John Cuthber

The best we have so far is sun-cream and a good diet.

I imagine he's referring to this https://quoteinvestigator.com/2015/05/08/stamp/

Doh! All the deceit decent furnaces I have seen have PID controllers. The cheap ones use bang bang, but if you want good temperature control- and control of rate of change etc- you need PID Very- for exactly the reasons that (1) you gave earlier and (2) people use PID controllers.. In that case I'd definitely try to include some sort of feed-forward if I wanted good control (OK- at the expense of a more complex system but that's always an issue) If I have a flow of x litres per minute and I want it at 50 C, but the incoming temperature is y then I can calculate the power requirement to a good precision. I'd use that to supply the bulk of the temperature rise, and use a PID to fine tune it.

All the deceit furnaces I have seen have PID controllers- but (obviously) no active cooling. "if PID controllers are so great at controlling temperature, why don't room thermostats use them?" Because bang bang is good enough and a lot cheaper. It has, of course, also been going since long before PID controllers were invented. As an amusing aside, you might even get away with feed-forward control in this case. If you know the temperature from which the water is starting and you know how much of it you have, you can calculate the energy needed. And you can deliver that very quickly indeed. it fails on the requirement.

What do you consider the word "universal" to mean?

Probably none. I suspect that other animals have the same preference (given a choice). In which case early man's instinctive preference for cool water would have kept them safer.

In a fantasy universe G needn't be the same for all materials- or even for all configurations of materials. You could, for example, make it "magically" temperature dependent in order o get the stars to work.

In our evolutionary past we would have been better off finding cold water- it's less likely to be stagnant and less likely to have had a chance for pathogenic bacteria to grow in it.

OK, why did anyone ever go to the trouble of inventing the PID controller? The whole point is that you can set the P, I and D terms independently and avoid the issues you mention. (Still a less interesting question that how does Sensei's "heater free heater" work? but I guess that's a lost cause).

I'm assuming that you have the gas rather than a solution in water. Saying "HCl" is ambiguous. If you have tons of it then you must do something with it- you can't just vent it to the atmosphere. Dissolving it in water to make hydrochloric acid- which can be sold- is probably the easiest way to deal with it. But you need to get proper advice from a chemical engineer. Posting on a web page won't do the job.

It is precisely because of the large time constant that people use PID controllers. I'd use bang bang control if I was just trying to keep my home-brew warm. But if I was trying to "Win" a school project, I'd get a PID I look forward to Sensei's explanation of how one might heat the water without a heater.

PID controllers have been widely used for a long time; rather longer than PWM has been common. If you use an arduino, doesn't it have PWM built in?

Without a heater that's not going to get very far (and I'm not sure what you think the history lesson achieves). A bang bang temperature controller is (usually) easy to implement, but a proportional one will give much better control. If you know how much water is in the tube then an interesting variation would be to measure the temperature of the water, calculate the energy needed to increase the temperature , charge a suitable capacitor the right voltage and then discharge it directly through the water itself. That gives you the possibility of heating the water in microseconds, at the expense of being much less practical.

How worth while it is depends on how much you have. If it's not much- say <100g I'd not bother except perhaps as an interesting project.

Liar. You did not say that. Now, the thing here is that I know what a thermistor is. It's used for measuring temperature. And it's seldom used as a heater. (It would be poor practice). It's hard to see how either of your posts here so far is in any way helpful. So, once more. You seem to have forgotten to include a heater in your suggestion. And you have now added stupid dishonesty.

The only Q the OP has asked seems to be "Would you dare asking this question to your science professors? And I would ask that. It would be pointless. His answer would be that his proff is long dead.

WTF? I have a feeling that the pointless history lesson is less helpful than telling him to google "PID controller" or even "thermostat". You seem to have include a silly assumption about the OP's sex and a 2000 year old career choice, but forgotten to include a suggestion for a heater.

The Michelson–Morley experiment proved that the ether (or aether if you like) didn't exist (at least- not as expected) in 1887. At the time Einstein was about 8 years old. Now it's possible that he's not as clever as some folks say, but the idea that he believed in something which had been proven not to exist when he was at school doesn't seem realistic.

The best Bond was the one in the books.

No. Imagine I made a statue of my arm from lead. When I put it in the bucket, it will displace just the same weight of water as my real arm does. So the weight of water will be the same, but a lead copy of my arm is obviously a lot heavier than the real arm.

What do you mean by "only"? The folk doing this took the trouble (and money) to produce lots of isotopically enriched silicon precisely because they want to compare the mass spectroscopic result with the "conventional" one. I could use a laugh; exactly what experiment would you like to see?

They plan to. https://www.nist.gov/si-redefinition/kilogram-silicon-spheres-and-international-avogadro-project

That's why a chose lead as an example. You can get nearly pure 208Pb by isolating lead from thorium minerals. You can compare it to lead from other sources and show that there are differences. Getting other pure isotopes is tricky. You can get monoisotopic mercury by neutron irradiation of (naturally single isotope) gold.

The atomic masses were determined (often to quite high accuracy) well before there were any mass spectrometers. For example, you can take a gram of hydrogen(mixed with nitrogen r something else that won't react), and pass it over hot copper oxide. The hydrogen will be oxidised to water. You can trap that water by passing the gas over phosphorus pentoxide. And you can weigh the water. You will get 9 grams of water and so you know that the mass of oxygen that combines with 1 gram of hydrogen is 8 grams . It gets a bit more complicated when you try to decide if water is HO H2O HO2 or what, but it was pretty much all worked out by classical chemistry.

Or with a questionable definition of "chemistry"... https://en.wikipedia.org/wiki/Mass_spectrometry It is possible to observe isotope mass effects with chemistry. According to this data https://en.wikipedia.org/wiki/Isotopes_of_lead you can get samples of lead that have between 51 and 56 % 208Pb That would be a big enough range to detect if you measured the molar mass by classical wet chemistry.