gee, no Luddites here, just a refusal to have a battery die in the middle of a fishing outing...Best Fishing Buddy always knows the temp before and better than i do if the battery is working. he recommends it very much and thinks you would be happier with one. he's also used one of those laser thermometers that measures surface temps, but we weren't sure if it was measuring the top of the water or the bottom of the streambed, so he went back to the dippable digital. i'll ask him what kind it is.
i feel a hijack coming on, but it relates to how to use a thermometer.
the zinger string is sort of short, so perhaps my thermometer is not staying in the water long enough. i swish it around for about 30 counted seconds. if my hand gets too cold to hold it under water any longer, it's cold enough/too cold for fish. i take my hand out of the water and watch the red stripe as best i can sideways unless my teeth are chattering too hard, in which case, bag it, just fish for a while and then go home.
if my hand doesn't get too cold, it's warm enough/too warm, and i can really watch the bright red stripe creep up and down the little number scale until i decide which condition it is. Wiflyfisher is right: those last two degrees are crucial, so it's good they're easier to read! if it's too warm, fall in the water on the way out of the stream and go find something else to do. the car is full of little field guides so weed, tree, flower, insect, and bird identification are all on the menu.
please forgive me if this was not helpful; how long do you keep your thermometer in the water?