Everyone does things a bit different. I'm not one to purchase the most expensive thing and put my faith in that. Trial and error really is the simplest method. I like simple and using notes or writing on masking tape fixed to incubator my calibrations. With the oral thermometer I have it's spot on. I've a forced air unit so just stick it down the vent hole and turn it on. It automatically stores the high temp reading then I pace around the incubator waiting for the light to turn on to know the heater kicked in and start to turn on and off the thermometer (digital medical show highest reading) until I find the low. Average the two. It's been spot on for the three years I've been doing it this way. Some hatch day 20, most day 21 and few late early day 22 if I kept an average temp at 95.5 F. This $6 unit has proven time and time again to be spot on accurate. I keep a cigar humidor combo unit in the incubator as quick temp reference and humidity gauge. If that ever read different than 97 F then I know the temp is off. LOL. Yeah, had early hatch the first year I used only that and being so early was not a good hatch rate. Knew to try 98F next round then got the oral and have not looked back. Was unnerving to see the 97F on cigar unit the first time I used the oral but it proved true.
Temp is the singular most important thing for a good hatch. Turning the second and humidity third I think. You got to do some horrible things with humidity to destroy a hatch. Biggest mistake is to have higher humidity during first 18 days of incubation. The drier is much better than too moist. If you ever get a hygrometer preform a salt test on it to calibrate. Easy to do and takes all guess work out. Again, any cheap hygrometer will do if you salt test it for calibration. As for RH, I like 30% for incubating and 70% for hatch.
Milk or juice cap filled with salt, add drops of water until saturated. I pour off any standing water.
Put cap and hygrometer into a sealed container and leave on kitchen table. I like quart zip seal bags and provide small pillow of air when sealing.
Wait 4 hours or more and take your reading. Subtract your reading from 75 and that's your calibration #. Write it on masking tape and stick to incubator.
Ex: Your reading is 84%, 75-84= -9. You'll always subtract 9 from readings for true RH.
I'm trapped waiting for birch sap to be syrup. Have all sorts of time this morning, just run out every 30 minutes to add more concentrate to boil. Got another hour or so....lawd, it took 8 days to boil on 2x3 pan just to get to this stage. So ready to be done with it!