A pressure gauge is an instrument that indicates the pressure of the fluid in a circuit . The fluid in the circuits can be both liquid and gas; Hydraulic oil and compressed air being the most common, respectively.
We use the pressure gauges for both control and regulation, or to obtain information from our hydraulic or air system. With the pressure gauges we can know the pressure of the circuit quickly and visually at any of the points we want.
Gauges have traditionally been analogue. They consist of a clock with values between 0 and the expected maximum pressure with an added safety margin; and a needle, which indicates the gauge pressure.
Nowadays, it is increasingly common to find digital manometers, which in addition to measuring pressure, can perform other functions such as having alarms or digitally transmitting the values represented in another location.
How a pressure gauge works
As we have indicated previously, a manometer measures the gauge pressure. But what is gauge pressure? It is the result of subtracting the atmospheric pressure from the real pressure.
In the photo above you can see how the oil or compressed air enters with pressure from the lower part, the one corresponding to the intake. It continues through the conduit surrounding the sphere until it reaches the end where it will generate its pressure by moving the needle.
In this way the needle will indicate to us by means of the sphere which is the pressure of the circuit. These gauges are called Bourdon tube. Although there are more types, this is the most common in the industry and covers almost all the possible needs that we are going to have.
In contact pressure gauges the operation is the same, but additionally we have the magnetic field that electronically operates a circuit. For example, if we take the previous photo and it is a normally open contact, while the needle is between 0 and 4, the circuit is open and does not send the electrical signal.
When we apply more pressure and it exceeds 4 bars, it reaches the set point, the circuit is closed and the electrical signal is sent.
If the contact is normally closed, it would work in the opposite way to that explained above. And if it were a switched contact, we would have 2 electrical circuits, as long as the needle is below the switched contact, circuit A will be closed and B open, once the contact is exceeded, it will be reversed and circuit A will be open and B closed.
Types of Pressure Gauges
We have on the one hand the analogue or also called mechanical pressure gauges and on the other hand the digital ones.
-Digital Pressure Gauges: As its name indicates, it will take the measurement and give us the pressure digitally. Due to the development of technologies and the needs of some applications, this type of pressure gauge has been developed that allows us to measure with greater precision and accuracy.
In addition, we can digitally send the measurement values to a remote point from where it is physically located.
-Analog / Mechanical: There are several types, but the most common is the Bourdon tube, followed by the membrane mainly when we require very low pressure measurements.
These manometers represent their measurement value by means of a needle on a clock that marks the different pressures. And in addition, they represent almost the entire use of a pressure gauge in industry.
-Contact: They measure the pressure in an analogue way and open or close an electrical circuit simultaneously.
So it provides us with a reliable measurement and offers us automation. Within the contact pressure gauges we can find 3 models, normally closed, normally open and switched.
The premise of all is that by means of a magnetic field, when the pressure reaches a previously regulated pressure, it will send an electrical signal or it will stop doing it depending on whether it is normally open or closed. In the case of the switched when the pressure reaches regulation, it will close one circuit and open another.
With this type of pressure gauge we achieve automation within our circuits in order to optimize our equipment as much as possible.
-With glycerin: Many gauges are filled with liquids to provide extra protection and permanent lubrication, which extends the useful life of the instrument. In addition, it provides extra protection against corrosive or extreme environments. Although glycerin is the most common liquid it can also be filled with mineral oil and silicone oil.
-Stainless steel: These gauges are designed for special environments or applications, either for hygiene or for the protection of the element. The main food, pharmaceutical, chemical and marine processing applications.
-With Vertical or dorsal intakes: Simply depending on the position that the pressure gauge is going to have with respect to the pressure connection, it must have the connection at the bottom or rear of it. It does not influence the operation at all, but in practice for the comfort of the operator.
How to read a pressure gauge and how to measure pressure with a pressure gauge
A pressure gauge has a needle that will indicate the current pressure of the circuit we are measuring. This needle can return different values depending on the unit of measure that it represents.
The measure used in the International System of Units is the Pascal (Pa). But in practice we find that this measure is too small to measure the working pressures, even more so if our system is hydraulic. Therefore, bars (Bar) are used.
1 Bar = 100,000 Pa
On the other hand, you will commonly hear about kilos, which corresponds to the measurement in kg / cm2:
1 kg / cm2 = 98,000 Pa
And finally we also have the Psi, which corresponds to the pounds per square inch:
1 Psi = 6.895 Pa
1 Bar = 14.5 Psi
What you should know before buying a pressure gauge
It is very important to know which pressure gauge you should buy before going to an establishment/ store where these products are sold. The characteristics that you should take into account more are:
-Use: Hydraulic or compressed air
-Range: You must take into account what pressure you need to measure and leave a safety margin. For example, from 0 to 100 bars.
-Thread: The threads of the pressure gauges are normally 1/8, 1/4, 3/8 or 1/2.
-Special features: Stainless, glycerin, bezel, vertical or dorsal socket, etc.
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