Lotus Elan

temp gauge accuracy

PostPost by: rdssdi » Sun Apr 22, 2012 1:21 pm

I have been plagued with a hot running engine since the completion of my +2 restoration years ago. A larger capacity electric fan did help a bit but I was unhappy with the 90 deg. C (and up) operating temperature.

I purchased temperature recording strips. They will record the highest temperature of a surface by changing color and are single use.

I placed the strip on the top tank of the alloy radiator just above the weld joint and adjacent to the radiator fan sender. What I discovered is that the radiator temperature never exceeded 200 F. The gauge (in deg C) reads between 10 and 12 degrees higher than actual temperature. So when the gauge reads 90 C it is actually around 80. This is where the temperature usually stays. The fan turned on around 190 F.

I wonder if the instrument voltage stabilizer is not functioning properly or do I need to add a resistor in line with the temperature sender to calibrate the gauge. I can just run it the way it is and consider 90 C a normal operating temperature.

Some more servicing a wash and wax and the +2 will be ready for a long trip.

Bob

1969 Elan +2
1968 TVR Vixen
1963 Jaguar MK2
coming 1960 Lancia Flaminia Berlina
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PostPost by: stugilmour » Sun Apr 22, 2012 3:27 pm

rdssdi wrote:I have been plagued with a hot running engine since the completion of my +2 restoration years ago. A larger capacity electric fan did help a bit but I was unhappy with the 90 deg. C (and up) operating temperature.

I purchased temperature recording strips. They will record the highest temperature of a surface by changing color and are single use.

I placed the strip on the top tank of the alloy radiator just above the weld joint and adjacent to the radiator fan sender. What I discovered is that the radiator temperature never exceeded 200 F. The gauge (in deg C) reads between 10 and 12 degrees higher than actual temperature. So when the gauge reads 90 C it is actually around 80. This is where the temperature usually stays. The fan turned on around 190 F.

I wonder if the instrument voltage stabilizer is not functioning properly or do I need to add a resistor in line with the temperature sender to calibrate the gauge. I can just run it the way it is and consider 90 C a normal operating temperature.

Some more servicing a wash and wax and the +2 will be ready for a long trip.

Bob

1969 Elan +2
1968 TVR Vixen
1963 Jaguar MK2
coming 1960 Lancia Flaminia Berlina


Sounds like a great method Bob.

You can easily check the Voltage Stabilizer if it is a modern solid state one. If it is the old type I would consider changing it on principle due to age; I understand you cannot accurately measure voltage output from them as it is not continuous.

I had a very similar result, and rectified the temperature gauge reading by adding a resistor in series with the sender. Actually found I needed two small Radio Shack resistors in parallel because one alone was heating up too much. If going this route, buy the large resistor selection at Radio Shack as it provides several resistors of each value in the selection. You can calculate the total resistance of several resistors in parallel if different resistor values are used, or just measure the total with your ohmmeter.

I calculated the exact amount of resistance to use by measuring the voltage to ground at the temp sender at three gauge readings and comparing to actual coolant temperature measured with a candy thermometer right at the gauge sender. I found the adjusted readings to be reasonably linear; selected my final resistance value to make the gauge behave correctly at normal operating temp.

I checked my notes and unfortunately didn't write down the exact resistance value I used, but I believe it was around 7 to 9 ohms total resistance. I think the additional resistance makes up for loss of spring tension in the old gauge. I would recommend trying your gauge check with 9 ohms resistance in place and see if you are happier with the gauge response. The thing is approximate anyway, so close is OK. You could dial in the exact resistance you are happy with by changing them out and comparing to your heat strip, but i think 9 ohms will be a good starting point.

I found the calibrated gauge way less stressful than mentally subtracting 10 degrees C from the gauge reading. After getting the gauge to work correctly, I kind of wonder how much of my extensive cooling system modification was actually needed (re-built radiator, radiator inlet baffle, modern fan, Cliveyboy thermostat housing, full pressure expansion tank, cooler thermostat). However, the cooling system now works very well, and did not go over 105 degrees C in bumper to bumper traffic for about 1/ 1/2 hours at 94 degrees F ambient temperature.

I suppose the other option is to send the gauge in for service and calibration. Here is a link briefly explaining the calibration marks on the gauge face. I assume the gauge shops have test benches that allow the dialing in of the gauge movement to three known input voltages.

http://tigersunited.com/techtips/Brenna ... Gauge1.asp
Last edited by stugilmour on Sun Apr 22, 2012 4:40 pm, edited 4 times in total.
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PostPost by: cal44 » Sun Apr 22, 2012 3:52 pm

Bob,

I bought a heat sensing gun (lazer) a couple of years ago. Point and shoot. This way I am able to check different spots on the engine. I also have an oil cooler on the Plus 2.

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PostPost by: rdssdi » Sun Apr 22, 2012 4:53 pm

Stu, Thanks for the info. My temperature gauge is new. Should I say was new at the time of restoration. The original was too far gone.

