Explanation of Various Terms for Pressure Gauges
- Pressure gauge: Based on atmospheric pressure, it is an instrument used to measure pressures that are either lower than or higher than atmospheric pressure.
- Pressure: In fact, it refers to the pressure in physics, which is the magnitude of the force exerted per unit area.
- Absolute pressure: Pressure that is higher than the zero point of absolute pressure.
- Positive pressure: Pressure that is higher than the atmospheric pressure, measured relative to the atmospheric pressure.
- Negative pressure (vacuum): A pressure that is lower than the atmospheric pressure, measured relative to the atmospheric pressure.
- Pressure difference: The difference between two pressures.
- Table Pressure: The pressure that is greater than or less than the atmospheric pressure, measured relative to the atmospheric pressure.
Within the measurement range, the indication error should not exceed the specified allowable error.
Within the measurement range, the return error should not exceed the absolute value of the specified allowable error.
3、Light tapping displacement
After lightly tapping the watch case, the variation in the pointer indication value should not exceed half of the absolute value of the specified allowable error.
4、Stability of pointer deflection
Within the measurement range, the pointer should move smoothly without any jolts or sticking.
Introduction to Pressure Gauge Accuracy Grades
The pressure gauge accuracy class is the abbreviation for the pressure gauge's precision class or accuracy class. The applicable standards areGB/T1226-2001"General Pressure Gauge", and JJG52-1999 "Spring Tube Type General Pressure Gauge, Pressure Vacuum Gauge and Vacuum Gauge".The accuracy grade of a pressure gauge is expressed as a percentage of the gauge's range that represents the allowable error. It is generally divided into0.5、1、1.5、2、2.5、3、4There are seven grades (levels 1 to 7 are not used for boilers), and the smaller the value, the higher the accuracy. For example, a pressure gauge with a dial range of 0 to 2.5 MPa and an accuracy of level 2.5, the allowable error between the indicated pressure value by the pointer and the actual pressure value of the measured medium shall not exceed 2.5 MPa × 2.5% = ±0.0625 MPa. When the pressure gauge indicates a pressure of 0.8 MPa, the actual pressure should be within the range of 0.7375 to 0.8625 MPa.From this, it can be seen that the actual error of the pressure gauge is not only related to its accuracy, but also to the range of the pressure gauge.When the range is the same, the higher the accuracy (that is, the smaller the number), the smaller the allowable error of the pressure gauge. When the accuracy is the same, the larger the range, the greater the error of the pressure gauge.Nominal diameter of the shell(mm Accuracy level
40 ；60 2.5；4.0
10 1.6；2.5
150；200；250 1.0；1.6
Note: For the 1.5-class pressure gauges in use, the allowable error can be calculated based on the 1.6-class standard, and the accuracy grade does not need to be changed.
The representation methods and classification of stress
There are two ways to represent pressure: one is the pressure expressed relative to an absolute vacuum, which is called absolute pressure; the other is the pressure expressed relative to atmospheric pressure, which is called relative pressure. Since most pressure measuring instruments measure relative pressure, relative pressure is also called gauge pressure. When the absolute pressure is less than the atmospheric pressure, it can be represented by the difference between the absolute pressure in the container and one atmosphere. This difference is called "vacuum degree". The relationship between them is as follows:
Absolute pressure = Atmospheric pressure + Relative pressureVacuum degree = Atmospheric pressure - Absolute bed forceThe legal unit of pressure in our country is Pa (N/㎡), which is called the Pascal and is abbreviated as "Pa". Since this unit is too small, its 10^6 times unit, MPa (megapascal), is often used instead.Pressure gauges are classified according to their measurement accuracy into precision pressure gauges and general pressure gauges. The measurement accuracy grades of precision pressure gauges are 0.1, 0.16, 0.25, and 0.4; while the measurement accuracy grades of general pressure gauges are 1.0, 1.6, 2.5, and 4.0.Pressure gauges vary depending on the reference point of the indicated pressure.，There are many different classification methods.：1、According to the measurement accuracy of pressure gauges, they can be classified as precision pressure gauges (GB/T1227) and general pressure gauges (GB/T1226)；2、According to the different pressure reference standards of the pressure gauges, they are classified as general pressure gauges and absolute pressure gauges.、Pressure differential gauge； 3、According to the measurement range of the pressure gauge, they are classified as vacuum gauges, pressure-vacuum gauges, micro-pressure gauges, low-pressure gauges, medium-pressure gauges and high-pressure gauges；4、According to the measurement range of the pressure gauges, they are classified as vacuum pressure gauges, vacuum pressure and pressure gauges, micro-pressure gauges, low-pressure gauges, medium-pressure gauges and high-pressure gauges.The structure and working principle of the pressure gauge
The open-type spring tube pressure gauge is composed of five main parts: the case section, the pointer, the scale plate section, the spring bend tube, the transmission mechanism section and the pipe joint. When the medium pressure is applied to the spring tube, its movable end extends outward. Then, the transmission mechanism drives the pointer to rotate, and the pressure of the medium is indicated on the scale plate.
