Controller calibration

[RogueDiscus]

I recently received some controller/heater sets from BRS The one I’ve set up is showing the temp to be 2-3 degrees F higher than my good quality laser temp gun (Milwaukee). I’ve never tried to calibrate that gun, although I did do the ice water test yesterday. It read 33 F. So if anything, the gun is reading already high, although I could understand if 32F was outside the calibration range. Anyway, what’s the best way to test the calibration of the controllers I just bought? Thanks.
Steve

[AquaticNerd]

It's a little pricey, but this is the thermometer I purchased: Thomas Traceable Long Stem Digital Thermometer (https://www.amazon.com/gp/product/B0069TUET8/)

It's NIST calibrated and comes with a calibration certificate. Item description says it's guaranteed for 1 degree C accuracy, but from the testing I've done and the certificate seems to indicate that it's accurate at .5 degrees F. BRS has the Hanna CheckTemp (https://www.bulkreefsupply.com/hi98509-checktemp-1-digital-thermometer-hanna-instruments.html) with similar specs. I have the Inkbird version of that temperature controller and it also seemed like it was a few degrees off as well.