Testo 550 clamp style temp probe issue.

[mark beiser]

Let me start by saying that, based on my testing in my house and in my garage, the clamp on temperature probes for the Testo 550 are very accurate.

Having said that, there is a serious flaw in the design that I plan to spend some time working on this weekend.
The flaw is that the sensor itself is embedded into one of the clamps, with no thermal isolation whatsoever, so there is very solid thermal contact, over a large area, with the clamp.
Also, when the sensor is clamped to a refrigerant line, there is a gap between the jaw of the clamp and the pipe.

The large thermal contact with the clamp, and freedom of ambient air to circulate around the sensor, makes this otherwise very accurate sensor extremely imprecise, even useless and dangerous to rely on, for taking refrigerant line temperatures when the ambient air temperature is significantly different from the line temp, without having to take extra steps to insulate the entire clamp after it is clamped on a pipe. Even then, you have to wait for the entire thermal mass of the sensor and clamp to reach the actual line temperature.

This totally kills the entire purpose of having a clamp on style probe, speed and convenience.

With the clamps uninsulated, I've been getting readings that are off by as much as 8ºF on suction lines in 90º weather. This after fiddling with the sensor position to get the very best possible reading.
Liquid line temperatures have not been off by quite as much due to the lower difference between the ambient and line temperatures, but they are still off by 1-5º.

I haven't had time to work on a solution more convenient than wrapping insulation around the whole clamp assembly yet, but I do have a few ideas.
Hopefully I'll be able to make them work precisely enough to use them without physically removing the sensor from the clamp to isolate it better, but it may be the only way to make them trustworthy.

For now I've gone back to using my Fluke 52 + pipe clamps like I always have.

I very strongly recommend that anyone else using the 550 verify the line temperature readings they are getting before relying on them, especially in high ambient conditions.

I haven't received the sensors for my Digi-Cool 1250 yet, so haven't had opportunity to test them yet. I sure hope they do better.

For some reason I find it amusing that the very best, in terms of accuracy, precision, and ease of use, pipe temperature sensor for a field service instrument of any kind I've ever seen from any manufacturer is the PT100 thermistor pipe wrap probe for the first generation Testo 523.

[itsiceman]

Back to the drawing board






Did testo even test them once outside the sales office

[jvillehvac]

I havent had a problem other than when im working in the sunlight i cover them with a rag, but in the shade it seems fine, verified with my fieldpiece. but every sensor i have used is affected when not in the shade. Also the screws on the clamp come loose after about a month, thread lock will fix that.

[mark beiser]

Got the clamps in for my 1250.
The part the sensor is embedded in is a rubber type material, that has air gaps molded into it, I guess to reduce its thermal mass and ability to conduct heat.
I haven't had the opportunity to test them on a running system yet.
the actual sensor is a little shorter than the Testo clamp too.

The Digi-Cool clamps are pretty big, but they will clamp on a 1 5/8 pipe, so I'm OK with the size of them. I largest suction lines for any of the equipment I work on are 2 1/8, but they are reduced to 1 5/8 before the compressor service valves.

The temps are spot on with the all my other myriad of pipe temp sensors in static tests in my house and in my garage.
I'll check them out on a running system in 90º+ ambient conditions this weekend.

I'm not sure I'll ever be happy with how slow NTC thermistors are compared to PT100 thermistors and good quality thermocouple probes...
But then again, PT100 thermistor, and high end thermocouple probes, cost $120 to $350 each to replace...

[itsiceman]

I havent had a problem other than when im working in the sunlight i cover them with a rag, but in the shade it seems fine, verified with my fieldpiece. but every sensor i have used is affected when not in the shade. Also the screws on the clamp come loose after about a month, thread lock will fix that.

A Fieldpiece What ...... Clamp ........ and what did you verify that with
Mark is right on except the PT-100 clamps might be less accurate than he thinks when used without insulation to ambient and when you insulate them they slowwwww downnnnn

[jim bergmann]

Weber,
The design of the 550 clamp has been thoroughly tested and is without a doubt accurate as designed if used properly. The 550 was tested for over 12 months prior to its release. The clamp was redesigned during initial testing to properly center the sensor on the pipe during the measurement process and provide better contact on smaller lines. After it was lab tested in Germany, it was field tested by myself, several engineers at Testo, and again field tested in a pilot of 10 persons for several weeks all over the country. Testo is known all over the world for accurate temperature measurement, and in fact since 2000 has been accredited as the first DKD lab for surface temperature probes.

