Measuring Pipe Surface Temperature

 One of the problems that often comes up in the field is the need to measure a temperature in a pipe at a location where there is not a well. The illustration in this screen shot, taken from the Functional Testing and Design Guide, illustrates an approach I used in the past with some success for a semi-permanent installation.

If you want some other ideas, Minco has a number of application guides on their web site, including one on HVAC sensor installation that has a section covering surface temperaure measurements.

I developed the detail above to solve a problem on a large central plant system that we modified. The system served a major Medical Complex and simply could not be shut down. We were able to totally reconfigure the plant with out an outage via a well thought out process that took advantage of some existing service valves and made some hot taps.

Most of the new temperature sensors we needed could be installed in the new piping or in portions of the existing piping that were taken out of service temporarily as part of the reconfiguration plan. But there was one location where we needed a sensor in a line
that would not and could not be shut down, thus the need for a surface temperature measurement.

Surface temperature measurements can be tricky for a number of
reasons. 

  • Good thermal contact is required between the sensor and the pipe it is measuring. This in turn,implies that the pipe must be free of corrosion at the location where the sensor will reside. Once the pipe is clean, heat transfer paste

Image Courtesy of Omega Engineering; http://www.omega.com/pptst/OT-201.html

… or  a thin film transmitter epoxied to the pipe

… can be used to ensure good heat transfer.

  • Freedom from moisture is important. This is harder to accomplish on a cold piping than hot piping due to the tendency for condensation. In the application I have been discussing, we waited for cold dry weather to install the sensor,minimizing the potential for condensation while we did the work. We also used heat guns to warm up the surface of the pipe.
  • Insulation is important. Covering the sensor with insulation puts the sensor on the pipe’s side of the thermal gradient between the pipe and ambient environment. Thus, even with less than perfect contact with the pipe, the temperature of the sensor will tend to approach the temperature of the pipe. With out insulation, the ambient environment can have a significant impact on the temperature measurement.
  • Vapor barrier integrity is important for cold piping. With out a vapor seal, moisture will migrate through the insulation and the cracks at joints and condense on the pipe, ruining the intimate contact between the sensor and pipe, causing corrosion, and compromising the integrity of the insulation system. Its important to seal all potential paths,including things like the inside of a conduit carrying the wire from the sensor to the transmitter or control system. Otherwise the conduit itself will provide a vapor migration path.
  • Surface temperature data should be “taken with a grain of salt.” For all of the reasons listed above, data from a surface temperature sensor should be assumed to be less reliable and persistent than data from other sensors. Providing a means to verify the integrity and accuracy of the sensor can be desirable, but introduces problems of its own, especially on cold piping where the joint associated with the access patch represents a potential vapor barrier breach.

Disadvantages aside, surface temperature measurements represent a means to assess system temperatures when no other option is available. And if a thin film, epoxied in place sensor is used, the sensing system can have a faster response characteristic than other approaches since there is no well or sensor mass to heat up or cool down.

Come back in a couple of days and I’ll outline the steps I used to install a temporary surface temperature sensor on a hot water piping system to serve a data logger on one of my projects, including pictures of the process.

 
David Sellers
Senior Engineer – Facility Dynamics Engineering

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