Useful information about pressure gauges
The term “pressure gauge” is used throughout the text, and the name pressure gauge is generic. This term also includes vacuum gauges and manovacuum gauges. This material is not related to digital instruments.
Pressure gauges are devices that are widely used in industry and housing and communal services. In the production process, enterprises need to control the pressure of liquids, steam and gas. Depending on the specialisation of the enterprise, there is a need to measure different media. For this purpose, pressure gauges for various purposes have been developed. The difference between the devices is determined by the medium to be measured and the conditions under which the measurement is performed. Pressure gauges differ in design, size, connection thread, units of measurement and possible measuring range, accuracy class, and material of manufacture, which determines the possibility of using the device in aggressive environments. The choice of a device that does not meet the tasks to be performed may result in failure of the device earlier than the expected service life, measurement errors, or overpayment for functions of the device that are not used.
Classification of pressure gauges according to criteria
Depending on the area of application.
Technical pressure gauges of standard design – used to determine the overpressure and vacuum of non-corrosive, non-crystallising media: liquids, steam and gas.
Technical special pressure gauges – this type of gauge is used to measure specific media (e.g. corrosive) or in special conditions (high vibration or temperature, etc.).
Special devices:
- ammonia pressure gauges;
- Corrosion-resistant pressure gauges for aggressive environments;
- Vibration-resistant pressure gauges – copper metal;
- Vibration-resistant pressure gauges – stainless steel;
- precision measurement pressure gauges;
- railway gauges.
Ammonia pressure gauges, as well as corrosion-resistant pressure gauges, have stainless steel and alloy parts and mechanisms that are resistant to aggressive media, which makes this type of device suitable for use in applications where interaction with aggressive media is required.
Vibration-resistant pressure gauges can be used in conditions of vibration frequency exceeding 4-5 times the permissible vibration frequency for a conventional pressure gauge.
The main distinguishing feature of vibration-resistant pressure gauges is the presence of a special damping device located in front of the pressure gauge. This device helps to reduce pressure pulsation.
Some types of vibration-resistant pressure gauges can be filled with a damping liquid. Resistance to vibration is achieved by a vibration-absorbing substance, which is glycerin.
Precision pressure gauges are used in the areas of state mercological control, heat supply, water supply, energy, mechanical engineering, etc. In addition, they are used as a reference for verification and calibration of pressure measuring devices in compliance with the requirements for compliance with the accuracy classes of the device used as a reference and the device to be verified.
Railway pressure gauges are used to measure the overpressure of media that are not aggressive to copper alloys in systems and installations of rolling stock and to measure the pressure of refrigerants in refrigeration machines in refrigerated wagons.
Depending on the application, the pressure gauge bodies are painted in the appropriate colours. Ammonia pressure gauges are yellow, hydrogen pressure gauges are dark green, red for flammable gases, blue for oxygen, and black for non-flammable gases.
Electric contact pressure gauges. The peculiarity of electrocontact pressure gauges is that they are devices with an electrocontact group. They are designed to measure the pressure of non-corrosive, non-crystallising media (steam, gas, including oxygen), as well as to close and open electrical circuits when a certain pressure limit is reached. The electrocontact mechanism allows for adjustment of the medium.
UNITS OF PRESSURE MEASUREMENT. GRADUATION OF PRESSURE GAUGE SCALES.
Pressure gauge scales are graduated in one of the following units: kgf/cm2, bar, kPa, MPa, provided that the device has a single scale. For pressure gauges with a dual scale, the first scale is graduated in the above units, and the second scale is graduated in psi – pounds per square inch. Psi is an off-system unit used in the United States.
Table 1 shows the ratio of measurement units to each other.
Table 1. Ratio of pressure units.
| Pа | кPа | МPа | kgs/сm2 | bar | |
| Pа | 1 | 10-3 | 10-6 | 10,197*10-6 | 10-5 |
| кPа | 103 | 1 | 10-3 | 10,197*10-3 | 10-2 |
| МPа | 106 | 103 | 1 | 10,1972 | 10 |
| kgs/сm2 | 98066,5 | 98,0665 | 0,980665 | 1 | 0,980665 |
| bar | 105 | 100 | 0,1 | 1,0197 | 1 |
| psi | 6894,76 | 6,8948 | 6,8948*10−3 | 70,3069*10−3 | 68,9476*10−3 |
Pressure gauges with a scale in kPa units are devices designed to measure low pressures of substances in a gaseous state. In their design, a diaphragm box serves as a sensing element. In contrast, high-pressure gauges have a curved or spiral tube as a sensing element.
RANGE OF MEASURED PRESSURES.
The following types of pressure are distinguished: absolute, barometric, overpressure, and vacuum.
Absolute pressure is the value of pressure measured relative to an absolute vacuum. The value may be negative.
Barometric is the atmospheric pressure. It is affected by altitude, humidity, and air temperature. At the zero mark of altitude above sea level, the barometric pressure is assumed to be 760 mm Hg.
