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.
Last edited by polyglot; 06-01-2011 at 12:57 AM. Click to view previous post history.