{"id":1221,"date":"2025-08-27T14:34:12","date_gmt":"2025-08-27T19:34:12","guid":{"rendered":"http:\/\/blog.wika.com\/us\/\/?p=1221"},"modified":"2026-02-02T11:00:13","modified_gmt":"2026-02-02T16:00:13","slug":"why-use-capillary-tubes-pressure-measurement","status":"publish","type":"post","link":"https:\/\/blog.wika.com\/us\/knowhow\/why-use-capillary-tubes-pressure-measurement\/","title":{"rendered":"What to Know about Capillary Tubes in Pressure Measurement Systems"},"content":{"rendered":"

Installed between a diaphragm seal and pressure gauge or transmitter, capillary tubing enables remote monitoring while protecting the instrument from extreme temperatures. Proper design and configuration minimizes the errors that capillary lines can introduce to pressure readings.<\/strong><\/p>\n

A diaphragm seal<\/a> separates the process medium from the pressure measuring instrument, thereby protecting both from each other. These barriers are essential when the medium is aggressive and would damage the pressure gauge or transmitter, as well as to prevent the contamination of hygienic media<\/a>.<\/p>\n

In most applications, the pressure measuring instrument is attached directly to the diaphragm seal via a threaded, flanged, or sanitary connection<\/a>. But in certain applications, a capillary tube (also called a capillary line) connects the two components.\u00a0<\/strong><\/p>\n\n

Diaphragm seal system with capillary tube<\/p><\/div>\n \n

Advantages of Using Capillary Tubes<\/h2>\n

There are two main reasons to opt for a capillary line rather than a direct process connection.<\/p>\n

1. Greater accessibility<\/h3>\n

When the measurement point is too tight to fit a pressure gauge or transmitter, or if the location is too far or inaccessible for the gauge to be read easily, a fluid-filled capillary line allows the instrument to be installed away from the process connection. This is known as remote mounting.<\/p>\n

2. Protection from extreme temperatures<\/h3>\n

High process temperatures is one of 8 common reasons for gauge failure<\/a>. A capillary tube acts as a powerful cooling element. As heat travels through the line, it dissipates from the process medium before the pressure reaches the instrument. The longer the tube, the more heat is radiated.<\/p>\n

Here is a practical example: As a rule of thumb for standard direct-mounted pressure systems, a diaphragm seal can protect the measuring instrument from temperatures between \u221240\u00b0F (\u221240\u00b0C) and 300\u00b0F (~150\u00b0C). Adding a heat-sink-type cooling element can boost the assembly\u2019s temperature tolerance up to 500\u00b0F (260\u00b0C). However, by mounting a flexible capillary line between the diaphragm seal and the measuring instrument, the assembly can withstand temperatures up to 750\u00b0F (~400\u00b0C).<\/p>\n

Note that a capillary connection measurement system requires a mounting kit or mounting bracket for the gauge or transmitter, as the instrument would not be a freestanding unit.<\/p>\n

Three Considerations When Using Capillary Lines<\/h2>\n

Capillary tubes are excellent solutions in extreme conditions and tight and\/or remote spaces, but there could be a dip in performance since they introduce variables to the pressure measuring system.<\/p>\n

1. Response time<\/h3>\n

The longer the capillary, the farther the pressure has to travel to reach the instrument, thus increasing the response time.<\/p>\n

2. Elevation or gravity <\/span>effect<\/h3>\n

When using a capillary, it\u2019s common for the diaphragm seal and the pressure instruments to be at two different levels. This difference in elevation and, thus, gravity can create false positive or false negative pressure readings. However, since the level difference is known, it can be compensated for during the calibration of the diaphragm seal assembly.<\/p>\n

3. Temperature effect<\/h3>\n

Changes in temperature cause the fill liquid inside the capillary tube to either expand or contract. This change in fluid volume creates an error in the pressure reading as a function of the total volume of the entire pressure measuring system. There are two components to the temperature effect errors.<\/p>\n