Pressure Measurement Manometer

Manometer

Now let us look at some of the instruments used to measure and indicate values of pressure. Probably the simplest and one of the most accurate instruments used for the measurement of pressure is the manometer. There are several forms and all are based on the simple U tube which is basically a glass tube bent into the shape of the letter U

When a pressure (greater than atmospheric) is applied to one limb of the U tube, with the other limb being open to atmosphere, it deflects the liquid down the high pressure limb and up the open limb until the weight of the displaced liquid, plus the value of atmospheric pressure, balance against the applied pressure. This is a simple application of a manometer where it measures the difference between the applied pressure and atmospheric pressure. Fig. 3.2 shows such a manometer.

The value of the applied pressure, P₁ can be calculated as before with the column of water; so that:

P₁ = rgh

where r = density of the liquid in Kgm^-3

g = acceleration due to gravity = 9.81 ms^-2

h = difference between the heights in both columns in mm

The value of atmospheric pressure need not be considered when making measurements in terms of gauge pressure.

If we now suppose that the liquid in the manometer is water, with a density, r , of

1000 Kgm^-3, and the difference in height, h, is 250 mm, then we can say that the pressure is 250 mmH₂0

or, p = 1000 Kgm^-3 x 9.81 ms^-2 x 250 mm

= 1000 Kgm^-3 x 9.81 ms^-2 x 0.250 m

= 2452 N/m^-2

= 24.52 mbar.

Should some liquid, other than water, be used with a density of r₂ then it is possible to convert its height into the equivalent height of water. Can you make a formula to convert the height of another liquid to an equivalent one for water?

Equivalent height of water = r₂ x height of other liquid.

r

The range of application of water filled manometers is from a few mbar up to about 100 mbar.

Manometers of this form are usually limited in application to low pressure local measurements and for instrument calibration purposes.

In any manometric system the liquid in the tube has a curved surface due to surface tension effects. In order to read the manometer accurately it is important to use the correct section of this curved surface, called a meniscus, to align with the scale as in Fig. 3.3.

For most liquids, including water, the curve is concave and the level should be read at the bottom of this curve.

For mercury the curve is convex and the level should be read at the top of this curve.