Accurate
Measurements Critical for Subcooling and Superheat
By Jack Sine
With system efficiencies
continually improving and environmental requirements becoming more stringent,
subcooling and superheat measurements become more and more important because
they are the truest indicators of system performance. One of the most critical
measurements and the one most often taken inaccurately is that of pipe temperature.
A difficult measurement to take
Although many methods exist for measuring pipe temperature, many methods in
use today are just plain inaccurate. This is because a temperature sensor
that is not properly thermally isolated will be affected by the temperature
of the environment surrounding the pipe resulting in a false measurement.
When the ambient temperature is higher than the pipe temperature, a false
measurement will provide a temperature reading higher than the actual pipe
temperature, indicating a falsely higher superheat. Since most superheat charts
call for accuracy of plus or minus one degree Fahrenheit, readings that are
off by typically five degrees Fahrenheit may cause the technician to overcharge
the system and put the compressor in danger.
No one is more aware of the problem than Adolfo Wurts. He is the senior research
specialist at Fieldpiece Instruments, a company that manufactures measurement
instruments for the HVAC/R market. One day his boss gave him the assignment
to find a better way of measuring pipe temperature.
“He wanted heat transfer from the pipe to the sensor to be as easy as
possible without exposure to ambient air. I’d seen so many temperature
measurement products that I figured that I’d wrap up this project before
lunch,” said Wurts. “Two months later, after numerous experiments,
consultation with HVAC teachers and other gurus, and several prototypes that
didn’t do the job, I was extremely frustrated.”
Some missteps with digital thermometers
“Of all the measurement errors made on measuring pipe temperatures,
pocket thermometers are probably responsible for the most,” said Wurts.
“I’ve strapped them to cold pipes and recorded temperatures that
were 15 degrees higher than the actual temperature. And while performing temperature
shock testing, I’ve taken them from an environment of 32 degrees F to
one of 120 degrees F and seen inaccuracies of 20 degrees F. The digital pocket
thermometer is an inexpensive solution for taking air temperature measurements
and can deliver good performance provided the ambient conditions are stable
and provided you can leave them in place long enough. But their inability
to make good thermal contact with pipes makes them a poor choice for pipe
temperature measurements.
“Yet one HVAC instructor I consulted insisted that I could take accurate
tests with a digital thermometer by using a clamp such as those used with
automobile jumper cables to hold it on the pipe,” said Wurts. “It
didn’t make sense to me for two reasons. One, you can only make point
contact with the pipe, leaving most of the probe exposed to ambient air. And,
two, the clamp is metal and will conduct heat away from the thermometer to
the environment.”
But Wurts decided to experiment with the pocket thermometer. He put a bend
in the thermometer to assure line contact between the pipe and the sensing
rod as opposed to point contact. Then he substituted a Velcro strap for the
clamp to hold it in place. Finally, he insulated the thermometer with neoprene
to keep out ambient air.
“I was finally able to take reasonably accurate measurements,”
said Wurts, “but it took too long to set up and stabilize and the thermometer
once altered was useless for other applications.”
Achieving accuracy with a beaded thermocouple
One of the most common methods of measuring pipe temperature is with a K-type
thermocouple. It was one of methods Wurts spent a lot of time analysing.
“My first attempts at this gave me readings that I knew were off by
three to five degrees F,” he said. “I was surprised because this
was such a common method for taking pipe temperatures. It makes sense, though,
if you have a spherical thermocouple bead contacting a tubular pipe surface
you end up with a point contact between the two. That slows heat transfer
and leads to inaccuracy.”
Wurts wondered if there was a way to accelerate heat transfer. He decided
to strip some of the insulation near the bead and bring the wire in contact
with the pipe.
“I took several Fieldpiece beaded thermocouples from stock and stripped
back insulation from the sensor at different lenghts,” he said. “I
wanted to see if having the wires near the bead come in contact with the pipe
would improve the accuracy of the thermocouple or decrease the response time.
It did both. After running experiments with the insulation stripped at different
lengths. I noticed significant improvement when 3/8 inch of wire was exposed
and brought into contact with the pipe. But after 3/8 inch the improvement
diminished with length.”
Even though the temperature measurement was only occurring at the tip of the
bead, the wire being in contact with the pipe increased the rate of heat transfer
to the bead. To remove the influence of ambient air temperature, Wurts strapped
neoprene insulation around the bead and wire.
“That gave us the accuracy and response we were looking for,”
he said. “We immediately began producing K-type thermocouples with 3/8
inch of insulation stripped back and we amended the operator’s manual
to explain in detail how to get the best pipe temperature measurements using
a beaded thermocouple. Since we are a small company compared to the giant
instrument manufacturers, we are agile enough to make product improvements
very quickly.”
