UEi PDT655 Differential Temperature Folding Pocket Thermometer, HVAC Digital Thermometer with Backlit Display and Magnetic Mount, Measures -58 to 572°F

The device is somewhat on the larger size, but for most users this is a plus. Its length is 6 inches and the width is 1.5 and it about ¾ inch thick. It will fit in a pocket but there will be no hiding it there, too long for that. However, it’s actually about ½ inch shorter that the Samsung phone I carry. The readout numbers are ¾ high and easy to read from 10ft away with good lighting. The display does light up, but only for about 4 seconds, just long enough to see the number if you’re in a dark basement or attic. The probe length is 5 inches and has enough mechanical resistance that it holds the position as you place it into a vent or hole. I was able to hang it onto a vertical return grill by the probe only, since in this case the grill was aluminum. The built-in magnets will hold it nicely on a flat metal area but probably not on a round duct. On those you could easily hang it by the probe though. It uses 1 CR2032 battery which helps keep the weight down. It’s a common enough battery that big box and drug stores sell them and they are cheap. It will self-power off after one hour, a nice feature, but it also means that any readings you have also disappear. It updates very quickly and follows the temp rise in a duct very near real time. It followed the rise I saw with my Fluke thermocouple and I was impressed with that. Accuracy seems reasonable and matches up nicely with other thermometers that I believe to be accurate. See photo

I do air conditioning and this is a great thermometer keeps track of the high and low temperatures and read out is very big numbers

It works good but it is much larger than the older ones definitely can’t keep two in your shirt picked anymore. Also one of them the back light does not work

Excellent tool

Buy 2. Very easy to read! I habe good eyes, but it's nice to see your temp split (Delta) From a distance.