Views: 19 Author: Site Editor Publish Time: 2022-10-11 Origin: Site
A manometer is an instrument used to measure pressure. The term pressure indicator can also be used.
In industry, pressure and temperature are undoubtedly the most measured and monitored process parameters, so pressure gauges can be found almost everywhere.
This buyer guide does not cover pressure sensors. Unlike sensors that generate electrical signals and need to be connected to a device or control system to make their measurements readable or usable, manometers can read pressure values directly on a dial or digital display. Some pressure gauges are purely mechanical, while others require a power source.
This question may seem trivial, but there are portable and stationary manometers available, and you must choose between the two depending on your needs.
Portable pressure gauges are often electronic, enabling you to make just-in-time measurements and controls in industry or laboratories.
Mechanical or electronic fixed pressure gauges allow permanent control of pressure in industrial processes.
Manometers can be divided into two categories: manometers with analog displays and manometers with digital displays.
Although liquid column manometers can still be seen in some applications, analog manometers typically use a mechanical process that moves a pointer on a dial under pressure.
advantage:
· No power supply is required, so the manometer can work continuously without changing batteries or installing a power supply.
· Robust construction and designed to resist shock and vibration, so it can be used in many industrial sectors · Even in harsh conditions.
· They enable quick visualization of stress levels.
shortcoming:
· Manometer readings are inaccurate and subject to errors (eg parallax errors).
· Measurements cannot be used in process control systems.
Digital manometers convert pressure into electrical signals and display the measurements on a screen, and therefore include an electronic circuit that requires power.
advantage:
· The digital display provides an easy and precise reading of the value.
· Due to the electronic technology of the pressure gauge, more functions are provided, such as the selection of the measurement unit, the storage of the maximum value, etc.
· Manometers can be equipped with an output for use with process control systems.
shortcoming:
· Requires battery or permanent power supply.
When discussing pressure measurement, it is first necessary to know what pressure is being measured.
There are 3 different types of pressure that can be measured with a manometer: absolute pressure, relative pressure and differential pressure.
Manometers and sensors typically measure pressure by converting the deformation of a surface (membrane or other) under the pressure to be measured. The surface is deformed by the pressure difference between its two sides: the pressure to be measured on one side and the reference pressure on the other side. The reference pressure determines the type of pressure being measured and the type of manometer used.
The reference pressure is a vacuum (the pressure is close to zero), so the absolute pressure corresponds exactly to the pressure to be measured.
The reference pressure is atmospheric pressure (for simplicity let's say the other side of the surface is in open air), so the given measurement is the difference between the atmospheric pressure and the pressure we want to measure.
The reference pressure is the second measurement point, so the differential pressure gauge has two inputs, and the measurement given is the pressure difference between the two points.
Within a manometer, there are different types of measuring devices, and the ability to deform in a certain way under pressure is exploited.
This type of manometer is an elbow with an oval profile. The pressure of the fluid to be measured acts on the inside of the tube, and when the pressure increases, the tube straightens. A mechanism amplifies the motion of the tube and translates it into the rotational motion of the needle.
Bourdon tube manometers are used for pressures from 0.6 to 4,000 bar and they are sensitive to overpressure. For aggressive media, the material of the Bourdon tube must be adjusted.
Fluid pressure is applied to one side of a thin corrugated diaphragm. A mechanical device translates the deformation of the septum into rotational movement of the needle.
Diaphragm manometers measure pressures from 16mbar to 40bar and can withstand very high overpressures. Diaphragms are more susceptible to corrosive liquids and can be covered with a protective coating or interlayer.
Bladder pressure gauges are assembled from two diaphragms on their circumference to form a tight cavity. The pressure of the liquid is applied inside the capsule, and the capsule expands according to the pressure change. A mechanical device converts the deformation of the capsule into rotational motion of the needle.
Capsule manometers are used for low pressures from 2.5 to 600 mbar and have limited resistance to overpressure.
The pressure of the fluid is applied inside a bellows (a thin-walled annular cylinder). A mechanical device translates changes in the length of the bellows into rotation of the needle.
Bellows pressure gauges are used for low pressures from 60 to 1,000 mbar.
Manometers with dials are sensitive to vibrations and pressure surges or drops because the mechanism converts the deformation of the sensing element into the rotational movement of the pointer. A solution to this problem has been found by filling the manometer housing completely with liquid. Therefore, when choosing a manometer, you should consider whether you need a dry or liquid-filled manometer.
Does not contain any liquid.
advantage:
· Dry manometers are less expensive than liquid filled manometers.
shortcoming:
· Manometers do not protect against vibration and pressure surges or drops, which will simply destroy the mechanism.
· Not suitable for use in cold and humid environments, as the moisture in the air inside the meter housing will freeze and cause the mechanism to fail.
application:
· Dry gauges are more economical and preferred in simple applications where vibration is not an issue. Usually used on air compressors.
Its shell has been completely filled with a liquid (usually pure glycerin or a mixture of water and glycerin), which will damp vibrations and pressure surges or drops.
advantage:
· Liquid-filled manometers are more resistant to vibration and pressure surges or dips.
· Is airtight, the presence of liquid prevents moisture from entering the housing and blocks the machinery.
· This type of manometer can work in sub-zero temperatures.
· They are eco-friendly because the liquid (glycerin) is non-toxic and can be broken down.
application:
· Liquid-filled manometers can be used in wet and cold environments, or where there is a lot of vibration.
· Manometers filled with pure glycerin can operate to -5°C. Glycerol can become viscous below 17°C (which makes the mechanism slower), and around -5°C, the gauge is completely blocked.
· For low temperatures, use a mixture of glycerin and water so that the pressure gauge can withstand -46°C.
Once the above points are established, there are other criteria involved in selecting a manometer.
· Pressure to be measured: Choose according to the measuring range of the manometer, for analog dial manometers, the working pressure must be between 1/3 and 2/3 of the range.
· Overpressure: The pressure gauge has a limited resistance to overpressure (pressure above the maximum value of the range), so it is necessary to determine that the pressure gauge can withstand any overpressure that may occur in the circuit. A normal manometer can handle 1.15 to 1.3 times its maximum pressure, beyond this range you will have to find a special manometer or install a pressure limiter upstream of the manometer on the circuit.
· Accuracy: expressed as a percentage of the range, the smaller the percentage, the more accurate the instrument is.
· Dial Diameter: The larger the dial, the more accurate the pressure reading, but the space available at the measurement location must be considered.
· Material Compatibility: Materials that come into contact with liquids must be compatible with them. Standard manometers are made of copper or copper alloys and are compatible with the most common fluids (water, air, oil, etc.), stainless steel manometers are used for more corrosive fluids. For very corrosive, viscous, pasty or extremely hot liquids, diaphragm separators are used, whereby the pressure gauge is isolated from the liquid, but the pressure is transmitted.
· Fluid temperature: manometers with copper parts can be used up to 65°C, above this you need stainless steel and can be used up to 150°C.
· Environmental Conditions: The enclosure material of the meter must be able to withstand the operating environment. Stainless steel pressure gauges are preferred in aggressive or corrosive environments, and waterproof housings are preferred for outdoor use.
Differential Pressure Gauge | Supplier: Sauermann - Belgium
Analog Pressure Gauge | Supplier: WIKA - Germany
Analog Pressure Gauge | Supplier: ifm electronic - Germany
Absolute Pressure Gauge | Supplier: OMEGA - USA
Absolute Pressure Gauge | Supplier: AMETEK Sensors - USA
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