Calibrate K-type thermocouple for DIY thermofilter

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#1: Post by jb-0101 »

Hey all,

Trying to get as accurate a read on my coffee machines brew temp. I have an Alex Duetto which I was told was scace calibrated when purchased - 9 years ago! I have also borrowed an 8 year old Breville dual boiler to compare.

The consistency of each machine is actually extremely good, with the Alex possibly being a touch better. (I haven't logged results and ensured the same exact procedure for both to ensure this statement is representative, it's just the impression I'm getting from running about an hour of roughly similar tests. The Alex, given a couple of minutes to stabilize, a flush, and then brewing when the PID comes to the temp I want, will hit the right temp quicker and stay there for 150- 200ml of flow through a pressurized basket. The BDB is similar, but usually holds a degree under, then after about 15 seconds climbs to the desired temp and holds for 150-200ml).

The above was a side point, the point of this post is about calibration of the thermocouple. So my Alex will bullseye the programmed temp on the thermocouple - 94 on the PID climbs quickly to 94.0 on the thermocouple, almost every time, and holds steady. There's little to no variance following the above procedure. The BDB, set to 94, goes to 92, holds for for 10ish seconds, then climbs to 93.0 (+\- .3) and holds steady. It's the same story for both with the PID set to different temps, from 88 up to 96.

I thought the Alex was just therefore better, but recall people saying the BDB is a scace killer, so thought I'd calibrate in a pot of boiling water. The thermocouple is around 99.0ish in boiling water (I'm 150m above sea level). So... does that mean the BDB is actually pretty bang on as my thermocouple is reading about 1C low? And therefore I need to drop the offset of me Alex down by 1C? Or is it normal for a TC to read 99 and the Alex is right as is?


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#2: Post by Jeff »

Reading a thermocouple to 1-2° C is pretty good. I think the as-delivered calibration of a typical Type K is around that.

As I recall from EricS years ago, you need to calibrate in saturated steam, not boiling water. If you think about it, boiling water would often be a bit below the boiling point.

Absolute accuracy, to me, isn't very valuable in espresso, unless you're calibrating all the machines in your five cafes to be the same. Even then, if they're all saying "96" but running 94.2, I don't think it often matters. Repeatability, yes, but accuracy?

jb-0101 (original poster)

#3: Post by jb-0101 (original poster) »

Thanks Jeff, yeah I wasn't sure exactly how precise the number would be, but it was very consistent which was good. But which machine do you think was more representative then? Because essentially for 150-200ml over about 1 min, the Alex Duetto, at 94 on the PID, would consistently show 94 on the thermocouple. Whereas the BDB would show the same consistency, but always 1C on the thermocouple below the PID setting?

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#4: Post by Jeff »

Do you have a US nickel?

Not only are they useful for estimating headspace, but in making decisions like this.

If it were me, I'd either leave them alone (most likely) or tweak the one that was easier to adjust to match the other. Even if they read the same, I don't know that you'd be pulling the same shot on both machines. My E61 has a baseline number of 201, my DE1 runs around 88. I have a feeling that +/- 2C might make a change in flavor around either of those baselines. Even if they were the same number, +2C would probably change the flavor differently on the two machines. There are even coffees that taste better to me on the warmer side on one machine and the cooler on the other.


#5: Post by vit »

Water boils at 99.5°C at 150m above sea level, plus or minus about 1° depending on the air pressure ...

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#6: Post by cafeIKE »

Jeff is correct, you want the TC in the steam of distilled water.

Boiling Point at Altitude Calculator compensates for altitude and pressure. I mean, really, let's be accurate :wink:

I agree with Jeff that other factors swamp absolute temperature accuracy. Dead nuts temperature and pressure control are swamped by bean and hand vagaries.


#7: Post by luvmy40 »

It's easier, and more reliable to calibrate your TC reader in an ice slurry, rather than steam or boiling water as there is no compensation for altitude or barometric pressure.

Completely fill a glass with CRUSHED ice and then fill to the top with water. Give it 5 minutes with the TC in the middle of the ice mass. Calibrate to 0.0 deg. C or 32.0 F.

Type K thermocouple is pretty darn accurate to .1 deg. C, depending on the resolution of your converter/reader. It's also quite responsive.