Eight common working principles of pressure instruments
1, piston pressure gauge
Piston manometer is based on the principle of static pressure balance, generally have single piston and double piston two kinds. According to its accuracy, the piston pressure gauge is divided into first class, second class and third class, and the accuracy grade error is 0.02 class, 0.05 class and 0.2 class, respectively. The effective area of the piston is generally 1cm2, 0.5cm2 or 0.1cm2. Voltage transmission media commonly used transformer oil and castor oil.
2, elastic pressure gauge
Elastic pressure gauge has spring tube type, diaphragm type, diaphragm type and bellows type, etc. There are three types of elastic pressure measuring elements commonly used in industry: spring tube, bellows and diaphragm. The elastic manometer works according to the principle that the deformation of the elastic element is proportional to the pressure. The greater the measured pressure applied to the elastic element, the greater the deformation of the elastic element. Commonly used elastic pressure gauges are spring tube pressure gauges, diaphragm pressure gauges, bellows pressure gauges, of which spring tube pressure is the most widely used.
The steel degree of the elastic element refers to the difficulty of deformation of the elastic element. The spring tube with large steel degree has small deformation after compression deformation. With stainless steel, alloy steel made of large steel, generally used to measure more than 20MPa pressure; The steel of phosphor copper and brass is small, and the pressure below 20MPa is generally measured. The spring pressure gauge is generally composed of a spring tube, a connecting rod, a sector gear, a spring, a pointer and a dial.
The spring tube in the pressure gauge is made of a metal tube bent into a 270° arc shape with an oval section. Because the elliptical section will tend to be circular under the action of the medium pressure, the spring tube bent into a circular arc will then produce a straight expansion of the deformation, so that the free end of the spring tube will produce displacement, and through the connection to drive the fan gear to amplify, drive the pointer to rotate, the Angle of the pointer rotation and the pressure path linear relationship, so that the size of the measured pressure can be read through the dial. The function of the spring is to create a reaction force. Diaphragm pressure gauge is generally composed of measuring diaphragm, transmission system, indicating system and case joint.
3, electric remote pressure gauge
When a remote pressure signal is needed, in order to be safe, convenient and reduce delay, the displacement or force change of the elastic element of the local pressure gauge is widely used to convert it into an electrical signal. The measuring range of the instrument is wide, and the pressure of 7×10-5Pa to 5×102MPa can be measured respectively, and the allowable error can reach 0.2%. Because the signal can be transmitted over a long distance, the automatic pressure control and alarm can be realized in the industrial production process, and can be used in conjunction with the industrial control machine.
It is composed of pressure sensor, measuring circuit and signal processing device. Commonly used signal processing devices are indicator, recorder, controller, microprocessor and so on.
4, Hall plate pressure sensor
The Hall plate pressure sensor is made according to the Hall effect, that is, the Hall element is used to convert the displacement of the elastic element caused by the pressure into the Hall potential, so as to achieve the measurement of pressure.
5, strain gauge pressure sensor
The strain gauge pressure sensor converts the change of pressure into the change of resistance value for measurement. A strain gauge is a resistance body made of a metal conductor or semiconductor whose resistance value varies with the strain generated by the pressure on a metal conductor.
6, piezoresistive pressure sensor
Piezoresistive pressure sensor is composed of piezoresistive effect of monocrystalline silicon. The monocrystalline silicon sheet is used as the elastic element, and a set of equivalent resistors are diffused in the specific direction of monocrystalline silicon by the technology of integrated circuit on the monocrystalline silicon membrane, and the resistors are connected into a bridge. When the pressure changes, the monocrystalline silicon produces strain, so that the strain resistance directly diffused on it produces a change proportional to the measured pressure, and then the corresponding voltage output signal is obtained by the bridge circuit.
7, torque balance type pressure transmitter
Torque balance pressure transmitter is a typical self-balancing measuring instrument, which uses the working principle of negative feedback to overcome the adverse factors such as component materials and processing technology, so that the instrument has a high measurement accuracy (generally 0.5), stable and reliable work, good linearity, small insensitive area and a series of advantages.
8, capacitive pressure transmitter
Capacitive pressure transmitter is an open-loop measuring instrument, with simple structure, strong overload capacity, good reliability, high measurement accuracy, its output signal is the standard 4 ~ 20mA (DC) current signal.
