When we have a fluid enclosed in a container, the first thing we must measure and know is the pressure ( force it exerts on the walls of the container ) and for this manometers are used ; too much pressure could burst the walls of it.
What is a manometer ?
A manometer is a measuring instrument that allows knowing the gauge pressure of a fluid (either gas or liquid) that is enclosed in a container.
Gauge pressure is measured relative to atmospheric pressure. The manometer uses atmospheric pressure as the reference pressure , since 0 (zero) of gauge pressure is equivalent to atmospheric. Gauge pressure above atmospheric? It will be positive . Under? It will be negative .
Since the manometer uses atmospheric pressure as a reference, and depending on how it takes this value, there are several types of manometers .
1. Bourdon tube pressure gauge
The Bourdon manometer is the most common manometer, it consists of a tube, closed at one end, flattened and spirally wound. The other end of the tube is connected to the container of the fluid whose pressure is to be measured.
The fluid is passed through the tube, which will cause it to unwind. With this movement of the tube, a needle moves that will mark the pressure of the fluid .
2. Manometer with 2 open branches (or glass in U)
This consists of a U-shaped glass tube that contains a liquid, where one of the branches is open to the atmosphere and the other is connected to the container that contains the fluid whose pressure is to be measured.
The fluid from the container enters the tube, generating a “push” to the liquid contained in this tube. Fluid pressure is easy to measure by calculating the hy value by applying Bernoulli’s formula, where:
Pfluid = Patm + ρ*g*h ;
where ρ is the density of the liquid in the tube and g is the force of gravity .
It is a type of Pitot Tube where, once inserted into a pipe, it allows us to measure the pressure of the flow that is available.
3. Truncated manometer
This is not a true pressure gauge as it measures absolute pressures and not gauge pressures. The operation is the same as the U-shaped glass manometer, except that the branch that is open to the atmosphere in this case is closed.
As it is closed, and the atmospheric pressure does not influence, it really starts from zero; It is used to measure small gaseous pressures of 1 Torr (remember: 51 Torr = 1 psi).
4. Metal pressure gauge
They are the ones that are mostly used in industries. These are modified barometers so that the pressure of the fluid to be measured acts inside it and the atmospheric pressure acts outside.
How does a manometer work?
The basic operation of the manometer is based on the Bourdon tube. As we already mentioned, the Boudron tube is based on a coiled tube, where one end is closed, and the other is connected to the fluid whose pressure is to be measured.
The fluid entering the tube generates a force that causes the tube to begin to unwind. This movement, when the tube is unrolled, makes a needle move, which indicates the pressure value of the fluid on a suitable scale.
Like everything related to pressure, it will expand until it finds its balance point, at which point we can make the measurement.
How does a bourdon gauge work?
The Bourdon tube is the most widely used. Therefore knowing its operation well is very important. Here is a video to finish understanding how it works.
Parts of a manometer
- Tubular spring : It is the coiled tube through which the fluid passes.
- Tie rod: Device that pulls the mechanism when the spring extends or contracts.
- Mechanism: Responsible for transforming the force exerted by the strap into a movement of the needle.
- Needle: Shows the value of the measured pressure.
- Sphere: Contains the aforementioned components.
Pressure gauge applications
Pressure gauges can be found in so many applications. Whenever pressure needs to be measured inside a container, there will be a pressure gauge.
- Car tire inflators : To detect the pressure of the tire.
- Gas cylinders : to measure the pressure of the gas inside a cylinder.
- Domestic pipes : to measure if the water pressure is adequate in the houses.
- Boilers : Measure the pressure inside a boiler as a safety measure.
- industrial applications. To measure the pressure of fluids in production processes where knowing the fluid pressure is critical.
Difference Between Pressure Gauge and Barometer
The basic difference is that a manometer measures the pressure in closed containers . While on the other hand, a barometer measures the pressure in the atmosphere or in open containers .
Difference Between Pressure Gauge and Vacuum Gauge
The difference between a pressure gauge and a vacuum gauge is not their operation but the measurement range. The manometer has a scale to measure positive pressures , while the vacuum gauge is applied to measure negative pressures .