jpender Senior Member Joined: 11 Jul 2011 Posts: 742 Location: California Expertise: I like coffee
Grinder: OE LIDO Vac Pot: S/S Moka Pot Drip: Aeropress
Posted Wed May 22, 2013, 10:36am Subject: Re: Water thermometer?
I notice you used 0C to test both Taylor thermometers against your precision thermister. Was that in an ice-water bath?
It seems that the variations of each were within 0.9C which is probably OK for coffee (and other cooking).
I wonder if those thermisters have the same error up around 100C which is where you would be doing a lot of testing for coffee.
I tested mine in boiling water and found a source of error in the gradient between the bottom and top of the boiling pot (currents?) and wonder if my ice-water bath had a similar gradient between the top and bottom.
The graph is for temperatures between 15 and 95°C. It shows the difference between the Taylor probe readings and the precision thermistor readings over the temperature range on the x-axis. So it appears that the two Taylor thermometers I have read low at lower temperatures and high at brewing temperatures. It's not a problem with the thermistors in these probes but rather the way that they are linearized.
I think in general NTC thermistors lose sensitivity as the temperature increases. For the thermistor I was using a change of 1°C results in about a 5% change in resistance at 0°C but only a 3% change at 100°C. The absolute resistance change also decreases with temperature. The R vs T curve is roughly logarithmic.
Testing with ice baths and boiling water is problematic. You really need finely crushed ice and an insulated container and you have to wait for it to equilibrate; otherwise it will be warmer than 0°C. For boiling water you need to know the barometric pressure. I wonder if the gradient you saw was really an error caused by a different reading due to how far you stick the long metal probe into the water.
Although the slow response is sometimes annoying I still think these are fine for coffee.
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