The sealed-type spring pressure gauge is composed of six main parts: the housing, the pointer, the scale plate, the spring tube, the elbow pipe and the transmission mechanism. The inner cavity of the spring tube is in a sealed form. The external pressure acts on the outside of the spring tube causing it to deform. The transmission mechanism drives the pointer to rotate and indicate the environmental pressure.
The numbers within the circle below the scale of the pressure gauge indicate the accuracy grade of the pressure gauge. They represent that the basic error of this gauge does not exceed a certain percentage of its full scale. The smaller the number, the higher the accuracy.
The pressure values within the measurement range are displayed by the pointer, and the indication range of the dial is usually 270 degrees.
Installation and setting of pressure gauges
(1) It should be placed in an area that is convenient for observation and cleaning, and should be protected from adverse factors such as radiation heat, freezing or vibration.(2) The pressure gauge should be installed vertically. If the installation position is at a higher level, it can be slightly inclined forward for easier observation, but the inclination angle should not exceed 30 degrees.(3) In some cases, a buffer elbow should be installed in front of the pressure gauge to prevent compressed air from directly entering the spring elbow and to allow condensate water to accumulate.(4) A three-way cock or needle valve should be installed between the pressure gauge and the buffer elbow, so that the pressure gauge can be replaced and calibrated.Proper selection and usage precautions for pressure gauges
Pressure measuring instruments are common measuring devices that are widely used in various fields. They can visually display the pressure changes at each process stage, understand the conditions formed in the product or medium flow, monitor the safety trends during the production operation process, and through automatic interlocks or sensing devices, build a rapid and reliable safety guarantee. They play an important role in preventing accidents and ensuring the safety of personnel and property, and are called the "eyes" of safety.
The range of the pressure gauge installed on boilers and pressure vessels should be adapted to the working pressure of the equipment. The range of the pressure gauge is generally 1.5 to 3 times the working pressure of the equipment, and it is best to choose 2 times. If the selected pressure gauge has an excessively large range, due to the same precision pressure gauge, the larger the range, the greater the absolute value of the allowable error and the deviation that can be observed with the naked eye will be. This will affect the accuracy of the pressure reading. Conversely, if the selected pressure gauge has an excessively small range, the working pressure of the equipment is equal to or close to the scale limit of the pressure gauge, which will cause the elastic element in the pressure gauge to remain in the maximum deformation state for a long time, prone to permanent deformation, resulting in an increase in the error of the pressure gauge and a decrease in its service life. In addition, if the range of the pressure gauge is too small, if the equipment operates under overpressure conditions, the pointer will cross the maximum range and approach the zero position, causing the operator to have a false perception and leading to a greater accident. Therefore, the operating pressure range of the pressure gauge should not exceed 60-70% of the scale limit.
Common Pressure Units and Conversion
Pa(N/m2);Kgf/cm2; bar;
mbar ; atm ;torr ;inH2O:
mmHg ;PSI(lb/in2)
1Mpa≈10Kg/cm 1Bar1Kq/cm
1Mpa=1000Kpa 1Bar=100Kpa
1Mpa=10000mBar 1Bar=1000mBar
1Mpa=7500mmHg 1Bar=750mmHg
For easier memorization, it can be simplified as the following rule:
1atm=0.1MPa=100KPa=1kg=1bar=10-meter water column=14.5PSI
1KPa=0.01kg=0.01bar
=10mbar=7.5mmHg=0.3inHg=7.5torr
=100mmH2O=4inH2O
Spring tube pressure gauge and installation tips
At present，The most widely used local pressure gauges in the production site are mechanical pressure gauges. Mechanical pressure gauges are mainly classified into spring tube (Bourdon tube) pressure gauges, diaphragm pressure gauges, bellows pressure gauges and diaphragm tube pressure gauges based on the different elastic sensitive elements. This article mainly takes spring tube pressure gauges as an example.
