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.