Glass bed temperatures

If you use a piece of glass on the heated bed of your 3D printer, how does the temperature measured by the thermistor compare to the temperature on the surface of the bed? The thermistor is underneath the bed, close to or in contact with the heater, and glass is a poor conductor of heat. So how do they compare?

I set up an experiment where I put a glass plate on a MK3 heatbed, and measured the temperature using the thermistor in the middle of the MK3 and a second thermistor taped to the top of the glass. This is not a very scientific test, but it gives a rough idea. It's a mistake to treat the values from the two thermistors as strictly comparable, as they they are not very precisely calibrated, but looking at the difference between them provides some insights. For the experiment, I started with the heater off and the bed cool and then raised the temperature 10C at a time from 30C to 80C, waited for the values to stabilize, read the temperatures and took their difference. I did the experiment on three different kinds of glass: window glass, thickness 2,2mm; mirror glass with a silvered backing, thickness 2.9mm; and borosilicate glass, thickness 3.1mm. The values are stable to within about +/-0.2C. In addition, for one reading on each material, I estimated how long it takes for the temperature on top to settle after the temperature on the bottom has done.

Here's the results. A delta of 1.9 means the reading on top was 1.9C below the reading underneath, so for example 28.1 if the reading underneath was 30.2.

Set temp	Delta (window glass)	Delta (mirror glass)	Delta (borosilicate)
30	2.1	2.5	2.3
40	3.7	5.3	4.0
50	6.2	8.4	6.5
60	7.8	11	8.1
70	11	13.7	11
80	13	17	13

I estimated the settling time at the 40C reading, and it was around 30s in each test, perhaps slightly more for borosilicate.

Some things to note. The delta gets larger at higher temperatures. The mirror glass delta gets larger at a greater rate than the window glass delta. This is important, as the deltas are not strictly comparable due to exactly how the top thermistor was attached to the glass. This wasn't apparent for borosilicate (and I don't know what happened with the last reading). To make the point, here are the deltas for mirror glass and borosilicate minus the delta for window glass:

Set temp	Delta (mirror glass-window glass)	Delta (borosilicate-window glass)
30	0.4	0.2
40	1.6	0.3
50	2.2	0.3
60	2.2	0.4
70	2.7	0.2
13	3.3	-0.6

What does this mean? First, don't trust absolute temperatures. For example, if someone says print on a bed at 60C, figure out what this means for your printer, as you might have a different bed configuration. This shouldn't be news to most people.

Second, if you are switching between different bed types, you probably don't need to worry about changing the temperature if you are printing at 50C (my usual value). It's only a couple of degrees different between the different types. But at higher temperatures, such as 80C and above, you might need to do some adjustment.

Thirdly, consider inserting a delay in your start gcode after the bed reached temperature and before you start printing. If you are heating the bed first and then the hotend, you probably have enough delay already.

You should treat all this with a healthy dose of skepticism. I was really interested in just one question: what should I expect when switching between the piece of glass that I have? It's not a detailed study or guidance on the kind of glass you should choose.