cdrikari wrote:Wanted to say first that I think the Scace Device is really a cool idea and I'm happy that he came up with it. Being able to consistently measure the temperature across machines and adjustments (even if there appears to be a lot of debate on what it might
mean) is awesome.
I did have a couple of questions about the accuracy that is being discussed though. Pardon me while I work through this.
I don't have any idea what thermocouple probe Greg is using other than Type T, so used the assumption that he's using high-tolerance T thermocouples with a tolerance of +/-.4%
Assume that you would plug that into something like a Fluke 50 Series which for temperatures <100C has an accuracy of +/- [.5% +.3C]. (Which I think most people would. Decent price for pretty decent accuracy.)
If we assume that water at 96C is being felt by the probe in the device, that would mean that the measured temperature would be 96 +/- 1.18C (.004*96 + 0.3 + 0.5) or 94.82-97.18C (202.7-206.9F).
So the numbers may not mean what people want them to. (For instance, if I got a Scace Device, went home and was desperately trying to get Schomer's '203.5F' I might succeed at that at the expense of having a nice temperature, but lousy espresso.) In fact, the original 203.5 really only means that Schomer's original rig read that when he got great espresso.
The point being, that me getting 96C on my device and you getting 96C on yours doesn't mean anything more distinct that we know that we are within 2.36C of each other. (And don't get me wrong, 2.36C is pretty darn good.) But people seem to be assuming a much tighter margin of error.
Which I think means I'm really missing something.

Help?
-Jesse
Hi there:
Here's the late response because I don't always go through all of the forums in hb, although I'm supposed to do this one at least...
Uncertainty in thermocouples doesn't actually follow a neat percentage of the temperature. I use Type T thermocouples because they are particularly good at boiling water temperature. This causes some headaches for folks who would rather buy a cheap type K readout device, but if you want accuracy you gotta do the type T program. There's an ASTM term called "Special Limits of Error Class 1", which refers to wire with enhanced accuracy meeting their spec. I use probes that meet this spec, and they have an uncertainty in temperature of 0.9 Deg. F.
Fluke publishes an uncertainty spec of 0.6 deg. F for their series 5X thermocouple readouts. This uncertainty fits suitably with the probe uncertainty, as you will see.
Imagine that you buy a bunch of probes - maybe a couple thousand, and you make it your life's work to calibrate all of these damn probes, plotting the results on a graph. You'll find that most of them cluster around the temperature you'd expect, with less and less of them further away. About 95% of them will be within the accuracy spec of 0.9 degrees and a few will be outside of that. If you plot the number of thermometers that deviate by a certain amount away from the expected value, against the deviation amount you'll be likely to come up with that bell curve thing.
The same deal probably applies to the readout device as well in that most of them are really close and a few are not.
The two uncertainties are "uncorrelated" in that the uncertainty of the thermometer doesn't have anything to do with the uncertainty of the readout. They are two different devices and you might have two perfectly accurate ones, or you might have a probe that reads high and a readout that reads low, or any combination of things you can think of that partially cancel or don't, or whatnot.
Since they are uncorrelated, you can calculate the combined uncertainty of the system of thermometer and readout by calculating the RSS (root sum squared) value of the two uncertainty components. Here's how.. Square the uncertainty of the probe (0.9^2 = 0.81) and the uncertainty of the readout (0.6^2 = 0.36). Add them together and then take the square root (0.36 + 0.81 = 1.17, and 1.17^0.5 = 1.08). Rounding 1.08 to the nearest tenth of a degree gives 1.1 degrees, which is the combined uncertainty of the probe and readout in degrees F. That's about 0.6 Deg. C for you SI junkies.