BulletThe types and specifications of diaphragm pressure gauges are diverse. They feature a simple structure, ease of use, high accuracy, durability, and low cost. As a result, they are widely used in various pressure measurement scenarios in industrial production.The spring tube pressure gauge is also known as the wave tube pressure gauge. It is divided into types such as C-type tube, disc spring tube, and spiral tube.Its structure is shown in the figure below.
Spring tube pressure gauge structure图
The phenomenon that the spring tube generates under the pressure of the medium inside the cavity.Elastic deformation utilizes its elastic properties to convert the pressure into the elastic displacement of the free end of the spring tube. Then, through the gear transmission mechanism of the pressure gauge, the pressure is amplified, and the pressure gauge will display the relative value (either higher or lower) compared to the atmospheric pressure.The pressure values within the measurement range are displayed by the pointer. The measurement range of the spring tube is generally from 0.1 MPa to 250 MPa.
When installing the pressure gauge, the following points should be noted:
1、Pressure Sampling componentThe installation location should be chosen in a place where the medium flow rate is stable.
2、 When measuring the pressure of turbid media such as dust, solid particles or sediment, the sampling component should be installed at an upward angle.
3、When installing the pressure sampling component on horizontal and inclined process pipelines, the orientation of the sampling port should comply with the following regulations:
（1）When measuring gas pressure, it is located in the upper part of the process pipeline.
（2）When measuring the pressure of a liquid, within the range of 0 to 45 degrees from the lower half of the process pipeline to the horizontal centerline of the process pipeline.
（3）When measuring the steam pressure, it should be done within the range of the upper and lower halves of the process pipeline, and within an angle of 0 to 45 degrees from the horizontal centerline of the process pipeline.
Pressure gauge calibration cycle
Pressure gauges are subject to either mandatory inspection or non-mandatory inspection. Not all pressure gauges are subject to mandatory inspection. In 2019, the State Administration for Market Regulation issued the "Announcement on the Catalogue of Measuring Instruments Subject to Mandatory Management" (No. 48 of 2019), and items 21 and 22 in the catalogue, which are pressure gauges for safety protection purposes, require mandatory inspection as shown in the figure.
Pressure gauges that require mandatory inspection
For non-mandatory calibration pressure gauges, regular calibration is also required, but the calibration cycle is determined by the enterprise itself.
For chemical enterprises, on October 10, 1988, the Ministry of Chemical Industry and the State Bureau of Technical Supervision jointly issued the "Regulations for the Classification Management of Measurement Instruments in the Chemical Industry" (Trial). The measurement instruments were managed in three grades: A, B, and C. According to this regulation, the "Directory for the Classification Management of Measurement Instruments in Chemical Production Enterprises" is as follows: The classification and calibration cycle of pressure gauges in chemical enterprises are as follows:
Grade A: Pressure gauges used for safety protection should be calibrated at least once every six months.
Grade B：The pressure gauges used for process control have a verification period that coincides with the maintenance period of the equipment.
Grade C：The pressure gauges used for monitoring have a verification period of 1 to 2 years; the monitoring pressure gauges do not require periodic verification; if they malfunction once, they should be replaced immediately.
Pressure gauges are indeed important, but they are not omnipotent. Simply using the pressure indicated by the pressure gauge to judge the pressure of equipment, pipelines, etc., is not reliable. On September 17, 2023, the 10-dead-3-injured accident of Ordos Yiding Ecological Agriculture Development Co., Ltd. was a typical case. Through this accident, it also reminds us that we need to strengthen the daily maintenance and regular inspection of monitoring and measurement instruments such as pressure, liquid level, and temperature to ensure their good usability. This is also one of the important aspects of equipment integrity.
