I was at the very end of my last brew day and cleaning up all of my equipment when I shattered my floating glass thermometer. This is the second one I have shattered in 3 years and I always worry about breaking it off in a mash or in a kettle. I was also a little suspect of the accuracy.
Besides the floating glass, I also own a CDN ProAccurate Digital Thermometer. Though I love how quick it gives you reading vs. the glass, I was always suspect of the readings because of the need to constantly recalibrate and sometimes its differences from the glass thermometer (Which one was wrong? Who knows).
I have become tired of the fear of accuracy as well as breaking the glass so I ponied up ($96) and bought a Thermapen. Everyone that has one raves about them so I figured why not. Even though they are supposed to be one of, if not the most accurate thermometer on the market, I ran my own test to verify. While I tested the Thermapen, I decided to do a side by side with the CDN to see if it was also accurate. Thought it would have been nice to test a glass thermometer as well that was not an option.
I made the ice bath just as the Thermapen directions suggested. The estimated temperature of the solution should be very close to 32. The Thermapen gave me a reading of 32.3 in a few seconds. Very happy about that. I put the CDN into the same solution.
Now I knew I'd have to calibrate the CDN for best results and temperature must be in the 32-34 range to do so. Well when I put the CDN into the ice bath, it read 29.6. If you try to calibrate at the temperature you get an error. So right away I was concerned with the accuracy. That almost 3 degrees could make a big difference in an hour mash.
I also had some water boiling. Based on my elevation and air pressure at the time of the test, water would boil at 210.6 degrees. I put the Thermapen into the boiling solution and it read 210.5 degrees. After both test I was now a Thermapen believer! I put the CDN into the boiling water and it read 208. Again, was around 3 degrees off in the same direction.
I then put the Thermapen and CDN back into the ice bath. The Thermapen again read 32.3. The CDN now read 32. That bugged me that it read a different temperature in the same medium. However, I'd now be able to calibrate the CDN. So I did and it read 32.1, very close to the Thermapen and expected temperature of the solution.
The CDN and Thermapen went back into the boiling water. The Thermapen again read 210.5 and the CDN also read 210.5. Interesting results.
So what this tells me in this very limited test is that the Thermapen and the CDN are very close to each other as long as the CDN was calibrated. However, as I said the first time, in the ideal ice bath solution I could not get the CDN to calibrate.
Though the Thermapen is around 5x the price I think over the long term I'd put my faith in it over the CDN. However, if you don't have around $100 laying around the CDN is a viable option as long as you can calibrate it. That's the key!