The biggest problem I've had in the past with digital thermometers is preventing them from being damaged by moisture.
I've had the best success with the models that have a probe on the end of a cord. Here is an example of something similar:
If I were to work in a shared darkroom, just for fun, I'd like to use this one :):
Is you want a really fast and spot on unit, This is for you !! - http://www.thermoworks.com/products/...thermapen.html
I've been using the Comark KM28 for the past year.
It Is Digital Thermometer Perfection !
I actually have three of those $1 thermometers. They work fine (they sit neatly in bottles/jugs of developer while warming in a jobo) and are very easy to clean. The display is a bit small though and I don't trust them to be waterproof.
For the larger display and even though it costs twice as much at $2, I prefer: aquarium thermometers. I plan to mount a row of 5 or 6 of them in a piece of tin and rivet that to the back of my jobo behind an acrylic splash-guard. Calibration is by writing an offset under each one with a sharpie. You do need to be careful not to fracture the cable where it goes into the back of the housing though - one of my older ones (used freehand, especially while cooling dev in the fridge when the tap water is 25C) I've had to resolder a few times.
As to Ralph's ethical objections to the price, this is what they all really cost, it's just in this case no western reseller has had the opportunity to slap a big margin on it. You see the exact same items for $2 on eBay and then $8-$15 from AU resellers and then $25+ in retail stores; buying the $25 version doesn't mean the factory is getting any more than $1. Same model, clearly comes out of the same factory. Personally, I have no issue with cutting two or three layers of middlemen out, in fact I view the opportunity with some glee.
In terms of consistency, I've found a difference of up to 1.2C between separate instances of all of these thermometers (I have about 9 all-up) but the difference is consistent to about 0.1-0.2C across 0C-40C. Zero-order calibration (adding an offset) is easy!
Edit: for colour processing, another good choice is the ear thermometer. Maybe not the cheap ones off eBay, but the good ones are designed to be accurate to 0.1C and have a scale centred on 37C for obvious reasons. They generally don't go down to 20C though, so no good for B&W.
Guys, this is way over the top and too clumsy to use.
This is all you need:
I do not shop in dollar stores for the same reason, by the way.
I'm actually quite frugal and compare prices before I buy, but I never quite understood the hunt for the lowest price at all cost.