Pressure instrument selection considerations
In order to protect the pressure gauge, generally under the condition that the measured pressure is stable, the maximum pressure value should not exceed 2/3 of the meter range; If the measured pressure fluctuation is large, its maximum pressure value should be less than 1/2 of the meter range. In order to ensure the accuracy of the actual measurement, the minimum value of the measured pressure should not be less than 1/3 of the meter range. You also need to consider the following:
1. Compatibility of tested media
In the selection of the medium to consider its pressure interface and sensitive components, must consider the pressure interface and sensitive component material, otherwise a short time after use will be corroded the outer diaphragm, corrosion caused by equipment and personal accidents, so the material selection is very important.
2, the influence of medium temperature and ambient temperature on the product
In the selection to consider the temperature of the measured medium and the ambient temperature, if the temperature is higher than the product itself temperature compensation, easy to cause the product measurement data drift, the selection must consider the actual working environment of the transmitter, to avoid the temperature caused by the pressure sensitive core measurement inaccurate.
3. Selection of pressure range
The working pressure level of the equipment should be considered in the selection, and the pressure level of the transmitter must be consistent with the application.
4. Selection of pressure interface
In the selection process, strictly implement the size of the pressure interface used by the user, select the appropriate and correct thread connection form, and confirm the product outline diagram, or the manufacturer can provide samples.
5. Selection of electrical interface
In the selection, it is necessary to confirm the user signal acquisition method and the field wiring situation, and the sensor signal should be connected with the user acquisition interface; Select appropriate and correct electrical interface and signal mode.
6, pressure type selection
An instrument that measures absolute pressure is called an absolute pressure gauge. For ordinary industrial pressure gauges, the gauge pressure value is measured, that is, the pressure difference between absolute pressure and atmospheric pressure. When the absolute pressure is greater than the atmospheric pressure value, the gauge pressure value measured is positive, called positive gauge pressure; When the absolute pressure is less than the atmospheric pressure value, the gauge pressure value measured is negative, which is called negative gauge pressure, that is, the vacuum degree. The instrument that measures the degree of vacuum is called a vacuum meter.
7. Pressure gauge is affected by vibration
Pressure gauge in the vibration environment will also appear inaccurate indication, severe vibration will directly cause the deformation of the elastic elements of the pressure gauge, affecting the sensitivity of the pressure gauge, accuracy and the user to read the indicator value of the pressure gauge. Pressure gauge anti-vibration measures are mainly installed buffer device and shock absorber.
Pressure gauge installation three precautions
1, the choice of pressure measurement point should be able to reflect the true size of the measured pressure
1, to choose in the measured medium straight flow of the pipe section, do not choose in the pipeline bend, fork, dead Angle or other easy to form a vortex place.
(2) When measuring the pressure of the flowing medium, the pressure point should be perpendicular to the flow direction, and the inner wall at the connection between the end face of the pressure tube and the production equipment should be kept flush, without protruding objects or burrs.
(3) When measuring the pressure of the liquid (gas) body, the pressure point should be in the lower (upper) part of the pipeline, so that there is no accumulation of gas (liquid) body in the pressure guide tube.
2, pressure pipe laying
1, the thickness of the pressure guide pipe should be appropriate, the general inner diameter is 6 ~ 10mm, the length should be as short as possible, the longest should not exceed 50m, in order to reduce the slow pressure indication. If more than 50m, the pressure gauge that can be transmitted over a long distance should be selected.
(2) When the pressure guide pipe is installed horizontally, it should be ensured that there is a inclination of 1:10 ~ 1:20 to facilitate the discharge of the liquid (or gas) stored in it.
(3) When the measured medium is easy to condense or freeze, it is necessary to add a thermal insulation tracing pipeline.
(4) A cut-off valve should be installed between the pressure opening and the pressure gauge to prepare for the maintenance of pressure timing. The cut-off valve shall be installed near the pressure opening.
3. Installation of pressure gauge
① The pressure gauge should be installed in a place that is easy to observe and repair.
② The installation location should strive to avoid the impact of vibration and high temperature.
(3) When measuring steam pressure, a condensate pipe should be installed to prevent high-temperature steam from contacting the pressure measuring element directly; For the pressure measurement of corrosive media, an isolation tank with neutral media should be installed.
④ The connection of the pressure gauge should be selected according to the level of the measured pressure and the nature of the medium, as a sealing gasket to prevent leakage.
⑤ When the measured pressure is small, and the pressure gauge and the pressure inlet are not at the same height, the measurement error caused by this height should be corrected according to Δp = ±Hρg. Where H is the height difference, ρ is the density of the medium of the pressure tube, and g is the gravitational acceleration.
For safety reasons, the pressure gauge measuring high pressure in addition to the selection of ventilation holes, the case should be installed to the wall or no one through the place to prevent accidents.