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.