Lotus Elan

Smiths dual oil/water temp gauge repair?

PostPost by: tvacc » Fri Dec 21, 2018 1:06 pm

All,
Please understand I am not suggesting any one buy bulk ether. When you buy the cheap gauge, there is ether in that gauge in that bulb. When you put the bulb in the ice bath, you are stabilizing that ether, it stays in the bulb while you solder. You are going to do this once or twice. Buy the cheap gauge, use it. Unless you are going to go into the business of fixing gauges, I don't see the need to buy bulk ether.
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PostPost by: billwill » Fri Dec 21, 2018 1:25 pm

When I looked into this a few years ago, I could not find any sources of 'cheap' temperature instruments in the UK, it seemed to be a USA only option.

I like the idea of using brass tube as the joining mechanism instead of trying to drill a fine hole in a brass rod. I had forgotten about the availability of brass tube.
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PostPost by: nmauduit » Fri Dec 21, 2018 5:26 pm

billwill wrote:When I looked into this a few years ago, I could not find any sources of 'cheap' temperature instruments in the UK, it seemed to be a USA only option.

I like the idea of using brass tube as the joining mechanism instead of trying to drill a fine hole in a brass rod. I had forgotten about the availability of brass tube.


it also possible to source for cheap samples of soft (ductile) copper in many capillary dimensions : that is what I opted for so that the solder on piece would just slip over the original tubing, it would hide easily under the spring protection, is pliable (though less than the stock inner tubing) and of similar color when tinned.
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PostPost by: Quart Meg Miles » Fri Dec 21, 2018 11:29 pm

tvacc wrote:The 37 degree thing has nothing to do with out side temp. That is the temperature that ether turns to liquid. You will find that plain ice in water will cool the bulb to 36 to 37 degrees. By adding salt to the ice, you get down to 32 or so. At that temp, the ether is a fairly stable liguid and will stay in the bulb in the salt/ice bath.

Hey guys, you're talking ice freezing at 32 °F but the boiling point of ether is 37 °C. If you look at your broken gauge you will see that the indicated temperature is 37° and, ideally, is what the temperature of the ether should be when you seal the tube. Sealing it while the ether is at 32 °F will give wrong readings but probably not badly at 90 °C where it matters.
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PostPost by: billwill » Sat Dec 22, 2018 12:14 am

Quart Meg Miles wrote:
tvacc wrote:The 37 degree thing has nothing to do with out side temp. That is the temperature that ether turns to liquid. You will find that plain ice in water will cool the bulb to 36 to 37 degrees. By adding salt to the ice, you get down to 32 or so. At that temp, the ether is a fairly stable liguid and will stay in the bulb in the salt/ice bath.

Hey guys, you're talking ice freezing at 32 °F but the boiling point of ether is 37 °C. If you look at your broken gauge you will see that the indicated temperature is 37° and, ideally, is what the temperature of the ether should be when you seal the tube. Sealing it while the ether is at 32 °F will give wrong readings but probably not badly at 90 °C where it matters.



No, I don't think you are right. That sort of error might occur if the whole bulb and tube and gauge are filled completely with ether, but provided that there is some liquid and some ether gas it should read correctly. Thats why you only half fill the bulb with liquid ether.


The 37 degC (it's CENTIGRADE NOT FARENHEIGHT and is actually 34.6degC) spoken about is the 'boiling point' of ether AT ATMOSPHERIC PRESSURE! Because it is a sealed system, as the temperature (of the engine coolant in this case) rises, more of the ether boils off until the vapour pressure is the pressure corresponding to the boiling point of ether at that new temperature. It is this pressure that the gauge measures by causing a bent tube to straighten out a little, while connected to the needle on the gauge face.

They don't bother to include temperatures below about 40 deg C on the scale as it is not of interest, but actually if the engine temperature is below 34.6 deg C, the pressure inside that closed system will be less than atmospheric and the needle would move backwards, relative to what it was just before you solder the whole system closed (but it usually hits a stop pin at those low temperatures)

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PostPost by: nmauduit » Sat Dec 22, 2018 12:18 am

Quart Meg Miles wrote:
tvacc wrote:The 37 degree thing has nothing to do with out side temp. That is the temperature that ether turns to liquid. You will find that plain ice in water will cool the bulb to 36 to 37 degrees. By adding salt to the ice, you get down to 32 or so. At that temp, the ether is a fairly stable liguid and will stay in the bulb in the salt/ice bath.

