The main role of capillaries in pressure measurement or differential pressure measurement is to transmit pressure over long distances and to help protect sensitive pressure transmitters or sensors from high temperatures, corrosive media or vibrations in the measuring environment. Capillaries are often used with diaphragm seals (also known as diaphragms) to transmit pressure through a capillary filled with conductive fluid to a pressure transmitter, ensuring measurement accuracy and sensor safety.
The main role and function of capillary
1. Long-distance pressure transmission (some occasions are not suitable for pressure tube)
When the measuring point is a certain distance away from the pressure transmitter, it may be difficult to directly introduce the medium (such as gas, liquid, steam) into the pressure transmitter. Capillaries can transmit pressure over long distances, placing the transmitter in a location more suitable for maintenance or monitoring. For example, when measuring steam pressure, the transmitter can be damaged by high temperatures, and the capillary can keep the transmitter away from the high temperature source.
2. Isolation medium (corrosive medium requires special diaphragm material) :
Capillaries are often used with diaphragm seals, which isolate the measuring medium from the pressure transmitter to avoid direct contact between the medium and the transmitter. This prevents corrosive or viscous media (such as acid-base liquids or high-temperature steam) from entering the transmitter and protects it from damage.
3. Control of thermal effect (beyond the limit range of the transmitter) :
In high temperature situations (such as measuring the pressure of boiler steam), directly connected pressure transmitters can be damaged by high temperatures. By using a capillary, the capillary can be filled with a suitable conducting liquid (usually a liquid with a low temperature expansion coefficient), effectively reducing the effect of temperature on the pressure transmitter. This liquid can transmit pressure signals without transferring heat, protecting the transmitter from high temperature damage.
4. Reduce vibration effects:
When there is severe mechanical vibration at the measuring point, the direct installation of the pressure transmitter may affect the measurement accuracy or damage the transmitter. With capillary tubes, the transmitter can be installed away from the vibration source, thus reducing the impact of vibration on measurement accuracy.
Examples of using capillaries
1. Boiler steam pressure measurement:
In boiler steam pressure measurements, the temperature of the steam is usually very high (e.g., over 200°C). If the transmitter is installed directly at the measuring point, the high temperature of the steam will cause serious damage to the transmitter. Through the use of diaphragm seals and capillaries, steam pressure can be transmitted over long distances and at lower temperatures, allowing the transmitter to operate at the right temperature while ensuring measurement accuracy.
2. Differential pressure measurement of corrosive media in chemical plants:
In chemical plants, certain media are highly corrosive. If this medium is allowed to come into direct contact with the differential pressure transmitter, the transmitter will be quickly damaged by corrosion. Therefore, by installing a diaphragm seal at the differential pressure measuring point and using a capillary to transmit the pressure signal to the differential pressure transmitter, the medium does not come into direct contact with the sensitive transmitter, thus protecting the device and extending its service life.
3. Differential pressure transmitter in liquid level measurement:
When a differential pressure transmitter is used for level measurement (for example, tank level), the physical properties of the liquid (such as high temperature, viscosity, or corrosion) may affect the proper operation of the transmitter. Capillary and diaphragm seals can hold the transmitter away from the liquid while transmitting the pressure signal through the conducting fluid in the capillary. In this way, the transmitter is not in direct contact with the measured medium, reducing the risk of damage.
In summary, capillaries play a role in pressure transfer, medium isolation and environmental protection in pressure and differential pressure measurement, especially for high temperature, corrosive and vibration environments.
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