The clamp thermocouple – the ultimate tool
But there was still one problem that needed to be addressed – time.
As accurate as the new method was, it still took a long time to set up and
stabilize. That’s when Wurts thought of the pipe clamp thermocouple
as a possible solution.
“We already manufactured a clamp thermocouple at Fieldpiece, but it
had room for improvement in accuracy and response time,” he said. “I
thought the lessons learned from the beaded thermocouple could be applied
here.”
The original design of the Fieldpiece clamp thermocouple consisted of thermally
isolative material with a beaded thermocouple in its center. This could only
measure temperature to within five degrees of the pipe temperature, much too
inaccurate for many of today’s applications. Wurts wondered if he could
change the shape of the contact point of the thermocouple by using a different
material as the junction.
The problem with thermocouple material is that it is brittle and makes a poor
spring. The shape of the sensor was much more important that the material
it was made out of. The most important characteristics of the shape is that
it must provide a line contact between the thermocouple and the pipe while
also isolating the sensor from the clamp.
“I started experimenting by forming brass into different shaped sensors,”
he said. “I was getting closer to what I wanted, but the results weren’t
accurate enough. Finally one shape gave me the results I needed. I used a
thin piece of brass in the form of a curled cylinder with small metal flanges
as standoffs. Not only did this provide adequate heat transfer, but the flanges
were an effective standoff between the sensor and the clamp.”
Unfortunately, while this approach gave Wurts the accuracy and response time
he needed, it proved too difficult to manufacture. But it led him to another
approach.
“I thought why not use the thermocouple’s own wires as standoffs.
I drilled four small holes in the base of the clamp and ran wires up and back,
knotting them at the ends so they wouldn’t slip back – no welding
or soldering. Over the wires, I placed a thin brass band on which the pipe
could rest without damaging the wires. The junction in the thermocouple would
only exist when the clamp jaws were placed around a pipe causing the band
to come in contact with the thermocouple wires and close the circuit.”
This approach worked beautifully. The thermocouple wires created a small pocket
of still air underneath the band, enough to keep the band at the temperature
of the pipe. It was so effective at giving accurate measurements in a short
time that Fieldpiece has applied for a patent. In its final production form
the wires were welded in place rather than knotted and a more resistant material
than brass was used for the band.
For Fieldpiece this journey of discovery has resulted in three new products.
In the first two clamps, the ATC1 and ATC2, the same principles described
above were implemented with the thermal junction between the two wires being
this thermally conductive band. Because pipes often have condensation on their
surfaces, these clamps feature a metal band that is highly conductive, very
thin, and resistant to moisture. The band allows for excellent thermal conductance
between the sensor and pipe. The thin wires serve as standoffs and create
a thin layer of still air while sealing out ambient air. To assure maximum
accuracy, the technician can also wrap the clamp and pipe in neoprene insulation.
“After we put our first two clamps into production, we kept up the research,”
said Wurts. “The result was our newest clamp thermocouple that features
a special spring to suspend the metal band. It has tiny holes drilled into
it and leaves the sensor surrounded by completely still air, effectively sealing
out ambient air. This clamp, the ATC3, is so effective that it provides high
accuracy with an extremely fast response time – 20 to 30 seconds. We
actually use this clamp as a standard to test the others.”
Wurts offered a couple of hints on taking accurate measurements.
“No matter whose temperature measurement system you are using, it is
critical that you calibrate it as soon as you take it out of the box,”
he said. “Our research shows the easiest and most effective method is
to use a bucket of ice water. Stir the water and ice to assure consistent
temperature throughout the contents, then insert the sensor into the mix.
After a short time, dial in 32 degrees on the calibration pot. When this is
done, all of the inaccuracies associated with the thermocouple and the meter
are eliminated and an accuracy of one degree is achieved. Also, when measuring
pipe temperature on a horizontal pipe, place the sensor at a 45 degree angle
or greater from the bottom of the pipe. This is because all systems contain
some amount of oil in the refrigerant and it tends to rest at the bottom of
a horizontal pipe, giving inaccurate measurements.”
With the challenges presented by new systems and environmental standards,
Wurts and his team appear to have found some elegant solutions.
Jack Sine is a freelance
writer specializing the HVAC/R market place. He is based in Beacon, NY.
Fieldpiece
Instruments, Inc.
580 W. Central, Suite A
Brea, CA 92821
(714) 257-9060 FAX (714) 257-9069
fpinfo@fieldpiece.com