I mentioned that the Fluke is pretty well suited to the probe in terms of accuracy spec. Notice that the squared values of probe and readout uncertainty differ fractionally more (.81 is more than twice .36) than their non-squared values (.9 and .6). If you used a readout device that had an accuracy spec of say 0.2 degrees, the squared value would be 0.04, and the combined squared values of probe and readout would be 0.85. The combined system uncertainty would be the square root of .85, which is .92. You'd spend a boatload of money to buy a readout with 0.2 degree accuracy and you'd only gain 0.2 degrees of accuracy in the system - .9 vs. 1.1 degree.
It's useful to differentiate between precision and accuracy at this point. The Fluke meter has a reproducibility spec of 0.1 degrees, and the reproducibility of the probe is prolly on the order of 0.01 degrees. So if the same probe and meter are used to measure the same constant temperature process, the repeated measurements will be within 0.1 degree. That means that if you use the same probe and readout you can transfer machine settings to the tenth of a degree level.
On the other hand, suppose you mention to your friend that you like coffee A at 200 degrees and he sets his machine up similarly using a separate Scace and readout. Now you have two uncorrelated systems with system uncertainties of 1.1 degrees F each. You handle them just as before and calculate the RSS value of the pair. (1.1^2 + 1.1^2)^0.5 = 1.55, which rounds off to 1.6 degrees, which is the uncertainty value for the brew temperature of the second machine, compared to 1.1 degrees for the first machines temperature uncertainty.
These uncertainty numbers are entirely reasonable at this stage of the game in espresso. There are very few espresso machines that have one-degree reproducibility. The uncertainty figures for the probe and readout of a single system (or pair of uncorrelated systems for that matter) are about the same size. We do know that if you have a very reproducible machine and very good technique, you can discern the taste difference between pairs of shots brewed at a half degree or so different. Note that I said discern. I didn't say that one will suck and one won't. You can just taste a difference. In this instance, using a multiple group machine with independent temperature control of brew boilers, it's likely that you will be using the same probe and readout device, so the reproducibility spec is entirely adequate.
Now for calibration - Thermocouples are used in Scace devices because they usually work or they just plain don't. The operating principle of thermocouples is pretty fundamental and generally no calibration is needed, although I've learned of a few bizarre modes of failure since I've been making these things.
You can calibrate the system as Eric S demonstrated. I posted how to do it somewhere on this site in the past, but basically the deal is to invert the Scace, and suspend it over furiously boiling distilled water that is boiling in a tall pan, like a huge steam pitcher, with the level of the boiling water maybe half way up the huge steam pitcher. You knew there had to be a legitimate reason for those huge 2 liter pitchers that get used in shitty espresso shops, right? YOU HAVE TO USE DISTILLED WATER!!! You want the Scace to be pretty close to the boiling water so that it is suspended in the steam above the surface. Here's the neat part. Steam, which is water vapor, contains a shitload of heat because it's in the vapor phase. When it changes back into the liquid phase it gives off most of that heat, which is why steam burns are so destructive. In our calibration vessel the released heat from condensation keeps the region just above the boiling water at very constant temperature. So you can get a pretty damn good calibration answer - with uncertainty of a few hundredths of a degree. Once you get the temperature reading you have to correct the boiling temperature for atmospheric pressure. 212 Deg. F is the boiling point at seal level at standard barometric pressure ( 14.696 psi). The quick and dirty way to get local atmospheric pressure is to assume that your local airport is at the same pressure. This may or may not be right, but it's prolly good enuff unless you are living on your private mountain and the nearest airport is in da valley somewhere. Boiling point corrections are easily found on the internet using your favorite search engine.
An interesting result from calibration is that the system of probe and readout are now highly correlated in that only that probe and readout will produce the calibration result you obtained. There's no way to know which of the devices - probe or readout - contributed most to the difference between measured value and actual value, but we dont' really care anyway.
You can now look at the difference between the real boiling point and measurement, and apply the difference as a correction to your measurements, giving you a high degree of accuracy. If you calibrate multiple systems using the method outlined here, the uncertainty propagated by using independent measurement systems goes way down, allowing you to transfer temperature settings to the tenth of a degree level if you really wanna.
-Greg