Thermostat white paper

I’ve published chapter 1, and will add about a chapter per month.

The reason that I’ve chosen to publish in this peculiar way is that research takes time and money, so I can only work so fast. I’m trying to pack in as much real information as possible, so you may find that even this simple introduction is packed with surprises…


Thanks Mike
Agree with every word except a comment to revise bellows usage from 3.8 Es to all S1 Es.

Excellent Mike. There is a long thread here which discusses the bellows thermostats and related housings:

Feel free to take anything (photos, diagrams, animations) you feel appropriate for inclusion in your work as it would be good to get a single definitive document on this subject.



Looking promising! Please include both metric and imperial values? Some don’t understand ºF and likewise ºC or inch vs mm…

… there are formulas…:yum:

Indeed. But then we assume those that don’t understand one or the other can actually USE those formulas. Providing both just makes the document more “international” surely? Ummm…Which assumes that is the author’s intention… I can’t win this one :woozy_face:

Be easy… it was intended as a light-hearted comment.

Thanks Mike,
I will look forward to future ‘publications’… I certainly didn’t realize what a complicated device the ‘humble’ thermostat was.

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Good job Mike! Good appropriate level of detail. The phase change part as applied to thermostats was definitely new to me. Thanks in advance for your efforts.

Perhaps mention that of utmost importance is the attention to the particular composition and the quantity of the wax compound used that make it perform the way it does when exposed to heat. Other than that, I’d say it is a passenger, i.e. it isn’t an active device. It is only because its heat versus movement characteristics have been deliberately chosen that it performs the function it does.

Put another way, a lot of people think that the tail wags the dog.

kind regards

So, that’s how thermostats work! Didn’t understand the dynamic before.

But is it accurate to say the material in the enclosed space containing it “boils”? Or do the molecules excited by heat cause the material to expand and put the enclosed space under pressure that actuates the valve?

Nick, I would say “boils” is the best term. Longer version is it goes from a liquid to a vapor. The key aspect is that as it changes to a vapor it immediately needs a larger volume to expand into, popping open the thermostat as described. When the “molecules get excited” that is more like a gradual thermal expansion. Also, being a closed space, when it cools below it’s boiling point, it sucks the assembly closed again.

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Don’t confuse thermostats which use solid-liquid phase change with the liquid-vapour ones. They both work the same way, but one them will use the terms “melt” and “freeze” and the other will “boil” and “condense”. The graph and hysteresis will look much the same because the energy used/released for the phase change in the middle of the graph is typically 20-100 times the energy used/given up at the edges of the graph.

Perhaps it’d be helpful to only consider the solid/liquid wax thermostats and always refer to the solid/liquid phase change only.

kind regards

Very informative Mike, thanks for that.

A few responses:

It’s correct to talk about the working material “boiling” if it’s a bellows thermostat, which works with an liquid alcohol charge. In the case of wax thermostats, “melting” is close, but a more correct term would be “liquifying”. As a point of interest, thermowaxes are amorphous solids, and don’t have a specific melting point. They transition from liquid to solid in a critical temperature range. (Think about leaving butter out on the counter. It doesn’t exactly melt, it goes soft and gets still softer as it warms. Get it warm enough and it’s a liquid.) That’s why we can talk about “start of open” vs “full open” for wax thermostats. You really can’t do that for bellows thermostats, because the transition occurs at a specific temperature.

It wouldn’t be completely incorrect to say that the materials expand due to exposure to heat. But a steam engine doesn’t work on warm water. The entire point is that the phase change…from liquid to vapor or from solid to liquid…is accompanied by great expansion. The expansion graph in Chapter 1 is really what happens to a thermowax as it warms. Because that curve is an immutable physical property of the wax, the thermostat will open at precisely the same temperature, every single time.

Peter Crespin’s comment is a common misunderstanding…the only E-Types that had bellows thermostats were the early 3.8’s. Jaguar transitioned to wax thermostats late in 3.8 production. In fact, the E-Type 4.2 S1 manifold was deliberately configured NOT to accept a bellows thermostat.“Bellows” refers to the motor used to open the thermostat, not to the configuration of the bypass sleeve. This will become clear by the time I’m done with the Bypass chapter. I can see that there are many surprises coming.

David. Thanks for referring to the E-type UK forum. That thread was one of the things that kicked this project off. That particular thread had a lot of valuable information, but also a lot of misconceptions. One of the reasons that I chose to publish this as a web book is that I don’t want to muddy the text with red herring discussions. So the discussion will be confined to Jag-Lovers, and I’ll incorporate only the information that’s accurate and consistent. That way it will become an enduring reference.

Marek. I placed a link to the IGI website in my Sources page. IGI is a major producer of thermostat wax. There may be enough info on their website to satisfy your curiosity. I’ll go into this a bit deeper in the September update.

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Mike, it’s always tricky arranging your technical explanation of any topic like this to work for the “fat part of the bell curve” of your intended audience. Good job so far. Looking forward to more.

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I was visualising what happens in an alcohol thermometer as temperatures rise. The alcohol, dyed red, expands as temperatures increase and contracts as they go down but at no time gasses off because the vacuum within the glass housing prevents it. Off course if you get the alcohol in a thermometer hot enough the pressure from the expanded liquid trying to occupy too small a space causes the bulb to explode and the alcohol immediately vapourises.

The phase change is important over and above mere expansion Nick and will become apparent in part two prevents the alcohol bellows being used above a few psi. Later higher pressure system prevent the boil, so that type can’t be used.

Apologies Mike for mixing wax sleeves and alcohol bellows as two similar but quite distinct slit closure devices.

Well, the month has flown by, and it’s time for a new chapter. This month, it’s an introduction to bellows thermostats. Whenever I go deep on a topic, I’m always surprised by what I find, Hopefully, you’ll find a few Easter eggs, too:

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Good stuff Mike.
I might be tempted to reword the last paragraph to emphasize that a system using a bellows thermostat is intentionally designed to operate at low pressures, typically less than 4 psi. Then in BOLD I would caution to never substitute a pressure cap with a higher rating in a mis-guided attempt to “fix” a system that is not cooling properly, as there is great risk that the bellows thermostat will be damaged.

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