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| [GMCnet] Temperature Gauge Calibration [message #104083] |
Sun, 24 October 2010 21:36  |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
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As I reported, during the trip to GMCMI DuQuoin, IL my new Cad500's
temperatures were indicating higher than I liked -- H2O over 250*F on the
steepest grade. That emphasized to me the fact that I STILL hadn't
calibrated the instruments I installed with the new dash over a year ago.
So, over the past few days I've been boiling water, etc., to calibrate a
handful of temperature senders I had on hand, the 3 temperature gauges in
the dash, 3 spares, and the EFI monitor with its sender.
Basically, I used the mounting bracket shown here
http://www.gmcmhphotos.com/photos/showphoto.php?photo=4425&cat=3443
<http://www.gmcmhphotos.com/photos/showphoto.php?photo=4425&cat=3443>to
mount the senders in a can (SHE gave away that green pot so I had to use a
coffee can) full of crushed ice and water on an electric hot plate.
Starting at that known ~32*F temperature, I heated the water to boiling,
recording Ohmmeter readings as the digital thermometer indicated each 10*F
temperature rise.
With 6 senders mounted, the switch shown allowed me to read each of the
senders' resistance in fairly quick succession. FAIRLY. As it turns out,
even while heating the water quite slowly, by the time I read and recorded
and switched through 6 positions, the water temperature sometimes rose 10*F.
That introduced some experimental error which shows up dramatically in my
graphs.
After running two batches of 6 senders through the hot water exercise, I
turned to the gauges themselves: I have a 1% precision decade resistance
box which allows me to select any value from 90 to 99,990 Ohms in 10 Ohm
increments. With carefully applied jumper wires, I connected the decade box
to each of the dash gauges and the 3 spares. For each of them, I used the
decade resistor box switches to determine the resistance required to cause
the gauge's pointer to align with a gradation on the face, and recorded the
two numbers.
I did a similar exercise with the sender for and the display of the EBL
What's Up Display on my computer. That was especially easy because I could
just step through the decade resistances and read the digital temperature
from the monitor.
With those 4 sheets full of numbers, I opened a spreadsheet and transferred
all of the data. When plotted on a graph, my meter errors were immediately
obvious: Almost all of the exponential curves for the senders were
clustered together; without the mentioned recording errors, they'd have been
almost congruent. The curves for the meters were also very close, with one
exception of a bad meter. BUT, the curves for the meters, which would
ideally match those for the senders exactly, were well below (lower
resistance at a given temperature) the sender curves. Which meant that, on
average, when a meter read 190*F, the actual temperature recorded by the
sender was 215*F!
The EFI sender/WUD combination was, as expected, MUCH better; its accuracy
is on the order of 2% or better -- probably better than my measurements.
Now I've got to calibrate the gauges so that they read the higher values.
That's really not as difficult as it sounds; it's merely a matter of adding
series and parallel resistance to the connections. It's not possible to
correct readings over the entire possible range, but for the range of
interest, say 180 to 260*F, I should be able to achieve 5% accuracy; better
than we can expect from OEM gauges.
The arithmetic to calculate the resistance requirement is not difficult, but
I stumbled across a spreadsheet someone else had prepared that works very
well. It can be downloaded from : http://alteredz.com/gagecalibration.htm
A 420 Ohm combination of resistors across the terminals of each of my meters
should be sufficient to provide the accuracy I want.
Ken H.
Americus, GA
'76 X-Birchaven w/Cad500/Howell EFI+ & EBL
www.gmcwipersetc.com
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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