Manometer basics & Micromanometer

instrumentation

#1

Manometer:

A manometer is a continuous pressure measuring instrument. The manometer measures the pressure by balancing against the gravitational force of the liquid head. Pressure applied to a liquid column of the manometer affects the liquid level in the column.

Using manometer we can measure high pressure and also negative pressure. They are basically differential pressure measuring device, the difference in the liquid level of both tubes is proportional to the pressure difference across the tube ends.

The tubes are made of transparent glasses or plastics. There are different types of Manometers classified mainly based on the shape of the tubes and usage purpose.

U tube manometer:

u tube manometer

A U-tube manometer can be used for both differential pressure measurement and absolute pressure measurement. Differential pressure is applied to both the tubes and the difference in the height know as Head is measured and it is proportional to the pressure difference.

To measure the absolute pressure, the pressure is applied to one end of the tube, the other end is vented to a sealed vacuum chamber. Such that the applied (atmospheric) pressure is always compared against a vacuum.

Well manometer:

well manometer

One leg of the Well manometer is tube, as usual, the other leg will be a large well. For a small change in level at well there will be a large change in tube. One need only to compare the height of one liquid column, not the difference in height between two liquid columns.

The Well manometer is commonly used for absolute pressure measurement.

Inclined manometer:

inclined manometer

In Inclined manometer, one leg is inclined about 10° angle. Inclined manometers enjoy the advantage of increased sensitivity. Manometer works on the principle pressure balanced by liquid height, and this liquid height is always measured parallel to the line of gravitational pull (perfectly vertical), inclining the manometer tube means that liquid must travel farther along the tube to generate the same change in vertical height than it would in a vertical manometer tube.

Thus, an inclined manometer tube causes an amplification in liquid motion for a given amount of pressure change, so that the resolution and range is increased.

Micromanometer:

micromanometer

The one leg of the micromanometer is the well and other is the inclined tube, consists of magnifier. The meniscus in the inclined tube is located at a reference level fixed by the hairline viewed through the magnifier.

Initialy both the well and magnifier are the same pressure. Application of pressure at the well end moves off the hairline but it can be restored to the initial position by raising or lowering the well (mercury sump) The difference in the initial and final micrometer readings gives the height of the mercury column and hence the pressure.

Advantages of manometer:

  • Simple and Accurate

  • Good repeatability

  • Wide range of fluid can be used

Disadvantages of manometer:

  • Not portable

  • Need of accurate leveling

  • Small range