For technical pressure gauges, this value is assumed to be zero. This means that the measurement results depend on the barometric pressure.
Overpressure is a value that shows the difference between absolute and barometric pressure. This is relevant when the absolute pressure exceeds the barometric pressure.
Vacuum is a quantity that shows the difference between absolute and barometric pressure in conditions when barometric pressure exceeds absolute pressure. Consequently, the vacuum pressure cannot be higher than the barometric pressure.
Based on the above, it becomes obvious that vacuum gauges measure discharge. Manovacuum gauges overlap the vacuum and overpressure regions.
The function of manometers is to determine overpressure.
As a result of the standardisation of the ranges of measured pressures, it is accepted that they correspond to a certain range of values (Table 2).
Table 2. Standard range of scale graduation values.
| Тип прилада | Діапазони вимірюваних тисків, кгс/см2 |
| Вакуумметри | -1…0 |
| Мановакуумметри | -1…0,6; 1,5; 3; 5; 9; 15; 24 |
| Манометри | 0…0,6; 1; 1,6; 2,5; 4; 6; 10; 16; 25; 40; 60; 100; 160; 250; 400; 600; 1000; 1600 |
| Манометри надвисоких тиску | 0…2500; 4000; 6000; 10000 |
ACCURACY CLASS OF PRESSURE GAUGES.
The accuracy class of the device refers to the permissible error, which is expressed as a percentage of the maximum value of the pressure gauge scale. The higher the accuracy of the device, the lower the error. The accuracy class is indicated on the scale of the device. Pressure gauges of the same type may have different accuracy classes.
DIAMETER OF THE PRESSURE GAUGE BODY.
The most common diameters of pressure gauge bodies are 40, 50, 60, 63, 100, 150, 160, 250 mm. But there are devices with other body sizes. For example, vibration-resistant pressure gauges manufactured by UAM, type D8008-V-U2 – an analogue of DA8008-VUF manufactured by Fiztekh, have a diameter of 110 mm.
DESIGN OF PRESSURE GAUGES.
A fitting is used to connect the device to the system. There are two types of fittings: radial (bottom) and axial (rear). The location of the axial fitting can be central or offset from the centre. Many types of pressure gauges are designed with a radial connection only. For example, electrocontact pressure gauges.
The thread size of the fitting corresponds to the diameter of the body. Pressure gauges with diameters of 40, 50, 60, 63 mm have threads M10x1.0-6g, M12x1.5-8g, G1/8-B, R1/8, G1/4-B, R1/4. Pressure gauges with larger diameters are made with M20x1.5-8g or G1/2-B threads. European standards use, in addition to the above-mentioned thread types, tapered threads – 1/8 NPT, 1/4 NPT, 1/2 NPT. In industrial environments, specific connections are used depending on the tasks and types of measured media. For pressure gauges operating at high and ultra-high pressure levels, an internal tapered thread or a cylindrical thread variant is typical.
Depending on the type of equipment, specify the required thread type when ordering the device. This will help to avoid additional unforeseen costs, which will entail the replacement of the fitting.
The design of the pressure gauge body is also selected according to the installation method and location. For open pipelines, the design of the devices does not provide for additional fasteners. For devices installed in cabinets or control panels, a front and rear flange is required.
Depending on the design, there are the following types:
- with bottom connection without flange;
- with back socket with rear flange;
- with bottom connection with front flange;
- with back connection without flange.
The standard version of pressure gauges has an IP40 rating. Special pressure gauges are available in IP50, IP53, IP54 and IP65 degrees of protection depending on the conditions of use.
To prevent unauthorised opening of the pressure gauge, the device must be sealed. For this purpose, an eyelet is made on the body, equipped with a screw with a hole in the head for installing the seal.
Protection against high temperatures and pressure drops.
The measurement error of a pressure gauge depends on the influence of the ambient temperature and the temperature of the measured medium.
For most devices, the temperature range of measurement is no more than +60°C, maximum +80°C. Some manufacturers’ devices have the ability to measure pressure at high ambient temperatures of up to +150°C or even 300°C.
For standard pressure gauges, operation in such conditions is possible only if there is a siphon outlet (cooler) through which the pressure gauge is connected to the system.
This is a specially shaped tube with threaded ends for connection to the mains and connection of the pressure gauge. The siphon branch creates a branch in which the measured medium does not circulate. Due to this, the temperature at the point of connection of the device is much lower than in the main line.
In addition, the durability of the pressure gauge is affected by sudden changes in measured pressure and water hammer. To reduce the impact of these factors, damping devices are used. The damper is installed in front of the device as a separate device or mounted in the channel of the pressure gauge holder.
If you do not need to constantly monitor the pressure in the system, you can install the pressure gauge through a push-button valve. This allows the device to be connected to the mains only when the tap button is pressed. This protects the device without the need for a damper device.