temp sender details

[rdssdi]

I have been attempting to determine why my Smiths electric temperature gauge reads 10 to 15 degrees C high. I replaced the voltage stabilizer. The gauge is a Smiths BT2240/00 "bimetal" gauge manufactured in 2000.

The power to the gauge is 10 volts as required and has a solid ground.

I was told by Nisonger, the U.S. distributor of Smiths gauges, how to check the gauge calibration. There are three sets of two dots at the bottom of the scale. Located at the top middle and bottom of the needle arc. The needle should point to these calibration points when 250ohms, 70ohms and 20ohms are placed inline in lieu of the sender. Using resistors to achieve these values I was able to determine the gauge was operating correctly.

These calibration marks coincide with the 50, 90 and 140 degree C readings.

It appears that the sender is at fault. Nisonger said that the senders available , EXCEPT theirs, have incorrect ohmic ranges. I considered that statement a bit of hyperbole in light of the fact that their sender is twice the cost. As I have few options at this point I ordered the sender and was then told it was out of stock and should ship in early December.

If and when this sender arrives I will test it and if the gauge continues to read high than I do not know what to do.

Bob

[elanman999]

Bob,
My +2 sender is 1.77 Kohm at 10 degC.
Cheers
John

[john.p.clegg]

..which is a long way off "250ohms, 70ohms and 20ohms"

John

[stugilmour]

I used my existing sender and gauge; assume they were original. Was also reading about 15 degrees high. Assumed gauge was at fault due to aging gauge movement and weak springiness in the bi-metal strip.

I resolved the issue by adding resistance in series with the sender wire. I don't recall the exact resistance added. I bought a $10 resistor selection from Radio Shack and varied the total series resistance value until the gauge matched my icing thermometer. The reference icing thermometer was immersed in a Clivey Boy thermostat housing right next to the gauge sender, so pretty accurate in my case but not easily repeatable with the stock setup.

Having the correct calibration values, if you are really stuck perhaps add resistance to get the mid range 90 degree value spot on with your sender. Perhaps not a perfect solution, but have found it works fine in the real world.

If you have to go this route, recall you can tailor the total series resistance by placing various resistors in parallel with one another. I found the Radio Shack resistors OK with two in parallel, but too light a wattage rating if only a single resistor was used (heated up). You may be able to get a higher wattage resistor in the correct value if you know what you are looking for. I placed the resistor pack in heat shrink with soldered spade connectors and fitted it on the sender terminal.

Here is a link I found with some resistance values. He is calling up a different gauge movement though. Anyway, he mentions the voltage readings that should be present at the three calibration points, which I assume are the same values for all of the Smiths gauge movements? Might help to calculate the resistance value that will get you accurate at the 90 degree calibration with your known 10v supply.

http://www.jensenhealey.com/forums/view_topic.php?id=14

HTH

Stu

[rdssdi]

I was certain to include the exact gauge description because I considered older and possibly current gauges would use different ohmic values.

That is part of the dilemma as one sender is recommended for most Smiths electric temperature gauges. I have been given many different part numbers by a variety of suppliers.

If the calibration information achieves the correct gauge reading then I am anxious to see if the sender will give more accurate information.

Bob

[Sea Ranch]

Perhaps I'm missing something, but . . .

In the absence of a CliveyBoy filler/swirl-pot thingy (which is a very cool bolt-on part, I confess), it would be relatively simple to find at least one known absolute calibration point: 212 degrees fahrenheit/100 degrees celsius - boiling point. Remove sensor/sender from boss in thermostat housing, place in pot and boil, read gauge and add resistance to correct. At the very least, we'd have that point on the scale nailed down, no?

Will be doing something like this in a couple years' time when I (hopefully) am closing in on completion of resto . . .

Randy

[stugilmour]

You have it right Randy. There were several guys here that checked either type of gauge & sender (electric or ether) that way.

When I was fiddling with mine I also used a voltmeter and a model train transformer. The train power pack has a variable DC voltage supply, so you can check the gauge movement easily using the specified Smiths supply voltage numbers and the calibration marks Bob mentions. Can also run 10v supply to the circuit with the sender in the boiling water. The power pack is easier to haul to the kitchen than the battery and voltage stabilizer.

Where are you at on the project now Randy? Hope to be down your way again next spring.

Stu

[Sea Ranch]

Ah, you suggest 10 volts because that's what runs through the instruments?

Good tip. Thanks, Stu! As always

I am a long way from that business Shell is stripped and on a rotisserie, pretty much all structural fiberglass repairs done, and 3/4 of the gelcoat cracks done. I'll send you a pix . . .

[bob_rich]

Hi Randy

Generally agree with all the good ideas in the post. As you seem to be some way off from needing a solution I would add the following suggestions. (1) For the 10V in the car use a modern semiconductor 10V series regulator 3 terminal chip. Rather than the bi-metalic regulator used originally--these can be very variable and can be affected by other gauges that may be fed from the 10V. (2) To check a gauge get hold of a 5W wire-wound variable resistor ( rheostat) with a resistance up to say 500Ohms. Wire this in series with the gauge and then for various setting record the resistance value using the resistance reading facility on a test meter. this will give the requirement for a sensor to be used with the particular gauge. (3) The test meter on ohms can also give a simple resistance v temperature reading by using boiling water then letting it slowly cool and read the temperature with a thermometer thus giving a temperature v ohms reading for a given sensor.

The resistor trick describe earlier works fine but generally will be spot on over a particular point. Make this around 90C where the engine should run normally then either side it may be wrong but will still give the required warning.

Hope this helps Best of luck

bob

[Sea Ranch]

Awesome, Bob. Thanks for the info and tips.

Truly, I look forward to running this test!!

Randy