## Introduction

There are variety of pressure sensing elements. Manometer is one of the pressure sensors used to sense very low level pressure scales.we are going to discuss about manometers in this session

## The U Manometer

The manometer is a simple device to measure small amounts of pressure. It consists of a glass tube of a fixed diameter. It is bent into a U shape with (vertical) sides. The sides are next to a scale. The manometer is filled with a liquid, e.g. water or mercury.

**Absolute Pressure**

One side is sealed with a vacuum above the liquid in the manometer. An unknown pressure is applied to the other side (limb) which forces the liquid down. The difference in height (H) of the liquid column will give the unknown absolute pressure.

For example, a 9 inch difference, with water as the liquid, will give an absolute pressure of 9 x 249.1 = 2242 Pa.

**Gauge Pressure**

One side is left open to the atmosphere. An unknown pressure is applied to the other side. This pressure will force the liquid in the tube down. The height of the liquid gives the gauge pressure of the unknown pressure.

**Differential Pressure**

If unknown pressures are applied to both sides the difference in level (H) will give the difference (differential) between the two in absolute pressure.

*Do you want to know more about different pressure scales*

## The Well Manometer

* *The well manometer is like the U tube manometer. It is used to measure very low pressures. The pressure is measured in inches of water (H_{2}O). This measurement is divided by the ratio of the areas A and B. This gives the unknown pressure. A single limb manometer in a workshop usually has its scale already calibrated to allow for the ratio of the areas A and B. So, no calculations are required.

## The Inclined Limb Manometer

The inclined limb manometer is another device for measuring very low pressures. The unknown pressure is applied to the well and the single limb is tilted. This makes the scale longer so the pressure can be measured more accurately. The actual pressure is the height, H. No calculations are required as the scale is set by the manufacturer to give accurate readings

## Dual tube Manometer

### Advantages of manometers

- can read negative measurements.
- compare 2 liquids.
- simple construction.
- Low cost hence easy to buy.

### Disadvantages of manometers

- Fragile in construction
- Very sensitive