All clamps require time some for stabilization. This should happen with the 550 clamp in 3 minutes or less. The plastic (fiberglass resin) that it is molded in is isolated from the large mass of the clamp, and to top it off is an insulator with negligible mass. That being said, the error might have been the operator. The probe should be shielded from direct sunlight, it needs to have good contact with a straight section of clean copper, and it needs to be straight and centered. The surrounding air which has almost no mass will have a negligible effect on the measurement. the plastic which is two piece and not a good conductor will have little impact. The copper line that it is attached to will have a much larger effect on the temperature measurement due to the mass or the copper, the thermal conductivity and/or conductive heating or cooling. The sensor itself is also very low mass and very conductive.
See the photos and the thermal images below. The probe almost is identical in temperature to the line. Within 10th's of a degree. Notice the ice on the thermistor clear to the top. The plastic is not transferring heat away from the sensor and influencing its reading. It is an excellent insulator. If it was not the frost would develop at the bottom but not at the top of the sensor itself. The ambient temperature during the readings was almost 90° and the hottest part of the sensor handle above 86°F.

Without an accurate reference it is impossible to verify the accuracy of the probes that you are measuring against. The k-type you are using are far more subject to error by design and by age. Typical k-type +/- 2.5°C tolerances where as the standard 550 NTC has a tolerance of .2°C

Note: All of the parts of the probes and line were painted with flat black spray paint to balance emissivity for the thermal measurement.

You are correct, the NTC is slower than the k-type, but it is far more accurate.

Hope this helps.

[beachtech]

read this post right after you posted.

have been backing my testo clamps up with my gual temp clamps, and everything has been the same.

[mark beiser]

Ok, I believe I've discovered what is causing my issue. To be generous, I'll split the blame between operator error and design/production flaw.

The clamps are pretty accurate taking line temps in high ambient conditions if, and only if, the sensor is nearly perfectly aligned with the pipe, and in solid contact with the pipe for its entire length.
The problem I'm having is that neither one of my probes likes to clamp on a pipe with the sensor aligned with the pipe, so don't make good thermal contact without a bit of fuss, and constant monitoring.
Not a desirable situation.
The jaws on both of my clamps are out of alignment with each other by just enough that the sensor likes to sit slightly out of alignment with the pipe.
Because of this, I have to fuss with the clamps a bit to get the sensor correctly aligned, and they pop out of alignment at the slightest provocation, including natural vibrations in the lines.

I haven't had much time to try different things, but I'm having good luck with a couple of small strips of weather strip on either side of the sensor, parallel to it, about 1/4" away from it.
The weather strip aligns the sensor with the pipe with no fuss, and prevents the natural tendency of the clamp to twist the sensor slightly out of alignment.

Just as an FYI, the sensor from the 550, first generation 523 PT100 pipe wrap probe, Digi-Cool 1250, and Fluke 52 with K-type thermocouple pipe clamp all agree with each other when everything is sitting in my house or in my garage clamped/strapped to a piece of pipe.

The first gen 523's PT100 probe and and the Fluke pipe clamp are the best 2 pipe probes I've ever used.
One thing they both have in common is that the design gives very good tactile feedback for when the sensor is centered up on the pipe like it needs to be. They just feel wrong when the sensor is not in the correct position.
I have grown to trust them both because they both agree with each other to within 0.5ºF under all conditions I've ever subjected them to. They also both settle into a reading very quickly.
The clamp probes for the 550 have inspired no such confidence yet, at least not as shipped.

The little bit of messing around with the 550 clamps I did over the weekend, I was checking them against both the PT100 pipe wrap and K-type clamps at the same time. I was only able to get the 550 clamps to mirror the readings of the probes I trust by adding the pieces of weather strip on either side of the sensor, and by insulating the piece of plastic the sensor is glued to, especially where the end of the sensor, opposite the wire sticks out.

One slight difference I noticed from the probe in Jim's pictures is that on both of mine the end of the sensor opposite the wire sticks out significantly further on both of my probes. On sticks out about 0.09", the other 0.135". Next time I am messing with them in their "as shipped" state, I'll have to see if that is one of the possible reasons they don't even really agree exactly with each other when I use them "as shipped".

