How to Read Manometer?

The manometer is an instrument needed to measure pressure using a column of liquid. We all know that pressure is the force per unit area.

When it comes to measuring the low air pressure in the most accurate way, nothing can beat balancing a liquid column of known weight against it and later record the balanced liquid column’s height.

As we dive further into the article, we will unfold a detailed method on how to read manometer. Before that, we should keep in mind that the measures commonly employed are inches of mercury (in Hg), taking few inches of water (in w.c.) and mercury as the fluid, using oil or water as the fluid.

So without further ado, let us dive in.

What Are The Components Inside A Manometer?

Let us talk about the U-tube manometer here, which is a U-shaped tube. It is from the early pressure measuring equipment that is still prevalent for its operating simplicity and inherent accuracy. Inside the tube, the liquid present is mercury.

As the best manometer lacks any moving parts, it needs no calibration. The U-tube manometer has two physical properties: functions of the liquid’s density and gravity.

How To Read An Open-End Manometer?

Remember, we are talking about U-shaped manometers here. An open-end manometer has one end open to the atmosphere.

1. Setting the Zero Reference

With each end open to the air, the liquid inside each column is of the same height. This happens because both legs of this open-end manometer are subjected to equal pressure.

As the liquid stays at the same height in both legs, the tube establishes a zero reference.

2. Measure the Difference in Height

Take it as the zero reference, and attach one leg to an unknown pressure, and the other is open to the air. If the pressure between the two ends differs, the liquid tends to move away from the higher pressure source. (Here, let us assume that the higher pressure is placed on the left portion of the manometer).

The liquid on the left leg lowers, while that on the right one rises.  As long as the liquid weight doesn’t balances the pressure accurately, the liquid keeps moving.  Use a marker to note the new levels. Measure the distance, h, between, between the two column heights.

3. Calculate The Pressure Difference

First, you need to convert the non-metric units to metric ones. Later, convert your manometer readings to the standard units of pressure.

The variation between the forces per unit area of a manometer’s liquid columns gives the resulting pressure, with units like newtons per square meter (pascals) or pounds per square inch (psi).

You will see that the manometer is frequently used to consider the variation in column levels to be one common unit. It is shown in centimeters or mercury inches at a particular temperature, which, afterward, can be converted to pressure’s standard units using a table for converting.

Every pressure measurement is differential. Take the reference to be any pressure – be it atmospheric pressure or zero absolute pressure (an entire vacuum).

With one end exposed to the atmosphere, the measured pressure is the one that surpasses atmospheric pressure, which is 76 cmHg, 101.3 kPa, or 14.7 psi.

Using the following formula, we can calculate the pressure difference:

Δp = P2-P1 = ρgh

Here,

Δp = Differential pressure

P2 = pressure at the higher pressure end

P1 = pressure at the lower pressure end

ρ  = density of the liquid (mercury) in the column  (at a particular temperature)

g = acceleration due to gravity (at a particular elevation and latitude)

h = difference in the column levels

4. The Summary On How To Read An Open-End Manometer

If the column of mercury is higher in the open end,

  • Gas Pressure > Atmospheric pressure
  • P (gas) = P (air) + the height difference

 

If the column of mercury is lower in the open end,

  • Gas Pressure < Atmospheric pressure
  • P (gas) = P (air) – the height difference

How To Read A Closed-End Manometer?

Read Manometer

This is also applicable for a U-shaped manometer. A closed-end manometer has one closed arm, a non-volatile liquid (preferably mercury), and another arm directly connected to the gas to be measured.

1. Marking the Zero Reference

Just as with open-end manometers, the liquid at rest has a level. This liquid level is considered as the zero reference.

Note that here, there is no direct contact with the air.

2. Measuring the distance the liquid travels

If you have worked with a barometer, you might notice the uncanny similarity between a manometer and a barometer. As a barometer, the distance the liquid travels inside the two arms of the U-tube, h, is directly proportional to the container’s gas pressure

When you want to find an unknown gas or liquid pressure, fill it up in the container. Note the zero reference. Apply positive pressure on one end (not the closed-end).

When you perform this action, the liquid is pushed down in one leg, resulting in the liquid rising up the other leg. The distance the liquid moves can be measured. This is the difference in height, h, from the zero reference.

3. Calculate the pressure difference

All the steps to be taken are similar to that of an open-end manometer. After converting to standard units, you can head on to the calculations.

For closed-end manometers, the manometer reading is relatively easy. You only need to know the density, acceleration due to gravity, and the height difference – and there you have it!

Using this simple method, you can easily calculate the pressure difference of any unknown gas or liquid.

Δp  = ρgh

How To Read A Manometer When A Vacuum Is Applied?

This is a special case. We will explain to you in laymen’s terms.

  • When you apply vacuum to one end of a U-tube manometer, the liquid in that particular leg rises. Thus, the liquid falls in the other leg.
  • Here, you simply calculate the difference in height, h, considering a zero reference. This difference is the sum of the measurements above and below zero that indicates the vacuum amount.

How To Read A Reservoir Manometer

Regardless of the shape or size of the tubes, the differential pressure represents the difference in the column heights. As we have seen above, the pressure varies because of a change in the manometer legs’ volume. The liquid travels different distances owing to this.

Let us carry this change in the sizes of tubes further. So far, all the manometers we considered were the U-tube manometers. There are reservoir manometers as well.

Because the U-tube manometers are considered to give an accurate indication of pressure, no matter the tubing’s internal diameter, the U-tube manometer is said to be a primary standard.

As you apply pressure to a reservoir manometer well, the liquid level slightly falls in comparison to the increase in the level.

To rectify the drop, the scale graduations of a column are compensated. It enables to form an absolute reading of the differential pressure. Compared to a U-tube manometer, reservoir manometers have connection guidelines written over them.

The steps you need to follow are:

  • Connect the pressures, which are much higher than the atmospheric ones, to the well. Those with a pressure lesser than atmospheric pressure can be connected directly to the manometer tube.
  • As for the differential measurements are concerned, attach the bigger pressure to the manometer well.
  • The well-connected can be employed for vacuum and gauge measurements when it comes to raised-well manometers.

Another variant of this well-type or reservoir manometer is the draft gauge manometer. With its bent indicating tube, a rise of one inch vertically is extended over hundreds of centimeters of scale length.

How To Read A Digital Manometer

Any limitations that your liquid manometer poses can be solved with a digital manometer. Available in portable sizes, they are convenient to use. They have outputs to control the transfer of measurement data.

When you take pressure measurements using liquid manometers, you need to manually compensate for the normal conditions of gravity and density. With digital ones, this process becomes easier as some correction factors can be easily ignored. Later, the rest can be balanced out for inside the software.

Featuring dual ports, a digital manometer only needs swapping sensors to change measurements related to absolute pressure and differential gauges.

Read Manometer Easily Now!

Now that you are aware of the various ways you can read a manometer, you are good to go! Remember that you can move on to higher levels of reading a manometer once you know the basics.

Do not leave any stones unturned to satiate the learning curiosity. Furthermore, learning is of no help if you do not implement it. If you have learned how to read manometer, it’s time you put your knowledge to use.

Get a manometer and try following our steps to read it. We are sure you can ace it! Good luck!

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