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ijonsson

Water temperature sender and gauge

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How can I make the temperature gauge show correct reading? I have a new "classic" electric Smiths gauge in my TR2, I've tried several senders and it always shows 20 degrees C to high reading, I have a voltage stabilizer that gives a little less than 10 V. I connected another 12V temperature gauge to the sender and checked with an infrared thermometer and both show same value, 20 deg C below the Smiths gauge reading. I put 4 senders in a pan with hot  water and made readings at 4 different temperatures and I am using the one marked 8730 with the highest resistance. Would a resistance in series with the sender help, and has anyone got an idea of the size of such a resistance? Can the gauge be calibrated?

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Adding a resistor in series can put the reading right at one point on the curve, but since the curve isn't linear it will still not read correctly elsewhere.

If the gauge is reading 100C when the temp is actually 80C then from the sender curves a 100 Ohm resistor might be approximately right so the sender total is 200 Ohms. However at 50C you are only going from 300 to 400 Ohms so the gauge might still read 10C high at that point.  Also, above 80 the gauge will under-read which is probably more of a problem. 

Better to adjust the gauge if you can.  This shows how to adjust the standard instrument on a TR4 -  perhaps yours is similar? 

 

1198594551_CalibratingSmithselectricTR4Tempgauge-PeterSherman.thumb.jpg.07f2743face55a4a4bb1a0c1917153a2.jpg

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If the gauge, sender and voltage regulator are all as supplied and new, then I'd be inclined to send it all back to Smiths - 20deg out is way beyond acceptable tolerance. As Rob mentions, the sender is a non-linear thermistor and adding a series resistor will have a gauge calibration effect.

Personally, I'm quite happy with the old capillary gauge, the only issue being the sensor's enthusiasm to remain part of the aluminium stat housing!

Rob, the excellent adjustment text you posted may be useful to more members if published in TRA, did you create this just for Inge?

 

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Not mine Mike, it was written by a chap called Peter Sherman and came from this link. I guess I should have said that in my post. 

https://drive.google.com/drive/folders/0B2H2NJt34OffMDVmOTZlZWItYTcyMy00ZDhkLTliOWEtOWU1OWRhNDZhYmRl

I would normally post just a link to documents like that rather than copy-and paste it but this is a collection of documents and you don't seem to be able to point to just one of them.

 

Edited by RobH

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For those who think in Celsius, the temperatures mentioned are shown here

185 = 85C

212 = 100C

TT

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on this subject, my gauge dose not move.[ electric ]  If i earth the wire from the sender direct it moves right up so the gauge seems ok . Tested the sender and tried a replacement all good but no movement of the needle .

The fuel gauge works ok but could it be the stabiliser 

Roy

Edited by roy53

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If the fuel gauge is OK then the stabiliser is probably ok

How did you test the sender ?

Bob.

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with a multi meter and also used a cup of boiling water to check reading changing.  Cannot remember for sure but think it was cold switch 800 reducing when getting hot.

Roy

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Is this something that has suddenly happened to a system that was working OK up till then?

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No a new rebuild using the old parts .  new loom

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Silly question perhaps but Is the engine earthed ? (Must be I suppose if you have started  it ) Perhaps the thermostat housing isn't ?

Edited by RobH

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I tested all of the senders simultaneously in a bowl of water, gradually increasing the temperature (checked with a digital thermometer) measuring the resistance with a multimeter at 25, 40, 60 and 80 deg C.

Fuel gauge is OK, and I've measured the voltage output from the stabilizer with a multimeter.

It didn't happen suddenly, it's been like that since the complete rebuild, nuts and bolts, of the car finished in 2018. New loom, new gauge, new sender (twice) as I suspected the sender as the culprit.

Earth is checked and good, the bolts holding the housing ensures that.

Thanks for all good suggestions

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9 minutes ago, ijonsson said:

I tested all of the senders simultaneously in a bowl of water, gradually increasing the temperature (checked with a digital thermometer) measuring the resistance with a multimeter at 25, 40, 60 and 80 deg C.

Fuel gauge is OK, and I've measured the voltage output from the stabilizer with a multimeter.

It didn't happen suddenly, it's been like that since the complete rebuild, nuts and bolts, of the car finished in 2018. New loom, new gauge, new sender (twice) as I suspected the sender as the culprit.

Earth is checked and good, the bolts holding the housing ensures that.

Thanks for all good suggestions

Then it still sounds as if you need to send the gauge back as being not fit for purpose.

Stuart.

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Perhaps the gauge is setup for a lower resistance sender. 

Bob.

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Went out to the garage, checked voltage (9,5V) from stabilizer and thought I'll test RobH tip about adjusting the gauge, so took it out and nudged the cold side to the "colder" side of the pointer just to test.  Ran the engine until warm, and out of luck, now it's marginally above what I can measure with the IR thermometer! Happy days

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22 minutes ago, Lebro said:

Perhaps the gauge is setup for a lower resistance sender. 

Bob.

OK, didn't test that. I have a couple of senders with lower resistance. But in my head lower resistance would deliver more power to the gauge and then the gauge would read even higher.

But I don't know how the gauge works so I'm probably wrong here ;)

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7 minutes ago, ijonsson said:

OK, didn't test that. I have a couple of senders with lower resistance. But in my head lower resistance would deliver more power to the gauge and then the gauge would read even higher.

But I don't know how the gauge works so I'm probably wrong here ;)

Sorry, I miss read the original post, I thought it was reading low. 

So to re-phrase my suggestion !  perhaps it needs a higher resistance sender ?

Bob.

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I think we are getting crossed posts here. I was replying to Roy - perhaps that needs a new thread to save confusion. 

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23 minutes ago, ijonsson said:

OK, didn't test that. I have a couple of senders with lower resistance. But in my head lower resistance would deliver more power to the gauge and then the gauge would read even higher.

But I don't know how the gauge works so I'm probably wrong here ;)

I see from your first post you describe it as a 'new Smiths Classic' gauge.  According to their website 

https://www.smiths-instruments.co.uk/classic

the Smiths Classic temperature gauge is a bimetallic device and needs sender TT6811-01 ( about halfway down the page). Is that what you have?

 

 

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23 minutes ago, RobH said:

I think we are getting crossed posts here. I was replying to Roy - perhaps that needs a new thread to save confusion. 

Yes, I too was originally replying to Rob, who perhaps just needs a lower resistance sender.

Confused of Leatherhead

Edited by Lebro

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i will re post in technical as it seems to have confused things

ROY

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