Manometer Explained | Working Principle
What is a manometer? In this article, we’re going to introduce you to the manometer which is one of the oldest pressure measurement devices still in use today.
A manometer is one of the most accurate devices for measuring pressure in the lower ranges.
Since manometers are so accurate, they are often used as calibration standards.
Manometer applications
Some typical industrial manometer applications include:
– HVAC systems maintenance
– Residential or commercial furnace gas valve pressure measurement
Manometer basics
Alright… let’s get started. All manometers operate on the principle that changes in pressure will cause a liquid to rise or fall in a tube.
There are several different types of manometers and we will discuss many in this article.
As mentioned earlier, all manometers operate on the principle that changes in pressure will cause a liquid to rise or fall in a tube.
Types of manometer
1) U-tube manometer
Let’s look at a U-tube manometer as it is probably the most common manometer in use today. We’ll discuss how it is used to measure pressure.
Types of fluids
As we said earlier, a manometer is filled with a liquid. Typical manometer liquids are mercury, water, and light oils.
It’s worth saying here that mercury was a common manometer fluid in the past, but has largely been replaced due to its environmental and health hazards.
Quite often the liquid is colored to help detect the fluid movement.
Measuring pressure
The tube is filled until both sides are approximately half full. When the pressures are equal, the column of liquid on each side will be at the same height. This is usually marked as zero on a scale.
With both sides of the manometer open to the atmosphere, the fluid level on one side will be the same as the level on the other side because P1 equals P2.
Ok… Now suppose one end of the U-tube manometer is connected to an unknown pressure P1 whose value must be determined. The other end is left exposed to the atmospheric pressure, P2.
The difference in the height of the liquid on the two sides of the tube is the differential pressure.
In this case, the manometer provides a gauge pressure measurement because it is referenced to the atmosphere.
OK… So the total difference in liquid height is 4 units. And this is where the pressure measurement becomes interesting! What does the 4 unit difference represent?
Let’s assume our U-tube manometer is filled with water. That means that our pressure differential is 4 inches of water column.
From our previous article, DP Flow Transmitter Testing and Re-Calibration, you know that inches of water column is a unit of pressure measurement. Using conversion charts, we could easily convert 4 inches of water column to 0.144 psi(g).
As you can appreciate, a U-tube manometer filled with water is only capable of measuring very small pressures. To illustrate, let’s apply a pressure of 5 psi to the P1 side of the U-tube manometer. Oops… we don’t want that to happen!
Increasing measurement accuracy
What if we wanted to accurately measure very low-pressure values?
If we replace the water with a Meriam Red Oil fluid we will get a greater difference in liquid levels. Why is that?
Water has a specific gravity or relative density of 1.0 while Meriam fluid is an oil and has a specific gravity of 0.83.
We will get a much larger difference in liquid level with Meriam fluid resulting in more accurate pressure measurement.
In some cases, vendors will provide a U-tube manometer scaled directly in pressure units such as kilopascals (kPa).
Meniscus error
Manometer users must be aware of meniscus error.
Meniscus occurs when the particles of the liquid stick to the sides of the glass tube. Depending on the direction of pressure applied, the meniscus can be concave or convex. Either way, a reading error may occur.
Let’s have a look at a couple of other types of manometers. One common type of manometer is a barometer used to predict weather conditions by measuring atmospheric pressure.
2) Barometer
A Barometer consists of a glass tube with one end sealed. An evacuated tube has its open end submerged in an open container of mercury.
The pressure exerted by the column of mercury is balanced by the pressure of the atmosphere. The glass tube is calibrated in pressure units.
Any liquid could be used in a barometer, but mercury is used because of its high specific gravity.
A mercury barometer needs to be at least 30 inches tall. A water-filled barometer would have to be more than 33 feet high!
3) Inclined manometer
A manometer that provides even better accuracy than the U-tube manometer is the inclined manometer.
This manometer has a well that contains the liquid and a transparent column.
The column is mounted at an angle.
The pressure is indicated by the vertical amount the liquid rises or falls in the column. Because of the incline, a small change in pressure will cause greater movement of the liquid in the column.
Summary
– A manometer is one of the most accurate devices for measuring pressure in the lower ranges.
– Typical manometer applications include measuring pipe fluid flows, HVAC system pressure, and gas pressure.
– All manometers operate on the principle that changes in pressure will cause a liquid to rise or fall in a tube
– Typical manometer liquids are mercury, water, and light oils.
– One end of a U-tube manometer is connected to an unknown pressure while the other end is left exposed to the atmospheric pressure. The difference in the height of the liquid on the two sides of the tube is the differential pressure.
– Other types of manometers include the barometer and the inclined manometer.
If you have any questions about manometer and its applications, add them in the comments below and we will get back to you in less than 24 hours.
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The RealPars Team
You might want to review 3 of our other articles:
1) DP Level Measurement Explained
2) Pressure Transmitter Explained
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