How Do Diaphragm Pressure Gauges Work

The diaphragm pressure gauge consists of a diaphragm isolator and a general-purpose pressure gauge. It is suitable for measuring the pressure of the medium with strong corrosion, high temperature, high viscosity, easy crystallization, easy solidification, and solid floating matter, and the direct measurement of the medium must be avoided. Enter the general-purpose pressure gauge and prevent the accumulation of sediment and easy plot occasions. Diaphragm pressure gauges are mainly used for the pressure of the flow fluid medium in the production process of petrochemical, alkali, chemical fiber, dyeing, pharmacy, food and dairy industries.

Diaphragm Pressure Gauge working principle

The diaphragm pressure gauge is composed of a diaphragm isolator and a general-purpose pressure gauge. The diaphragm seals the diaphragm. When the pressure P of the measured medium acts on the diaphragm, it deforms, compresses the working fluid filled in the system, and makes the working fluid form a △P equivalent to P, with the help of work. The conduction of the liquid causes the free end of the elastic element (spring tube) in the pressure gauge to produce a corresponding elastic deformation and displacement, and then display the measured pressure value according to the working principle of the pressure gauge that matches it.
The temperature characteristics of the diaphragm pressure gauge: Since the diaphragm pressure gauge system is filled with the sealing liquid as the medium for transmitting pressure, the temperature and the expansion coefficient of the sealing liquid cause the pressure meter to increase with the temperature of the pressure receiving part. The amount of temperature influence is related to the expansion coefficient of the sealing body, the stiffness of the diaphragm and the temperature under pressure, especially for pressure instruments with a low range. Generally, the temperature error of the pressure receiving part is specified to be no more than 0.1%/degree, so the total temperature influence of the diaphragm pressure gauge is generally the sum of the temperature influence of the general-purpose instrument and the temperature influence of the pressure part of the diaphragm device.

Calibration of diaphragm pressure gauge

Before installing the diaphragm pressure gauge, you must carefully check the model, diaphragm material, and measurement range to check whether the diaphragm is damaged; when there is no pressure, the pressure gauge indicates whether it is at the zero position, and at the same time, find out the reason, re-adjust and calibrate. It can be used after passing the inspection. The steps for calibrating diaphragm pressure gauges are the same as those for calibrating conventional pressure gauges, and will not be repeated again, but the following points should be paid attention to:
(1) Before calibration, first prepare a matching joint according to the diaphragm device model of the diaphragm pressure gauge to be calibrated, so that it can be used in a general pressure calibration bench. The diaphragm device of the diaphragm pressure gauge is generally flanged. According to the size and model of the flange, the matching pressure joint can be prepared. The corresponding flange and general pressure gauge joint can be found on-site for processing. This is also the most direct and convenient method.
(2) Pay attention to the process of fixing the calibrated diaphragm pressure gauge on the standard calibration platform: handle with care and do not damage the diaphragm. When rotating the calibrated diaphragm pressure gauge with a wrench, the wrench must be stuck The connecting nut of the diaphragm device is not stuck on the universal pressure gauge, because the universal pressure gauge and the diaphragm device on the instrument are a sealed system. Do not disassemble or replace other types of pressure gauges at will to prevent the working fluid filled in the system from leaking. And it affects the performance or even scraps, especially the new students who have just joined the work, this must be explained clearly before calibrating the diaphragm pressure gauge.
(3) Result processing: In the process of boosting, compared with the standard table, if it is out of tolerance, first check whether there is oil leakage, sealing, etc., if there is oil leakage, the sealing is not good, it can only be returned to the factory for processing or scrap; if not In the above situation, you can remove the indicator needle and re-set the indicator needle. Long-term experience is that when the needle is fixed, it is generally reasonable to re-set the needle at 1/2 of the measuring range; if it is still out of tolerance, it can only be degraded for use or scrapped.

Application of diaphragm pressure gauge

Before using and installing a diaphragm pressure gauge, be sure to clarify the characteristics of the monitored medium, because the diaphragm pressure gauge is designed for special media. When monitoring the pressure of general gas, water and oil, you can choose a general pressure gauge. For nitric acid, phosphoric acid, strong alkali pressure, stainless steel pressure gauges can be used, but when the monitored medium is very corrosive such as: hydrochloric acid, wet chlorine, ferric chloride; has a high viscosity such as latex; easy to crystallize such as: Salt water; easy to solidify, such as hot asphalt; solid floats such as sewage, the above pressure gauge is not acceptable, because the SUS316 stainless steel pipe will be corroded by hydrochloric acid, and the floating matter of asphalt and sewage will also block the pressure gauge’s guiding hole , And make the pressure gauge lose its function. Since the diaphragm pressure gauge has a diaphragm diaphragm, it can prevent high-viscosity, easy-to-crystallize, and easy-to-solidify media from flowing into the pressure guide hole, ensuring that the pressure gauge reading can normally reflect the pressure of the measured medium.
Several points should be paid attention to when using the diaphragm pressure gauge:
(1) The installation position of the diaphragm pressure gauge must correspond to the installation position in the precautions of the pressure gauge instruction manual.
(2) The pressure gauge should select the appropriate interface form, diaphragm material, and gasket material according to the nature of the measured medium. It is required to be equipped with an anti-vibration pressure gauge when used in situations with strong mechanical vibration and medium pressure pulsation. When the pressure gauge is put into operation under pressure, the valve should be opened slowly and evenly.

Failure cause analysis of diaphragm pressure gauge

1. When there is pressure, the pointer has no indication
The reason for this failure is generally that the connection between the pointer and the central gear shaft is loosened due to strong vibration, and the displacement of the spring tube after compression cannot drive the pointer to deflect. Secondly, the silicone oil in the spring tube leaks too much, and the pressure cannot be transmitted.
2. The measured pressure is inaccurate, the pressure indicator changes at the low end, and the pointer does not change when the pressure rises.
The reason for this failure is that a small part of the silicone oil in the spring tube is lost, which allows air to enter. Because the gas can be compressed, when the pressure increases, the pressure indicated by the pointer is inaccurate or does not move.
3. In the process of pressure measurement, the pointer changes leaps and bounds
This kind of failure often occurs in occasions where the diaphragm pressure gauge is in large vibrations (such as the outlet pressure of the pump). Due to the fluctuation of the pressure, the pointer also swings back and forth, causing the central gear of the pressure gauge core to be severely worn, the central gear and the sector. The gears did not fit well, which caused the pointer to change abruptly. To deal with the jumping fault of the pointer, a small assorted file and a scrap steel saw blade can be used to file and repair the worn center gear. When the shaft hole of the pointer becomes larger, you can tap it with a small hammer to shrink it until the pointer can be closely integrated with the center gear shaft.