Hey guys, you're talking ice freezing at 32 °F but the boiling point of ether is 37 °C. If you look at your broken gauge you will see that the indicated temperature is 37° and, ideally, is what the temperature of the ether should be when you seal the tube. Sealing it while the ether is at 32 °F will give wrong readings but probably not badly at 90 °C where it matters.


it does not matter the exact temperature at which the diethyl-ether is when the capsule is sealed, provided that there is some of it left (in sufficient quantity and pure enough) after soldering. Then it will evaporate when the capsule is heated by the engine, and the gauge will read out the pressure inside the setup and translate that into a temperature. The cooling of the capsule during soldering is merely for keeping the ether liquide while heating a highly thermally conductive copper part of the yet unsealed setup - iced water is just a convenient way of doing that.
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PostPost by: billwill » Sat Dec 22, 2018 12:29 am

I tried to find a handy graph on the internet showing the vapour pressure of ether at 100 deg C (boiling point of WATER) but most of the graphs don't go that far.


This one is drawn on paper with a logrithmic vertical axis of mm of mercury and you can just about work out that the vapour pressure at between 30 degC and 40 deg C brackets one atmosphere (760 mm mercury). The vapour pressure at 100 degC seems to be about 1,500 mm mercury (i.e about 1.97 atmospheres which is about 30 pound per square inch total hence 30-14.7 = about 15 psi above atmosphere)

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So our engine temperature gauges are 'really' pressure gauges working in the approximate range -5 psi to + 20psi (say).
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PostPost by: billwill » Sat Dec 22, 2018 12:38 am

nmauduit wrote:
it does not matter the exact temperature at which the diethyl-ether is when the capsule is sealed, provided that there is some of it left (in sufficient quantity and pure enough) after soldering. Then it will evaporate when the capsule is heated by the engine, and the gauge will read out the pressure inside the setup and translate that into a temperature. The cooling of the capsule during soldering is merely for keeping the ether liquide while heating a highly thermally conductive copper part of the yet unsealed setup - iced water is just a convenient way of doing that.


There must be both liquid and vapour in the sealed system over the whole temperature range for it to read the vapour pressure correctly, if all the ether turns to vapour, it won't read correctly.
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PostPost by: tvacc » Sat Dec 22, 2018 3:43 am

As Elton John sang "All the science I don't understand, its just my job 5 days a week"

I don't really know why or how it works. All I can tell you is that I have fixed more than a half dozen of these combo gauges exactly as I described and they all work fine. I test them with boiling water, putting the bulb in the water without touching the bottom and all the gauges read 100 degrees centigrade.

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PostPost by: StressCraxx » Sat Dec 22, 2018 4:38 am

tvacc wrote:As Elton John sang "All the science I don't understand, its just my job 5 days a week"

I don't really know why or how it works. All I can tell you is that I have fixed more than a half dozen of these combo gauges exactly as I described and they all work fine. I test them with boiling water, putting the bulb in the water without touching the bottom and all the gauges read 100 degrees centigrade.

Tony V


The mechanical temperature gauge is actually a bourdon tube pressure gauge reading in degrees C or F. Ether is a hydrocarbon fluid with a known vapor pressure curve that goes from 0mm/hg at 0C to 7500mm/hg at 120C. The temp/pressure curve is well understood and the gauge is calibrated to read in degrees to the relative pressure. That's about the best I can 'splain it.

This is the page I used:
http://ddbonline.ddbst.com/AntoineCalculation/AntoineCalculationCGI.exe
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PostPost by: billwill » Sat Dec 22, 2018 11:48 am

Ether is a hydrocarbon fluid with a known vapor pressure curve that goes from 0mm/hg at 0C


Minor correction:
According to the graph I showed above the vapour pressure of ether at 0 degC is about 200mm/hg not zero.
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PostPost by: StressCraxx » Sat Dec 22, 2018 4:51 pm

billwill wrote:
Ether is a hydrocarbon fluid with a known vapor pressure curve that goes from 0mm/hg at 0C


Minor correction:
According to the graph I showed above the vapour pressure of ether at 0 degC is about 200mm/hg not zero.


Thanks Bill,
I misread the chart and should have used the table value
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