Choosing the Right Pressure Gauge

OMEGA portable pressure gauge

The question might seem trivial, but there are both portable and fixed pressure gauges available, and you will have to choose between the two according to your needs.

Portable pressure gauges are often electronic and allow you to carry out punctual measurements and controls in industry or in a laboratory. 

Mechanical or electronic fixed pressure gauges allow the permanent control of pressure in industrial processes.

WIKA digital pressure gauge

Pressure gauges can be divided into two main groups: gauges with an analog display and gauges with a digital display.

Analog pressure gauges
Although liquid column gauges can still be found in some applications, analog gauges generally use a mechanical process to move a pointer on a dial under pressure.

Advantages:

• They do not require a power supply, so they can operate continuously without needing batteries to be replaced or a power supply to be installed.
• They are solidly built and designed to resist shocks and vibrations, so they can be used in many industrial sectors, even in severe conditions.
• They enable quick visualization of the pressure level. 

Disadvantages:

• Their reading is less accurate and subject to errors (e.g. parallax errors).
• Their measurement cannot be used in a process control system.

Digital pressure gauges

Digital pressure gauges convert the pressure into an electrical signal and display the measurement on a screen, so they include an electronic circuit that requires a power source.

Advantages:

• The digital display offers an easy and precise reading of values.
• Thanks to their electronics, they offer more functions such as the choice of the unit of measurement, the storage of max values, etc.
• They can be equipped with an output for use with a process control system.

Disadvantages:

• They require batteries or a permanent power supply.

When discussing pressure measurement, it is of course important to know what pressure will be measured. There are 3 different types of pressure that can be measured by pressure gauges: absolute, relative and differential.

Pressure gauges and sensors generally measure pressure by converting the deformation of a surface (membrane or other) under the effect of the pressure to be measured. This surface deforms under the effect of the pressure difference between its two faces: on one side is the pressure to be measured and on the other is a reference pressure. This reference pressure determines the type of pressure being measured and the type of pressure gauge used.

Absolute pressure: the reference pressure is a vacuum (pressure close to zero), the absolute pressure therefore corresponds exactly to the pressure to be measured. 

Relative pressure: the reference pressure is the atmospheric pressure (for simplicity’s sake let’s say that the other side of the surface is in the open air), so the given measurement is the difference between the atmospheric pressure and the one we want to measure.

Differential pressure: the reference pressure is a second measurement point, so differential pressure gauges have two inputs and the measurement given is the pressure difference between the two points.

LANSO Bourdon tube pressure gauge

Inside a pressure gauge, there are find different types of measuring devices whose ability to deform in a certain way under the effect of pressure is used.

Bourdon tube pressure gauges: this type of pressure gauge is a curved tube with an oval profile. The pressure of the fluid to be measured acts inside the tube and the tube straightens out when the pressure increases. A mechanism amplifies the movements of the tube and transforms them into a rotary motion for the needle.

Bourdon tube pressure gauges are used for pressures from 0.6 to 4,000 bar, they are sensitive to overpressure. For aggressive media, the material of the Bourdon tube must be adapted.

Diaphragm pressure gauges: the pressure of the fluid is applied to one side of a thin corrugated diaphragm. A mechanism transforms the deformation of the diaphragm into a rotary motion for the needle. 

Diaphragm pressure gauges measure pressures from 16 mbar to 40 bar and can tolerate very high overpressure. The diaphragm is easier to protect from aggressive fluids, a protective coating or an intermediate film can be applied to it.

Capsule pressure gauges: the capsule is made of two diaphragms assembled on their circumference to form a tight cavity. The pressure of the fluid is applied inside the capsule which expands according to the pressure variations. A mechanism transforms the deformation of the capsule into a rotary motion for the needle.

Capsule pressure gauges are used for low pressures from 2.5 to 600 mbar and have limited overpressure resistance.

Bellows pressure gauges  The pressure of the fluid is applied inside a bellows (a thin-walled ringed cylinder). A mechanism transforms the changes in length of the bellows into rotation for the needle.

Bellows pressure gauges are used for low pressures from 60 to 1,000 mbar.

BENE INOX liquid-filled pressure gauge

Pressure gauges with dials are sensitive to vibrations and pressure surges or drops because of the mechanism that transforms the deformation of the sensing element into a rotational movement for the pointer. A solution to this problem has been found by filling the pressure gauge housing completely with liquid. Therefore, when choosing a pressure gauge, you should consider whether you need a dry or liquid-filled one.

Dry pressure gauges do not contain any liquid.

Advantages:

• Dry pressure gauges are less expensive than liquid-filled ones.

Disadvantages:

• They are not protected against vibrations and pressure surges or drops which can simply destroy the mechanism.
• They are not suitable for cold and damp environments as the moisture in the air inside the gauge housing can freeze and cause the mechanism to fail.

Applications:

Dry pressure gauges are more economical and preferred in simple applications where vibration is not an issue. They are often used on air compressors for example.

Liquid-filled pressure gauges have casing that has been completely filled with a liquid (usually pure glycerine or a mixture of water and glycerine) which will act as a damper for vibrations and pressure surges or drops.

Advantages:

• Liquid-filled pressure gauges are more resistant to vibration and pressure surges or drops.
• They are airtight, the presence of liquid prevents moisture from entering the housing and blocking the mechanism.
• This type of pressure gauge can work with sub-zero temperatures.
• They are environmentally friendly because the liquid (glycerin) is non-toxic and decomposes.

Applications:

Liquid-filled pressure gauges are used in wet and cold environments or when there is a lot of vibration.

A pressure gauge filled with pure glycerine can work down to -5°C. Glycerine becomes viscous below 17°C (which slows down the mechanism) and around -5°C the pressure gauge is completely blocked.

For low temperatures a mixture of glycerine and water is used which allows the pressure gauge to withstand up to -46°C.

Once you have determined the points outlined above, there are still other criteria involved for choosing your future pressure gauge.

• The pressure to be measured: a pressure gauge is chosen according to its measurement range, for analog dial pressure gauges, the operating pressure must be between 1/3 and 2/3 of the range.
• Overpressure: pressure gauges have a limited resistance to overpressure (pressure higher than the maximum of the range), so it is necessary to be sure that the pressure gauge will be able to withstand any overpressure that may occur in the circuit. Common pressure gauges can handle 1.15 to 1.3 times their maximum pressure, beyond this you will have to look for a special pressure gauge or install a pressure limiter upstream to the gauge on the circuit.
• Precision: expressed as a percentage of the range, the smaller the percentage, the more accurate the gauge.
• The dial diameter: the larger the dial, the more accurate the pressure reading will be, but the space available at the measurement location must be considered.
• Material compatibility:  the materials in contact with the fluid must be compatible with it. Standard pressure gauges are made of copper or copper alloy and are compatible with the most common fluids (water, air, oils, etc.), stainless steel pressure gauges are used for more aggressive fluids. For very aggressive, viscous, pasty or extremely hot fluids, a diaphragm separator is used, the pressure gauge is thus isolated from the fluid but the pressure is transmitted.
• The temperature of the fluid: pressure gauges with copper components can be used up to 65°C, above that, you will need stainless steel which can be used up to 150°C.
• Environmental conditions: the gauge housing materials must be able to withstand the operating environment. Stainless steel pressure gauges are preferred for aggressive or corrosive environments and waterproof housings are favored for outdoor use.