Capillaries and pressure tubes

In my opinion, pressure guiding tubes should be used for cleaning media, while capillaries are suitable for media with high viscosity that tend to crystallize

A pressure guide tube is just an ordinary tube. There is a separating fluid inside the capillary, so the transmitter does not come into contact with the measured medium.

Both capillaries and pressure conduits can transmit pressure; the only difference lies in their diameter. Capillaries have an inner diameter of 0.5–2 mm, which is very small, while pressure conduits have a diameter of 6–14 mm, or even larger. Capillaries are used in refrigeration or as capillaries in pressure transmitters.

Differential pressure (level) transmitter a) Generally, differential pressure transmitters are used for level measurement. b) For corrosive media, viscous liquids, easily vaporizable liquids, liquids containing suspended solids, etc., certain selection methods apply. c) For liquids that tend to crystallize, have high viscosity, are prone to gelation, or cause precipitation, plug-type flange differential pressure transmitters are suitable. d) When the substance being measured has condensates or precipitates, double-flange differential pressure transmitters should be used. e) When a differential pressure transmitter is used to measure the liquid level in a boiler drum, a temperature-compensated balance vessel should be employed. f) Differential pressure transmitters used for measuring liquid levels must be equipped with a migration mechanism, and the positive and negative migration amounts should be specified when selecting the instrument. g) For liquids whose density changes significantly under normal operating conditions, differential pressure transmitters are not suitable. h) When a diaphragm-sealed differential pressure transmitter is used to measure liquid level, the lengths of the two capillaries should be equal

Differential pressure level transmitter a) Under normal circumstances, differential pressure transmitters are used for level measurement. b) Flanged differential pressure transmitters should be used for corrosive media, viscous liquids, easily vaporizable liquids, and liquids containing suspended solids. c) For liquids that are prone to crystallization, have high viscosity, exhibit gelling properties, or tend to precipitate, an insert-type flanged differential pressure transmitter should be used. d) When condensate or precipitates form on the object being measured, a double-flange differential pressure transmitter should be used. e) When a differential pressure transmitter is used to measure the liquid level in the boiler drum, a temperature-compensated balance vessel should be employed. f) The differential pressure transmitter for measuring liquid level must be equipped with a drift mechanism, and its positive and negative drift amounts should be determined when selecting the instrument’s range. g) For liquids whose density changes significantly under normal operating conditions, differential pressure transmitters are not suitable. h) When a diaphragm-sealed differential pressure transmitter is used to measure liquid level, the lengths of the two capillaries should be equal.

This post was last edited by fpiliuwei on 2019-4-22 at 12:38. Capillaries are generally used in diaphragm-type pressure or differential pressure transmitters; they are filled with substances such as silicone oil, which is used to transmit the pressure of the medium from the diaphragm and silicone oil to the sensing element of the transmitter. Pressure transfer tubes (divided into Tube or Pipe types) are generally used in ordinary pressure or differential pressure transmitters; they are also employed in applications such as sample collection in analysis booths. It is similar to a piping system, used to transport the medium to the detection end or sampling inlet. In engineering design, the material of the capillary is generally provided together with the transmitter, as an integrated unit by the transmitter manufacturer. The pressure guide tube requires that the length of the material be calculated based on the distance from the pressure sampling point to the transmitter, while its dimensions are determined according to the specifications of the pressure sampling port and the transmitter’s sensing port. Select pressure guiding tubes of appropriate specifications based on the properties of the medium, as well as process conditions such as temperature and pressure. The pressure guiding tubes used in the sampling pipeline are primarily chosen based on considerations such as the fluid medium, flow rate, and heat tracing, as well as other related engineering requirements. I hope the above reply can provide you with a general overview.

The pressure guide tube is thick and unobstructed; the capillary tube, on the other hand, is thin and prone to various blockages. Using capillaries for pressure transmission can lead to pressure distortions and losses. Therefore, capillaries should only be used in situations where high pressures are present. When dealing with low pressures, using capillaries results in terrible accuracy losses – it’s absolutely horrible, as if one were forced to eat flies... Who has ever eaten a fly? Raise your hand; I’ll send you a big WeChat red envelope. :lol

A capillary is a relatively thin pressure-conducting tube. The pressure guide tube is used for guiding pressure, and the larger its diameter, the less likely it is to get clogged. 30 years ago, φ14×2mm pressure guide tubes were the standard for use in industrial installations; nowadays, those who choose to use pressure guide tubes opt for φ32mm tubes ； The larger the diameter of the pressure guide tube, the more medium or isolation fluid is required, and its volume changes depending on factors such as pressure and ambient temperature. In industry, the selection of pressure guiding pipes is often determined by factors such as clogging prevention, cost, and support strength. In situations where the media is very clean, it is also common to use pressure guide tubes with a smaller diameter. For example: φ6 pressure guide tubes are commonly used in laboratory apparatus ； For measuring the refrigerant system of a refrigeration unit, even φ3mm pressure gauge tubes can be used. For flanged transmitters, the flexibility of the pressure guiding tube must be taken into account, and less silicone oil should be used. Especially since the volume of the pressure guide tube can only be adjusted using the small amount of silicone oil in the diaphragm box as pressure, ambient temperature, and other factors change, it is better for the volume of the pressure guide tube to be small. Therefore, flanged transmitters use very thin tubes as pressure transfer pipes. In fact, “capillary” is just a *common term; if the pressure guide tube truly had a significant capillary effect, it would definitely affect the measurements.