How to Calibrate a Digital Thermometer with a Dry-Block Calibrator and with Freezing and Boiling Water

Video transcript:

This video demonstrates two methods for calibrating a digital thermometer: first, with ice and boiling water, and second, with our Fluke dry-block calibrators. Both methods are easy and fast. However, the water method has inaccuracies that are unknown caused by variations in altitude, atmospheric pressure, and water purity. The dry-block calibration method provides a better measurement because we’re working with instruments that have a known accuracy.

Let’s experiment with a couple different methods of calibration. We’re going to use the common methods here of ice water and boiling water. I’ve got everything set up here, and we’re going to do a quick check with these, compare against our dry wells, and see how we measure up.

I’ve got the unit under test sitting in an ice bath and right now you can see we’re measuring approximately 0.6 degrees, a little bit off from where we want to be. We’re going to make a quick adjustment to get this dialed in to our ice bath. After a quick adjustment, we’re right about zero degrees.

Now I’m going to move over and try this at the boiling point and see how we’re doing on the linear side going up. It looks like we’re bobbling around about 95 degrees and some change, so let’s make an adjustment on the top end.

You can see I’ve made the adjustment on the top end, getting it close to 100 degrees Celsius.

So now what I’d like to do is repeat this, but this time we’re going to use our 9100S and 9102S and see how we measure up…

Just a few seconds ago you saw me make the adjustments using the water method. Now you can see we’re a little bit out of spec again. Again, those common methods can net you some errors in the neighborhood of several degrees. The benefit of moving to a dry well is that these are calibrated, certified, and you know the uncertainty you are working with. Boiling water depends not only on elevation, but also barometric pressure. Ice water and building an ice bath, there is some technique to it, and the purity of the water will also make a difference.

So let’s make another adjustment to the thermometer, and see what our results are.

I’ve moved right over to the boiling point because that’s where we left off, and you can see right here from the boiling water, to the dry well, we’re almost four degrees and some change high. So you can see now I’ve made the high side adjustment, and again, we’ve pulled it back from the boiling water, and now I’ve got it down to 100 degrees Celsius. Next, we’re going to move to our zero point, and see how we measure up.

Alright, we’ve allowed a few minutes for the device under test to come down to zero and stabilize. We’re actually right on with what the dry well is reading. So again, we’ve had to make a slight adjustment of about four degrees at our top end versus the boiling water. At our low end, we’re pretty linear.

So we know at this point we’re within 0.25 degrees Celsius of our reading, which is well within a Type K uncertainty, and again, we’re confident even at the top end we’ve got an uncertainty of 0.25 degrees as well.

You can see the difference between using water versus a dry well. The big benefit is you’re working with a known uncertainty with dry wells, versus an unknown with water.