectional area (about 100 times) as compared to the area of the tube is introduced into one limb of the manometer.
- For any change in pressure, the change in the liquid level in the reservoir will be so small that it may be neglected, and the pressure is indicated approximately by the height of the liquid in the other limb.
- Only one reading in the narrow limb of the manometer needs to be taken for pressure measurement. The narrow limb may be straight or inclined.
The inclined type is useful for the measurement of small pressures as they are more sensitive than the vertical type.
B. Differential Manometers:
To measure the pressure difference between any two points in a pipeline or two pipes or containers, a differential manometer is employed.
In general, a differential manometer consists of a bent glass tube with two ends attached to each of the two gauge points between which the pressure difference needs to be measured.
Following are the common types of differential manometers:
(i) Two-Piezometer Manometer:
- It consists of two separate piezometers which are inserted at the two gauge points between which the pressure difference is required.
- This method is useful only if the pressure at each of the two points is small.
- It can not be used to measure pressure differences in gases.
(ii) Inverted U-tube Manometer:
- It consists of a glass tube bent in U-shape and held inverted. Thus, it is as if the two piezometers described above are connected with each other at the top.
- When the two ends of the manometer are connected to the points between which the pressure difference is required to be measured, the liquid under pressure will enter the two limbs of the manometer, thereby, causing the air within the manometer to get compressed.
- An air cock is usually provided at the top of the inverted U-tube which facilitates the raising of the liquid columns to a suitable level in both the limbs by driving out a portion of the compressed air. It also permits the expulsion of air bubbles that might have been entrapped somewhere in the pipeline.
- Inverted U-tube manometers are suitable for the measurement of small pressure differences in liquids.
Sensitivity of inverted U-tube manometer can be increased by replacing air with a manometric fluid of specific gravity such that S2 <S1 In this case, gauge equation can be written as;
(iii) U-tube Differential Manometer:
- In this type, a U-tube is connected between two gauge points.
The lower part of the manometer tube contains a manometric fluid that is heavier than the liquid for which pressure is to be measured (i.e. S2>S1).
- The pressure gauge equation in the case when the two points are at the same level can be written as
- In case, the points A and B between which the pressure difference is to be measured are not at the same level (Figure) and the fluids in A and B are also of different specific gravity then, the general gauge equation can be written as
- For the measurement of very small pressure differences, or for the measurement of pressure differences with high precision, special forms of manometer called Micromanometers are used.
- Micromanometers either magnify the reading or permit the readings to be observed with greater accuracy.
- The manometer contains two manometric liquids of different specific gravities and immiscible with each other and with the fluid for which the pressure difference is to be measured.
3. Mechanical Gauge:
- Mechanical gauges are those pressure measuring devices, which embody an elastic element, which deflects under the action of the applied pressure P, and this movement after being (mechanically magnified), operates a pointer moving against a graduated circumferential scale.
- Generally, these gauges are used for measuring high pressures and where high precision is not required.
- Following are the types of mechanical gauges :
- Bourdon Tube Pressure Gauge
- Diaphragm Pressure Gauge
- Bellows Pressure Gauge
- Deadweight Pressure Gauge
- Strain gauge transducers and piezoelectric transducers are also used to measure pressure. Quartz or Rochelle salt is an example of materials that are used in piezoelectric transducers.