Errors in temperature and pressure measurements

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versalab
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Postby versalab » Oct 11, 2005, 4:02 pm

Anytime that anything is measured, there are errors, to some degree, in that measurement.

Now that some people have become very interested in various measurements regarding espresso extraction, I thought it was time to present what these measurement errors can amount to. This will enable experimenters to understand why their numbers don't match.

Let's look at temperature first, as it is the most popular. [For reference a degree F = 1.8x a degree C.]

The first measurement limitation is the measuring instrument. Below are three typical cases. In all cases, unless otherwise noted, I am providing the total possible error band - not plus or minus.

Keithley 2700 Multimeter and Data Acquisition System -
the most accurate device for reasonable money ~ $1500
accuracy with type E or T thermocouple = .36 degree F

CN77000 PID controller ~ $230
typical of high accuracy controllers
accuracy with thermocouple .7 degree F

DVM typical digital volt meter less than $200
accuracy with thermocouples stated as ± 1% which equals 3.96 degrees F @ 198 F

Next to consider are thermocouples themselves.

Conventional thermocouples
Standard limits E = 3˚F T = 1.8˚F J or K = 4˚F

Better thermocouples use "Special limits of error" wire
Accuracy E type 1.8 degree F
Accuracy T type .9 degree F

The combination of the instrument and thermocouple errors are as follows.

Keithley 2700 with E type (special limits) = within 2.16 degrees F
with T type (special limits) = within 1.26 degrees F

High quality PID controller
with E type (special limits) = within 2.5 degrees F
with T type (special limits) = within 1.6 degrees F

DVM typical with E type standard = within 6.96 degrees F
with T type standard = within 5.76 degrees F
with J or K type standard = within 7.96 degrees F


I particularly mention the DVM with J or K type thermocouples because these are typical inexpensive systems such as most people will start out with.


Calibration
It is possible to get thermocouples by themselves, and even together with the measuring instrument, calibrated. The limits of this calibration are typically .7 degree F total.



Now we come to pressure measuring. This is yet to be a big issue but when people finally get the chance to adjust the extraction pressure in fine degrees it will become a big deal.

A typical gauge that might be fitted to a high quality espresso machine has an accuracy of ± 2%. At 9.2 bar this equates to an error of about ± .2 bar, or a range of .4 bar.

We know routinely that we can taste the difference of .05 bar.

In a machine such as the M3, which uses a calibrated pressure transducer (a thing that turns pressure into electricity) and an analog to digital converter to tell the computer what the pressure is in digital bits, there are errors here as well. There doesn't seem much reason to go into all those errors yet.

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malachi

Postby malachi » Oct 11, 2005, 4:11 pm

As noted previously, a good argument for very large data sets.
"Taste is the only morality." -- John Ruskin

lennoncs

Postby lennoncs » Oct 11, 2005, 4:26 pm

Poor measurement practices are another whole can of worms.

sean

DavidMLewis

Postby DavidMLewis » Oct 11, 2005, 6:02 pm

Hi John,

Nice to see you posting here. I had one question about your notes, which by the way, I greatly appreciate. The question is: what is the repeatability of the various methods, as opposed to the absolute accuracy? While the latter is clearly important if you're developing a machine for production, if you're an individual user closing the loop with your taste buds, which covers most of us, it seems like the former is more crucial.

Best,
David

versalab
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Postby versalab » replying to DavidMLewis » Oct 11, 2005, 7:18 pm

Usually repeatability is not worth worrying about. There are many tiny errors that are below what I have sketched out here, but we have yet to find them important. On the other hand, if you are using an inexpensive DVM it would be worth contacting the manufacturer about repeatability - particularly temperature coefficient (changes in gain due to warmup internally and also ambient - room temp.). Anyone has to cut corners on inexpensive equipment, and the input amplifiers on the DVM could have thermal sensitivity.

Accuracy is an issue when someone says that this blend needs 203.5 and 8.5 bar. How was it measured? With what overall accuracy? For instance, we have finally realised that we have about a 1 degree difference in temperature reading between our M3 machine and that at Kaladi in Denver. Makes a big difference what each of us says about a particular nuance change in a roast.

