Gauge accuracy

[europa88]

So I have read most of the threads on this subject on the forum and cant find the answer to a very simple thing! Perhaps I am being a bit dense, but what is the actual water temp marking at the first mark past 90 degrees. I have owned my Europa for over 40 years without ever giving it thought, as it has always read steadily between these two points. The marking is not equidistant like the one between the 50 and 90. I am assuming (Always a bad idea) that it represents 100 degrees. As I have recently seen it go past this mark with heat soak when stopping the engine.

[BDA]

I have a diagram from Lotus on my computer. I’ll post it when I get a chance, but the hash marks mark the half way point between the numbers on the face.

Edit: added water temp gauge graphic

[europa88]

I have a diagram from Lotus on my computer. I’ll post it when I get a chance, but the hash marks mark the half way point between the numbers on the face.

Edit: added water temp gauge graphic

Thanks for that BDA and that would make perfect sense if it wasn't for the fact that my gauge only goes to 130 degrees and the mark isn't halfway between 90 and 130. I guess it could represent a 20 degree difference and the mark would read 110 degrees? It's a factory fit gauge too...I'm confused now 🤔

[BDA]

It is a bit confusing. I have both a 130 and a 140 degree gauge. It could be that I’m jumping to conclusions but my take is that if the hash marks denote the half way point on one range, it almost certainly marks the half way point on similar gauges with different a different range. As for the fact that the half way point of the reading isn’t half way between the numbers… that used to bother me but I’ve gotten over it. Sort of like getting over not sitting straight ahead, a little water leaking through the doors, and getting in and out (now that I’m older). It’s all part of the charm!

[jbcollier]

Any gauge in a car is an indication of what is going on.  It indicates whether conditions change or remain the same.  Accuracy has to be checked with a known good gauge or standard.   A classic example of this is to but the water temp sending unit into boiling water.  Apply the local altitude correction and you can relate your gauge’s readings to actual conditions.

Long way of saying that any mark on a mass market automobile gauge is an rough approximation unless you calibrate the gauge/sending unit/power supply.

[EuropaTC]

So I have read most of the threads on this subject on the forum and cant find the answer to a very simple thing! Perhaps I am being a bit dense, but what is the actual water temp marking at the first mark past 90 degrees. I have owned my Europa for over 40 years without ever giving it thought, as it has always read steadily between these two points. The marking is not equidistant like the one between the 50 and 90. I am assuming (Always a bad idea) that it represents 100 degrees. As I have recently seen it go past this mark with heat soak when stopping the engine.

I've also assumed 100C but as John says, unless you calibrate with a kettle of boiling water then it's a 5% gauge at best. 

On the assumption front, I guessed that the first mark between 50-90 was around 70-80C where the lower thermostat would open (78C quoted in the manual but 72C used to be available so take your pick). For the mark above 90 I'd also gone with 100C. 

It seems reasonable with the heat soak after switching off that the coolant would go over 100C without boiling as it's under pressure from the expansion tank cap.

Brian

[europa88]

Thanks for every ones input on this, It would appear its time to boil that kettle!

[jbcollier]

At work I used a kettle with a digital read out that was checked against a Fluke meter.  It was then easy to check thermostats by slowly raising the water temp until it opened.  Once you knew the thermostat’s opening temp, it was simple to check the temp gauge against your now known thermostat opening point.

Yes, we also have Fluke IR temp gauges.  They are very useful for checking temperature differences in a system.  For example, a cooler exhaust primary pipe quickly shows you which cylinder is misfiring.  BUT, actual accuracy of the reading is biased by the surface being read.  For example a black coolant hose will not give the same reading as an aluminium housing even with coolant of the same temperature flowing through both.

[europa88]

At work I used a kettle with a digital read out that was checked against a Fluke meter.  It was then easy to check thermostats by slowly raising the water temp until it opened.  Once you knew the thermostat’s opening temp, it was simple to check the temp gauge against your now known thermostat opening point.

Yes, we also have Fluke IR temp gauges.  They are very useful for checking temperature differences in a system.  For example, a cooler exhaust primary pipe quickly shows you which cylinder is misfiring.  BUT, actual accuracy of the reading is biased by the surface being read.  For example a black coolant hose will not give the same reading as an aluminium housing even with coolant of the same temperature flowing through both.

Unfortunately I don't have access to higher end equipment any more, but I do have a dual laser IR gun that has compensation for emissivity. Would a reading taken from the top hose be reasonably accurate compared to the aluminium thermostat housing or head?

[jbcollier]

Go to your local grocer and buy a cooking thermometer (cheap).  Put it in boiling water and note the reading.  Check the reading against your local correction factor (water boils at lower temps the higher the altitude).  Now you have a correction factor for the cooking thermometer (some you can turn the dial to calibrate).  Suspend your thermostat in a pot of water and slowly heat it until the thermostat starts opening.  Note the temp.

Install the thermostat in the engine, bleed the filing system properly and run it up to operating temp and bleed it again just to be sure.  Let the engine cool and then run it again.  Hold the coolant hose next to the thermostat.  It will feel a little warning due to the coolant going through the bleed hole.  You will feel a sudden surge of heat when the thermostat opens.  Note the gauge reading.  Quickly use your IR meter to get readings on various surfaces on the cylinder head and coolant out hose (block and other hoses may be cooler).  Now you have a reasonable notion of the accuracy of your gauge and the best areas to scan to get a reasonably accurate reading of the coolant temp with your IR gauge.