I will check the voltage stabilizer. It was also replaced at the time of restoration but I believe it is of thermal design and not solid state. I do have a solid state stabilizer to install. I should try that first.

As to the heat sensing gun. It is my understanding that they are "calibrated" to work when reading from a matte black surface finish and other surface colors will cause a deviation from the actual temperature. That is why I chose the strips as they offer an accuracy of 1 percent.

I will install the new voltage stabilizer today and see what happens. It is also possible that the sensor/sender is incompatible with the gauge.

Bob
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PostPost by: Baggy2 » Sun Apr 22, 2012 5:05 pm

It is also possible that the sensor/sender is incompatible with the gauge.


In my experience this is the case. My car ran for years showing the 'correct' temperature. Then the connector tag dropped off the sensor. Put a new sensor in and instantly the indication is that its running hot.

Hope this helps

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PostPost by: cal44 » Sun Apr 22, 2012 7:31 pm

Heat sensor instruments are color blind. Having come from the natural gas industry and using "the guns" all the time to check iron temps when installing poly pipe the non-stick surfaces came in many colors. In vintage car racing they are used all the time to check temps.
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PostPost by: rdssdi » Sun Apr 22, 2012 11:29 pm

Then I shall purchase a heat sensor gun.

Should be helpful when calibrating the temp gauge.

thanks

Bob
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PostPost by: elansprint71 » Sun Apr 22, 2012 11:38 pm

I think this might have been discussed before once or twice since 1962 :roll: the sender sits over the exhaust manifold; therefor any readings might just be slightly higher than the true temperature.
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PostPost by: 1964 S1 » Mon Apr 23, 2012 3:52 am

Before buying guns and sensors, I'd try running the car around town with the radiator cap loose. If it doesn't boil over...

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PostPost by: rdssdi » Mon Apr 23, 2012 12:57 pm

The sensor position above the exhaust manifold was addressed by me as the first attempt to sort out the coolant temperature issue. I placed a heat shield between the manifold and the sensor.

The fact that it has been discussed since 1962 has no relevance. I was blissfully unaware of Lotus TC engines in 1962 and for many years afterwards.

"Baggy" I found your experience interesting. I am running a new Smiths gauge and a sender sourced from one of the usual suspects. It may well be a mismatch. Aftermarket reproduction components are often out of spec. If replacing the thermal voltage stabilizer with a solid state unit does not resolve the over reading problem, I will replace the sender.

Bob
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PostPost by: Robbie693 » Mon Apr 23, 2012 2:38 pm

Bob - forgive me if you have commented on this elsewhere but, did the heat shield under the sender make any difference to your temperature reading at the gauge?

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PostPost by: rdssdi » Mon Apr 23, 2012 3:29 pm

It made no discernible difference. I am considering a larger "insulated shield". It appears to me that heat will soak into that area of the head anyway but shielding it from direct manifold heat is most likely a good idea. In my opinion the problem is worsened by the use of headers.

Bob
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PostPost by: mikealdren » Mon Apr 23, 2012 3:57 pm

Bob,
as Stu says, you can't measure the performance of the voltage regulator easily while things are running because too much is changing.

However, you are interested in the performance of your gauge and you can check this with the engine off (not running). See what voltage you are getting at your gauge with the engine off and 12V from the battery. See what temp your gauge says and then see whether it changes with the engine running.

This should give you an indication of whether your regulator is a problem. Hope I've made this clear, if not please say so!

Mike
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PostPost by: holywood3645 » Mon Apr 23, 2012 4:33 pm

Temperature calibration 101 using 2 know reference points.

Take the sensor out. Place it in a kettle of boiling water, and still connect to to your indicator.
Boiling water = 100dec C.
then place it in an ice bath (water and ice cubes)
Ice bath = 0 deg C.

Make your adjustment accordingly to have the gage read correctly (more important at 100)

If there is any voltage issues fix them.

re-check a couple of times, you should be done.

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PostPost by: ricarbo » Mon Apr 23, 2012 7:14 pm

Just to pick up on Cal44's point on infra red sensors being colour blind.

They are not.

It's a characteristic called emissivity. To quote from the instructions for my tester "the thermometer is set at 0.95 before shipment, because in 90% of the time, objects emissivity is set at 0.95". A bit of poor English, but we know what it means. It then gives a chart for many materials, ranging from polished brass (0.1) to water (0.98). My meter has an adjustment range of 0.17 to 1.00. The instructions suggest you either adjust it according to the chart or "apply black tape, black matt paint or black magic marker to the object if safe" and then use it set at 0.95.
My meter also has a contact probe. When I compare the readings I get using the infrared and contact methods, I can see the emissivity does indeed affect the infrared readings.
So, for my money, get an adjustable one that comes with a chart and/or pay extra for the contact probe. Or just get one with a contact probe.
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