Then there are issues about what is being measured with a particular technique. This will also foil the individual.

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HB
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Postby HB » Oct 11, 2005, 9:07 pm

John, good to see you on HB, I feel like I already know you a bit from your interview with Abe. :)

David brings up a good point about repeatability. I'm pretty slack about calibration and tend to work in terms of x+1.5F or x-2.0F from a given setpoint rather than worrying about accuracy compared to "true" standard because of the problems you noted. That prevents me from comparing precise temperatures with others, but then again, I'm skeptical that's a realistic expectation unless the two comparison machines are identical in design.

(Add to that the influence of Abe, Chris, Steve, Peter and others who have persuaded me to adopt a more holistic approach to espresso than my natural tendency to break out instrumentation at the drop of a pin. Go figure).

John wrote:We know routinely that we can taste the difference of .05 bar.

You mentioned a similar point in your interview. I'm curious whether this is a perceptible difference that your everyday taster could discern and appreciate, or does this require a more refined palate?

Conventional wisdom has it that even 10x the difference you cite isn't worth discussing, but all the talk of the relationship between clarity and pressure delivery has encouraged me to rethink it. In fact, two of the early posts on this board are on this subject (Received wisdom about brew pressure and The Next Breakthrough in Espresso Technology) and it's come up several times in different forms (e.g., The Elusive Clarity in the Cup and The Air Pump Driven Espresso Machine). Of course I wouldn't be the first to suggest that uber-precise temperature control is passe and pressure technology is the new frontier.
Dan Kehn

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AndyS

Postby AndyS » Oct 11, 2005, 10:55 pm

HB wrote: Of course I wouldn't be the first to suggest that uber-precise temperature control is passe and pressure technology is the new frontier.


Just like I wasn't the first to suggest that espresso tastes better when pulled in Celsius and bars rather than Fahrenheit and PSI.
-AndyS
VST refractometer/filter basket beta tester, no financial interest in the company

swines

Postby swines » Oct 12, 2005, 11:42 am

Absolute accuracy is not necessary if the device can be proven to repeat a measurement. For example, if you found that your thermocouple + meter showed +2 degrees higher temperature (when compared to a calibrated thermometer) everytime you took a measurement you'd feel confident that your measurement was accurate each time you used the instrument. You would know that the temperature was actually 2 degrees less than what the instrument was showing and would apply this offset to the measurement.

Conversely, if you tested the setup against a calibrated thermometer and found that the temperature reading varied each time you took a reading, then the variability renders the readings useless if absolute accuracy is needed.

I had a US Army research program where we ran into this exact problem using an array of thermocouples used to measure soil temperature across a defined area and at various depths over the area. We ended up using a NIST traceable process thermometer to find each thermocouple's temperature offset over the temperature range within which we needed to make the measurements. Once the offsets were know and verified as being repeatable for each thermocouple, we could generate the data sets knowing the offset required for each measurement.

versalab
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Postby versalab » Oct 12, 2005, 1:40 pm

HB wrote:[quote="John"]We know routinely that we can taste the difference of .05 bar.

You mentioned a similar point in your interview. I'm curious whether this is a perceptible difference that your everyday taster could discern and appreciate, or does this require a more refined palate?[/quote]

Let me preface the following with... By way of being in dialogs in Home Barista I must talk about my own experiences. The possible problem is that these experiences are with equipment that we have for sale. And therefore anything I say may be thought to be advertising. I hope that I don't transgress here...

The set up was: the M3 system at the Denver Nascore show last month. We encouraged every serious visitor to have two shots. We varied the pressure .05 bar between them. Since temperature is a much more time consuming adjustment, I thought the rather instant availability of pressure adjustment might cause a stir. Every person, several editors of magazines, local roasters, even a young woman (there with her boyfriend) with no real interest in coffee, all immediately aware of the difference. Our friends who visit us here taste it. So I don't think it takes an extraordinary palate.