[itsiceman]

Weber,
The design of the 550 clamp has been thoroughly tested and is without a doubt accurate as designed if used properly. The 550 was tested for over 12 months prior to its release. The clamp was redesigned during initial testing to properly center the sensor on the pipe during the measurement process and provide better contact on smaller lines. After it was lab tested in Germany, it was field tested by myself, several engineers at Testo, and again field tested in a pilot of 10 persons for several weeks all over the country. Testo is known all over the world for accurate temperature measurement, and in fact since 2000 has been accredited as the first DKD lab for surface temperature probes.

All clamps require time some for stabilization. This should happen with the 550 clamp in 3 minutes or less. The plastic (fiberglass resin) that it is molded in is isolated from the large mass of the clamp, and to top it off is an insulator with negligible mass. That being said, the error might have been the operator. The probe should be shielded from direct sunlight, it needs to have good contact with a straight section of clean copper, and it needs to be straight and centered. The surrounding air which has almost no mass will have a negligible effect on the measurement. the plastic which is two piece and not a good conductor will have little impact. The copper line that it is attached to will have a much larger effect on the temperature measurement due to the mass or the copper, the thermal conductivity and/or conductive heating or cooling. The sensor itself is also very low mass and very conductive.
See the photos and the thermal images below. The probe almost is identical in temperature to the line. Within 10th's of a degree. Notice the ice on the thermistor clear to the top. The plastic is not transferring heat away from the sensor and influencing its reading. It is an excellent insulator. If it was not the frost would develop at the bottom but not at the top of the sensor itself. The ambient temperature during the readings was almost 90° and the hottest part of the sensor handle above 86°F.

Without an accurate reference it is impossible to verify the accuracy of the probes that you are measuring against. The k-type you are using are far more subject to error by design and by age. Typical k-type +/- 2.5°C tolerances where as the standard 550 NTC has a tolerance of .2°C

Note: All of the parts of the probes and line were painted with flat black spray paint to balance emissivity for the thermal measurement.

You are correct, the NTC is slower than the k-type, but it is far more accurate.

Hope this helps.

The way I read Mark's post was he is not referencing a K thermocouple but he said he is more happy using them than the testo clamp.
Not sure I'm impressed by all that hooha and I doubt those shots were from "the field" and can you really tell the difference between 18ºF frost and 28ºF frost by looking at a frosted suction line and sensor.
Jim during your field test did you or the other ten only use a I/R meter to verify the clamps in the field ? Maybe its just me but the pic shows a color variance from sensor to pipe.
Any one with a dual sensor meter can easily verify with their unit. Just install sensor #1 by original clamp and sensor #2 pulled from the clamp and insulated hooked to a suction line outside at a C/U or liquid line in a cold space and use the display on the 550 or whatever dual input meter to compare the two probes styles on the same pipe. This is probably what Mark has done and I have to go with him on this one. He really sounds like he is doing a very good unbiased job comparing the temperature probes in the field.

[mark beiser]

You are correct, the NTC is slower than the k-type, but it is far more accurate.

PT100 sensors are slower than k-type.
NTC sensors are slower than Christmas!

BTW, do you happen to have the raw radiometric jpeg files for those IR shots, and would you be willing to share them?
I'd like to see what they look like zoomed in on the pipe to sensor contact area, with a much narrower temperature range.
Glad to see you don't use the "rainbow" pallet for your IR images.

[itsiceman]

This was color boost and fade on the entire pic then zoomed in.

[itsiceman]

You are correct, the NTC is slower than the k-type, but it is far more accurate.

PT100 sensors are slower than k-type.
NTC sensors are slower than Christmas!

I have to disagree in part with both on this.

If you have apples to apples with mass and contact area and then compare sensor types even the manufactures list NTC thermistors at the top as far as reaction time.

RTD are fragile, more accurate in a range but slower than NTC.

They list thermocouples (K-Type probes) the slowest with reaction time and accuracy but are well suited for high temperature measurements vs the other two.

[mark beiser]

I have to disagree in part with both on this.

If you have apples to apples with mass and contact area and then compare sensor types even the manufactures list NTC thermistors at the top as far as reaction time.

