Ive used the same digital thermometer for year an have had great hatches with many different home made incubators. So I assumed it was accurate. We all know what they all say about assuming things. Anyway, It hit me that I had I had a new $3 oral digital thermometer in the cabinet. I checked it to make sure it was accurate. It showed 97.9* in the front of my mouth an 98.4* in the back of my mouth, perfect. Then I took the oral thermometer an put it in the water weasel/wiggler/sock an adjusted the thermostat till the temp read 99.5* then looked at my main thermometer. It read 98.8*. Now I know that I need to keep my temp showing 98.8* on my thermometer. Now that's not that much of a change but being consistently off by .7* has to hurt hatch rates especially when ya start hatching more sensitive birds like duck an button quail. I mean if you had a fever of 99.3 for 3 weeks or more ya might not die but there's a good chance you would wish ya would. I"m about to try my first hatch with my new home made fridge-u-bator. (friends barred rocks) Being the best incubator I've ever built an having my temp calibrated hopefully I'll get my best hatch ever.