It is our feeling that these are not minor flavor changes. We have learned, just this summer, about just how sensitive this process is. We were getting lots of variations in the cup that we thought was the natural result of the variability in the beans from one shot to the next or one day to the next. What it was was the small variations in brew parameters that we didn't imagine would affect the flavor, and that we therefore paid no attention to from shot to shot or day to day. Now we have serious repeatability.

We are not imagining things - nor exaggerating - when we say that the flavor change of .05 bar on the M3, 100 rpm grind speed on the M3 grinder, or 1-2 degree change on the Sivetz roast temperature on one of three beans in a blend - is a very serious flavor change. Any single one of these can make or break the flavor in a cup. And any one of these is repeatable.


I am quite hopeful about Home Barista. Glad to be here. Earlier this year I was very concerned that the entire web presence for espresso was in unfortunate circumstances - that is now changed very much for the better.

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HB
Admin

Postby HB » Oct 12, 2005, 8:31 pm

John wrote:The possible problem is that these experiences are with equipment that we have for sale. And therefore anything I say may be thought to be advertising. I hope that I don't transgress here...

Not to worry, that hasn't been a problem to-date because the participating sponsors / professionals understand the difference between sharing information and promoting their products. While there are no formally published rules for the board yet, Team HB has collectively created a mission statement and guidelines in the spirit of moderators as your hosts and discussion facilitators, not enforcers. Since moderatoring in the common use of the word hasn't been necessary (*knock on wood*), I've seen no need to post rules.

It is our feeling that these are not minor flavor changes. We have learned, just this summer, about just how sensitive this process is. We were getting lots of variations in the cup that we thought was the natural result of the variability in the beans from one shot to the next or one day to the next. What it was was the small variations in brew parameters that we didn't imagine would affect the flavor, and that we therefore paid no attention to from shot to shot or day to day. Now we have serious repeatability.

We are not imagining things - nor exaggerating - when we say that the flavor change of .05 bar on the M3, 100 rpm grind speed on the M3 grinder, or 1-2 degree change on the Sivetz roast temperature on one of three beans in a blend - is a very serious flavor change. Any single one of these can make or break the flavor in a cup. And any one of these is repeatable.

I consider myself an average espresso lover and thus assume such minute subtleties would either be beyond my abilities to detect or acceptable variance. To put it in basic terms by way of example... I like red grapes more than white grapes, but I'll happily eat either for lunch. But if I'm really in the mood for grapes and we only have bananas in the house, well, I'm disappointed. I am struggling to grasp if the differences between such minute pressure changes would mean nicely ripe bananas, hard green bananas, or simply red grapes / white grapes. I don't mind a little serendipity in my espresso as long as it's within an acceptable range. In contrast, based on the writings of some professionals, I imagine them flying into a rage if the temperature is off by 0.4F from the previous extraction. That's not me. ;-)

More seriously and directly to your point: Is the tight control you describe within .05 bar of a fixed pressure, or do you intentionally vary pressure through the extraction to produce a "pressure profile"? Jim Schulman postulated on the positive effects in Received wisdom about brew pressure (excerpted below) and I'm wondering if his findings are consistent with your own.

another_jim wrote:There's two disputable assertions on lever groups - that they have greater clarity and less crema, and that the reason is lack of vibration.

The first is an oft repeated observation -- I get it reliably and invariably when I set the La Peppina and Tea side by side, Mark mentions it in the Elektra review. Almost all owners of lever machines, home or commercial attest to it. Obviously, it could be wrong, but it's about as solid as any other fact in this game.

The reason for it being lack of vibration is mostly unresearched. I was the one who raised the pressure profile as an altrnative possibility; and, so far, am the only person I know of who's tried it. Most people just don't have big variacs lying around into which they can plug their machines or pumps. of course, even modified the E61's profile has the 10 second ramp up, so that's still a point of difference to the typical lever having a no pressure presoak for around the same length of time.

...

Thinking about this. I guess there's a lot of received wisdom about pressure, most of it virtually untested, and perhaps wrong. The knowledge about temperature variations is hugely more reliable, since adjusting temps is a part of lots of people's daily shot making practice. Adjusting pressure is usually done for maintenance or gross defects, not taste adjustments. So there's several orders of magnitude less data on it.
Dan Kehn