Capillary Tube

There are many reasons to think about using capillary tube. Before considering those reasons, it is important to understand the role of the capillary. Capillaries are used in instrumentation to move pressure gauges, transmitters and switches away from the process tapping. This can be done for many reasons. You may want to place your pressure instrument in an area with good visibility, such as the instrument panel. In other cases, it may be necessary to reduce the pulsation of the instrument.  Its diameter is very small. Capillaries are mainly filled with glycerin but may change depending on the process.

Capillaries are also used to isolate devices from very hot or cold temperatures. The amount of temperature reduction they provide is dependent on the situation and varies. Make sure you have selected right capillary for your needs.

The diameter of the capillary tube is very small. It is used to isolate pressure equipment such as gauges or transmitters from process tapping/connections.

Capillary tubes are primarily used to separate equipment from process tapping. Sometimes the transmitter is also used to make it easier to place. For example, a capillary tube is used to raise the level transmitter.

Principle:-

“Capillary tubing is used to isolate the direct connection between the measuring instrument and the process tapping point.”

Capillary tube uses :-

In Pressure Gauge

A diaphragm pressure gauge indicates pressure and reflects the temperature of it’s filled system, its diaphragm and filled capillary has an error in the process pressure indication, because the trapped fill fluid expands and contracts with temperature and that temperature expansion/contraction in a filled system results in pressure changes which are temperature related, not process pressure related. to eliminate this error, we use capillary tube between sensing element and remote seal diaphragm. Capillary isolate the pressure sensor element from process medium. It may be used for isolating sensing element, reduce temperature effect and physical damping of pulsating pressure signal.

In differential Pressure Transmitter:-  

Differential pressure transmitter is used to measure Differential pressure, Level, Flow etc.  Normally transmitter are used in this services directly but sometimes capillary are used. There may be some reason to use capillary.

  1. For Isolation of sensing elements from process medium.
  2. For easy Approach.
  3. For physical damping of pulsating signal.
  4. For reducing process temperature effect on sensing element.

Sizing :-

1/16 inch to 1/8 inch Dia capillary tubes are used in instrumentation applications. In This dia range capillary size and length depends and varies on application.

Effect on Capillary tube:-  

Mounting effect:-

Transmitter mounting positions effect its pressure.  A non remote seal transmitter and remote seal transmitter reading will be different at same mounting position. It can be eliminate by calibration.

Process Temperature Effect:-

Process temperature may effect on capillary because Capillaries are used to isolate instruments from very hot process tapping. Its filled material may expand with process temperature. The value of expansion is dependent on the situation and varies. To eliminate this error, Make sure you have selected right capillary for your needs.

Ambient Temperature Effect:-

Capillaries are made from many materials. It is mounted between the remote seal and the sensing element. If the capillary is placed in a very hot area, the effect of temperature can be on the capillary and it can also affect the reading. To overcome this, proper insulation should be done on the capillary or it should be said that the capillary should be selected properly.

Capillary Tube Advantage

There is some advantage of capillary:-

  1. Mounting the transmitter at the desired location
  2. Isolate and protect the sensing element from the process fluid.

Capillary Tube Disadvantage

  1. Accuracy decreases with longer capillaries.
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