Kind of hard to discuss differences of a few degrees though with such a method. Not criticising - just a comment.
Yes - I know that feeling about engineers all too well (and I won't say that it isn't oftentimes deserved). Sometimes, however, it is an excuse to avoid something necessary being done (not accusing you of that at all). Two quick examples: I was in charge of some process batch ovens in a factory one time. It was a 2 man-day job to replace the heater elements in an oven when they burnt out, and the heaters in each oven typically failed every 2 to 4 weeks of normal usage, so the equipmant down-time as well as tangible expense (labor and heating elements at several hundred dollars a pop) was considerable. I observed that the obsolete process controllers could not be properly tuned, and as a result, they would drastically overshoot temperature while trying to keep up with the profile, and to keep the duty cycle required to maintain the temperature profile, the heaters would go from 100% on for
5 and 7 minutes (literally red hot) at a time to full off for comparable periods of time. I know the solution was to use self-tuning (proportional tuning) controllers (under $400 each) that would optimize the duty cycle algorithm to the time consant of the oven (system) to prevent the wild swings and abuse of the heater elements. The maintenance supervisor, knowing how "impractical" and ridiculous "college boys" are, refused to invest the man hours to replace the controllers even though they were sitting on the shelf waiting to go in, and I assured him that the payback would be a matter of a couple of months when he didn't have to replace the heaters nearly as often (he didn't believe me). Finally I raised enough sand (actually contracted it out to an outside vender since he refused to do his job and follow orders and there were no consequences for that) and got it done. The heater elements lasted on average well over a year each after that (plus I found a third party vendor to supply heating elements at $300 a pop rather than the $800 that the oven manufacturer had been charging).This same maintenance supervisor also refused to repair a 100% hydrogen fitting leak on a gas mixer (100% nitrogen and 100% hydrogen in, 93/7 out) going to the same ovens. Again, he was allowed to do that without consequence to himself. Technically I could (and really should) have declared a safety hazard and shut the area down until it was fixed, but that would have come down on me - "over-reacting", you know. Anyway, after six weeks of my submitting several work orders to fix it that were ignored, he had an outside contractor welding above the mixer where the leak was - the mixer was draped with a canvas tarp for "safety". I happened to be walking thru the area with sparks falling onto the tarp, and all of a sudden, a 3 foot flame shot out of the leaking fitting. Keep in mind that this was about 5 years after one of those ovens had exploded due to over-ridden hydrogen safety interlocks (before I was hired) almost killing one guy, and Factory Mutual threatening to refuse to cover the plant until the safety interlocks were restored and upgraded. Fortunately nothing more happened, and we were able to evacuate the area and shut the hydrogen off. Even so, I was accused of over-reacting. Even another engineer told me that there was no danger - after all there had to be just the right mixture of hydrogen and air to be explosive, and the chances of that happening were "minimal". I said
- yeah - I guess that's how that oven exploded a few years ago, and maybe we ought to move his desk over next to the mixer and let it run and a welder be overhead until the leak could be fixed. He still didn't get it. IOW - I see both sides of that "engineers are anal" coin.
No problem.
However, what Ted described is, by definition *hysteresis*, so on that point either there is hysteresis and he's right, or there isn't hysteresis and he's not right (on that particular point of observation of a shift in going from hot to cold/open to closed vs. gfoing from cold to hot/closed to open). Perhaps what you meant was that you are not denying that the hysteresis is there, but are making the point that it just doesn't have significant effect on the end result. I still can't help but wonder if what was also a time soak phenomenon giving the false impression of the hysteresis that Ted thought he was observing.
There's also the observation of others of a temp spread in transitioning from full closed to full open - probably a thermostat manufacturer spec. sheet would clear all of that up (i.e., hysteresis as well as temp. spec. from full open to full closed). I do not have first-hand observations on this - only have the (obviously sometimes conflicting) statements of others to work from.
Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")