RTD are fragile, more accurate in a range but slower than NTC.

They list thermocouples (K-Type probes) the slowest with reaction time and accuracy but are well suited for high temperature measurements vs the other two.

True, but when you get down to finished products, NTC thermistor probes virtually always have many times the thermal mass, and require much more contact area than a thermocouple or high end PT100 probe.

[pacnw]

Mark,

would it be possible to post some photos of what you are referring to and the "fixes".

I believe I understand and when mine get here I will compare them with my instruments and see how consistent they all are.

Thanks

[jvillehvac]

mark, im wondering if you got some bum 550 clamps or something. whenever i compare mine they are really close. i must admit i dont have that many instruments and if i did that in my house my wife would think im crazy!!!

[itsiceman]

mark, im wondering if you got some bum 550 clamps or something. whenever i compare mine they are really close. i must admit i dont have that many instruments and if i did that in my house my wife would think im crazy!!!

Really close to what? If they all read the same equalized and at rest at Marks house what could be different about yours and his clamps

[jvillehvac]

Really close to what? If they all read the same equalized and at rest at Marks house what could be different about yours and his clamps

I didnt say mine were different than his, did I? As with any other mass produced product you plan on them reading the same. Real world, who knows. How do we know all that other stuff on there is right, oh thats right, digi cools and flukes are never out of calibration! I already tried all this when I first got mine 2 months ago cause like him I didnt want to blindly trust a new tool. Why mine arent off I dont know, go ask a testo engineer and let me know.

[mark beiser]

I didnt say mine were different than his, did I? As with any other mass produced product you plan on them reading the same. Real world, who knows. How do we know all that other stuff on there is right, oh thats right, digi cools and flukes are never out of calibration! I already tried all this when I first got mine 2 months ago cause like him I didnt want to blindly trust a new tool. Why mine arent off I dont know, go ask a testo engineer and let me know.

Well, when 6 different probes in 6 different instruments using 3 different technologies from 3 different manufacturers are all within a degree or so of each other, and 2 probes in another instrument give readings that vary significantly from the other instruments, and from each other, guess which probes I'm calling into question.

I don't doubt the accuracy of the 550 or its probes.
When I have all that mess clamped on a piece of copper in my living room or garage, everything in those pictures, including some additional air temp probes and instruments, all of it, except the Fluke clamp I pulled out of my junk drawer, the difference between the lowest and highest readings displayed on any of the instruments is less than 1ºF.

The problem I'm having with the probes for the 550 is getting precise readings when there is a significant difference between ambient temperature and the pipe temperature, especially when there is not an absolutely straight section of pipe to clamp the probe to, which is the norm with residential equipment.

Accuracy is not the same as precision, they are related, but still 2 different things.

[pacnw]

I have to agree with Mark on the lack of confidence that the 550 temp probes provide.

I just received a replacement unit from TTT and the probes are anywhere from even( on a piece of copper sitting on my dinning room table) to 4*-5* different on the same pipe and way slooooooooooow to react!

[Vettgetsmwet04]

ive used my testo 550 for about a month now, u have to make sure that the liquid line and suction line u clamp onto is perfectly striaght or the readings will be off, if the pipe is straight and u have the probe making good contact then its perfectly accurate compared to my fluke clamps and my fieldpiece probes, ive compared over and over, if its off its because the copper is slightly bent or i didnt have the probe saddled on there right.

now on my two clamps i leave them plugged in all the time, i have a spot on my truck thats padded and i hang them them away from other things so nothin bothers them. i was finding that if u touched either clamp it would move off the copper pipe and lose good contact, so i decided to make the clamps better, i took my smallest step bit and on the clamp for the liquid line i drilled the side opposite of the senser on the clamp until i almost made a semicircle, about the same size and 3/8 copper, this makes it to where as long as the copper is striaght, it will make good contact and not move off even if it gets hit, i did the same with the suction clamp, but with a bigger sized step bit and not as dip, but just pushed down on it and drilled it out until a 7/8 pipe would fit on it good. and the ease of use is now much better, just as long as the copper is clean and striaght.

i do 90% resi and 10% commercial, so i rearly see anything over 7/8 and 3/8, sometimes 1 1/8 but not much, but if your like me do this and the clamps will saddle the copper much better for better results.