Calibration of a group thermometer

[JimG]

This is to share a simple observation regarding a very accurate means of calibrating a group head sensor and thermometer (see Eric's original thread here). I have checked the method extensively on a Silvia, but it should work on any single boiler machine with one of Eric's adapters installed.

1. With the portafilter and empty basket in place, run a little water through the group - just enough so that there is some free water in the group plumbing.
2. Turn on the steam switch.
3. Watch the temperature display for your group sensor. It will steadily rise with the steam switch on.
4. When the temperature in the group reaches your local boiling point (212F at sea level, lower with altitude and low barimetric pressure), you will see a temperature plateau.
5. The temperature will remain very steady for 30 seconds to 2 minutes, and you will probably hear a quiet "pssst" sound coming from the group.
6. The plateau temperature will be nearly exactly equal to the boiling point of water at your locale.
7. Once the water in the group plumbing evaporates, the temperature will start to steadily climb again.

If you hunt around online you should be able to find a BP calculator that permits you to enter your elevation and sea-level-adjusted barimetric pressure from a local weather outlet.

With distilled water, this method will be nearly exact. But there will be a little error associated with impurities in your water.

(Don't forget to turn off the steam switch).

Jim

[AndyS]

Cool, thanks for sharing.

[Jeepin' Geo]

Thanks Jim,

I'll give this a try.

Here are a few links I found;

Your local pressure can be found here:
http://www.weather.com/

Your city elevation can be found here:
http://geonames.usgs.gov/pls/gnis...014028675542937098

(type in your city for Feature Name, State and County then Send Query)

Boiling Point Calculator link(s):
http://www.csgnetwork.com/h2oboilcalc.html
http://www.thermoworks.com/software/bpcalc.html

George

[cafeIKE]

Your city elevation can be found here:
http://geonames.usgs.gov/pls/gnis...014028675542937098

You may be able to get your exact elevation on the Terra Server.

[Jeepin' Geo]

You may be able to get your exact elevation on the Terra Server.

Ian,

That's a neat site. I was able to find my exact house coordinates and could even see my Jeep in the driveway, but I couldn't figure out how to get the elevation. They mention DEM, Digital Elevation Models but I didn't see how to get them.

George

[JimG]

This graph shows how pronounced the temperature plateau is on my bench Silvia. Local BP here is within a few tenths of 211F. So this tells me my digital thermometer is still very well calibrated.

Jim

[cafeIKE]

Ian,

That's a neat site. I was able to find my exact house coordinates and could even see my Jeep in the driveway, but I couldn't figure out how to get the elevation. They mention DEM, Digital Elevation Models but I didn't see how to get them.

George

"Topo Map" link at the top right of the image window

[erics]

Jim, this is a very nice topic and I would be remiss for not including E-61 data.

This particular graph below shows the temperature of the thermosyphon piping (10 mm OD copper on Anita) immediately as it enters/exits the group. Data was taken using stick-on Type T thermocouples and an Omega HH506RA at 10 second intervals.

The next image is simply a "magnified" view of the 32-42 minute time period, detailing the plateau. The "ripples" in the upper graph represent the normal pstat cycling.

It indicates about a 0.8 degree error in the thermocouple(s) and/or the meter

Edit - I added an Excel file to my FTP site which allows for boiling point look-up for a large range of altitudes and barometric pressures: http://users.rcn.com/erics/

[cafeIKE]

Boiling Point Calculator