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Measuring The Pressure Of Hot Media

Jul 25, 2019

Installing pressure sensors close to high temperatures can lead to significant measurement errors and irreparable damage to the sensing elements and electronics.

The cheapest and simplest approach to protect a pressure sensor is to isolate it from the heat source. This can be done by extending the process connection with a short length of pipe and mounting the pressure sensor on the end instead.

Assuming the ambient air temperature is much lower than the process temperature and below the ambient operating temperature of the pressure sensor, the “stand off” pipe will dissipate enough heat so that the temperature is kept under the required value at the sensor process connection.

Some experimentation will be necessary to find the optimal length of pipe, since the thermal difference between the process and ambient environment will vary. Using a material for the pipe which has a high thermal conductivity such as metal will help increase the cooling efficiency.

Since the pipe is dead ended there will be less circulation of heat and a thermal gradient should be established along the length of the pipe.

In some applications such as super heated steam the pressure sensor is isolated from the steam using a “U tube” which allows condensed water to gather in the bottom providing a barrier to prevent steam reaching the pressure sensor.

Using “pigtails” or a coiled pipe is another similar method to the U tube which extends the length of the pipe and generates a greater thermal gradient between the pressure sensor and process media. The pig tail also allows steam to condense and gather in the bottom of the coil.

The temperature can be further reduced by mounting the pressure sensor underneath the heat source to minimise the contribution from convection.

Some pressure sensors are purpose built for measuring hot processes. There are three distinct ways in which pressure sensors can be adapted to high temperatures:

  • Extend the pressure connection by using a cooling element with fins to maximise the heat exchange surface area.

  • Extend the pressure connection by using a media isolated cooling element filled with a liquid behind a thin membrane.

  • Sensing technology that can withstand high temperatures by incorporating material and components such as silicon on insulator semiconductor devices, high melting point solder, wide temperature range sealants, coatings and potting compounds.

All of these options are significantly more expensive to manufacture than a standard pressure sensor but in many instances are more convenient and cheaper if the overall installation cost is considered.

To ensure the best possible accuracy it is advisable to isolate the pressure sensor from the heat source since a narrower temperature range will lead to smaller thermal errors and less measurement drift due to thermal stress on the components.