Glass Thermometer Variances and Alternatives

[Coastie Reefer]

So I have 6 glass thermometers (all same brand) that I use in multiple tanks./sumps. 3 I've had for a while and 3 I bought a couple weeks ago. The older ones ready roughly 79/80 and the newer ones run about 81/82. I like to run my temp around 78 and have been trying to adjust my temps down, but I honestly don't know which one to trust.

Anyone have recommendations for a accurate and reliable thermometer?

 

[Kristophf]

I have 8 and all my thermometers glass or digital read different. I just have 2 or 3 in a tank and find a medium