Installing a Surface Temperature Sensor for a Data Logger Deployment

In my previous post, I discussed measuring surface temperatures on pipes.  In this post, I’ll show you how I went about doing this for a temporary data logger deployment on a heating hot water system associated with a project I am currently working on.  I needed top pick up several temperatures and there were no wells available, thus the surface temperature approach.  The first step was to mark the pipe where I wanted to install the sensor.

To do this, I  made sure that for starters, I selected a point on the pipe that would give me the temperature I really intended to measure.  That seems obvious, but in the maze of piping you encounter in the field, the right location is not so obvious. 

That’s one of the reasons I like to develop system diagrams very early in my commissioning process;  they untangle the piping system and make it easier to understand how it works and what the parameters I am measuring at different points in the system are telling me.  More on that in a subsequent post.  (If you want to hear about my introduction to system diagrams, watch for my article on Mentoring Control Engineers and Technicians in next month’s edition of CSE.)

Returning to the the current discussion, I selected and then verified a spot on the piping that would give me the supply temperature to the loads.  This particular boiler room is very tidy, so I tried to pick a spot that was hidden from view to someone who was in the equipment room so that the minor patch I would install in the installation would not normally be visible to the casual observer. 

Truth be told, I think neatness counts anyway because it speaks about your professionalism and your concern for the property of others.  That’s why I’m holding a box of spare data logger batteries over the spot on the pipe where I wanted to install the sensor.  I used it as a pattern to outline the cut I would make in the insulation.  Incidentally, I felt around on the insulation and selected a location that was next to a seam but clear of the mechanical coupling that might lie below the seam.

The reason I thought that there might be a mechanical coupling there is that the insulation to the left and right of the seam felt fairly firm, but not so firm at the seam, indicating a void of some kind.  In the past I had seen insulators use a technique where the fittings were covered by seam tape in hot water insulation systems where the insulation thickness matched the thickness of the mechanical couplings. 

This approach made the insulation job look very neat and smooth since there are no bulges at the coupling locations.  From a practical standpoint, it also makes the insulation job easier (i.e. saves time and money).  And, if they pack the space around the fitting with loose fiberglass, the solution is probably a reasonable approach on a hot water system with relatively low surface temperatures.  But on a chilled water system, the insulation thickness at the couplings might not be adequate enough to prevent condensation.

Next, I cut the insulation on the lines I had marked and removed the insulation from the hole I cut as shown in the next two pictures.

I wore gloves to do this because fiberglass can cause allergic reactions in a lot of people, including your’s truly.  I saved the insulation I removed for use later after I had installed the sensor. 

The picture below shows the opening I cut, ready for the sensor to be installed.  You will note that my suspicion about the mechanical coupling was correct;  you can see it to the right of the opening (its the orange component just visible on the right side of the hole).

My next step was to get the sensor ready for installation under the insulation.  To do this, I simply dipped it in heat transfer paste, as illustrated below.

Don’t be afraid to be generous with the paste;  its key to success and relatively inexpensive (especially compared to what it will cost you to come back and do it over again if you get bad data because of poor thermal contact between the sensor and pipe).

Once the sensor was thoroughly coated with heat transfer paste, I inserted it under the insulation on one side of the hole I cut, as illustrated below.

A couple of important points here:

  • The size of the hole you cut should consider the space you will need to insert the probe under the insulation. This comes down to the length of the inflexible portion of the probe and the space you will need to maneuver it into the hole and slide it under the adjacent insulation. You could just try to hold the probe against the pipe in the opening and put the patch back in place, but that can be tricker than it sounds and result in a sensor that is not in intimate physical contact with the pipe, which will result in bad data.
  • Be careful to make sure that the probe REALLY is inserted under the insulation, not into the insulation. You want the probe to be in contact with the pipe, held in place by the insulation. Its remarkably easy to insert the probe in a manner that puts some insulation between it and the pipe (I know because I have made that mistake), again,resulting in bad data.

 Once I had the probe in place and was sure that it truly was in contact with the pipe, I reinstalled the fiberglass insulation that I removed earlier to plug up the hole.

Finally, I secured everything in place with a patch of aluminum HVAC tape, providing both a seal and a finished appearance as well as a bit of retention for the cable leaving the sensor.

If this installation where on a chilled water line, it would have been much more difficult, per my discussion in the previous post

For one thing, I would  have needed to concern my self with the impact of condensation on the pipe once I exposed it.  For another, I would have needed to vapor seal the patch I made. 

While the adhesive on the aluminum HVAC tape may actually be capable of providing such a seal, I usually carry a little tube of silicon caulk with me and run a bead of it around my cut before installing the tape on a surface temperature sensor probe installed to measure temperature on a cold pipe.  I am especially careful to make sure that no voids are left around the cable where it penetrates the insulation as they would provide a path for moisture to migrate under the insulation.

So, there you have it, my surface temperature sensor installation.  In the next post, I will show you how I went about deploying the actual logger and then follow that up with the first data I retrieved and the clue it provided about an opportunity for optimization of the facility’s heating hot water system.

David Sellers
Senior Engineer – Facility Dynamics Engineering

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