tmassey
Contributor
That’s the point to take away here.
You need high precision and accuracy especially when you are comparing the results to others (or to a standard) and in a situation where small changes will make a big effect.
Your compressor has neither of those conditions. First of all, you’re going to get a variation of multiple degrees just by what the ambient temperature is around the compressor. So being accurate to within a couple of degrees is meaningless when you won’t know if those couple of degrees is from a thermocouple variation or from changes in environmental conditions.
Second of all, the idea of trying to compare your compressor to what the manual says the temperature will be seems unnecessary as well. The manual is going to give you a maximum temperature not to exceed, but only if it’s measured in a specific location. That location is chosen to give you the best average of the entire operating area, such as on top of the head. And in the end, you don’t really want to know if you’re bumping up against that maximum do-not-exceed temperature. You want to know when the compressor is deviating from its normal, consistent temperature range. Even if you’re 10° below the maximum, if you’re 10 or 15° above your normal range, that’s a problem you should deal with before it reaches that maximum temperature!
So what you really need is consistent reproduceability. Who cares what the actual number is. The number is meaningless because of all the reasons we’ve described. What matters is the repeatability and the consistency. If you’re getting numbers that are within a 10° range, then you know what range your compressor needs to be in. If a deviates outside of that, shut it down and figure it out. And that’s true whether the number you’re getting is in Monarch units or scientific units, whether there’s no decimal point or three digits after the decimal point, whether they are two digit numbers or three digit numbers.
And from that perspective, any of the techniques that have been pointed out will probably be good enough. For me, personally, I didn’t want to have to use a handheld device and go look at the temperature of my compressor. I want it to be continuously monitored and all I have to do is glance at the continuous monitoring. For me, that meant thermocouples attached “permanently” (as permanent as any wire tie is…
) attached to consistent locations best likely to give me the temperature closest to the internal gas temperature. Now that meant a little bit of effort for me: electrically isolating but not thermally isolating the thermocouples, wiring them to a single thermocouple reader, getting the data from the thermocouple reader into an electronic device, and then writing some software to provide a very basic temperature graph. Now, I actually enjoy that process, so that was more entertainment than work. For others, a commercial off the shelf thermocouple unit might be a better solution.For others, none of that sounds like fun. All they want to do is spot check from time to time. A handheld FLIR unit certainly makes a lot of sense in that case. Plus, it makes all kinds of pretty pictures, and gives you a good idea of a large area, rather than one particular thermocouple point.
And frankly, none of this is exactly necessary. If it were, the high-end compressor makers would do a better job of instrumenting their compressors. Even (small, like ours) military and public service compressors don’t have these kind of features, and you can bet if there was a reason to sell this, they would.
(Temperature of filter stacks is much more important, and I believe the Bauer Securus system does indeed monitor the temperature of those. Ironically, that’s one of the things that us amateurs usually don’t monitor!
)In other words, the level of fine detail that you are agonizing over is literally meaningless. There are way too many variations to be able to derive that level of precision — or even accuracy. Like I’ve said a couple times: you don’t need accuracy. You need reproduceability – consistency.
But don’t let “need” get in the way of a good time. If you’re having a good time, by all means go for it. My computer-monitored Internet-connected wireless enabled graphical thermocouple system is also way beyond what is required, but I enjoy doing it. I guess my point is: don’t let the details overwhelm you. Anything you do is way beyond what will actually give you the value you seek.
ETA: also don’t forget that your compressor operating temperature will swing through a rather large range of temperatures. My compressor might start out at 7°C, and end up being 145°C by the time it’s done. But it is neither a straight line, nor a consistent curve that gets me there. There is the initial rapid warm-up curve, then it begins to flatten as it reaches something like working temperature. But it never even comes close to flattening completely: the pressure increases, and therefore has to work harder. But, of course, my auto drains kick in every 15 minutes, which drastically drops the temperature, because of the sudden loss of pressure, then it rapidly goes back up most of the way, but not all the way. It then starts to creep back up to the higher temperature, and then of course keeps going because the pressure keeps increasing. (never mind that I have literally six different thermocouples, which all believe it or not react differently to the increase in pressure, because they see the increase in pressure differently!
)In other words: what is this so-called “temperature” that your compressor is supposed to be running at? The temperatures are all over the place. And that’s discounting environmental conditions and other such issues.
To answer my own question: for me, what I’m looking for is what temperature is the compressor at when it reaches, say, 3000 psi. For me, that’s in the neighborhood of 140 to 145C. As long as I’m in that neighborhood, I’m not worried. As I get closer to 4000 psi, I might get closer to 150C Even that is not worrying me. Now, if I see 160C, then I would probably step in: I’ve never seen it get that hot. Or, if I were at 150 C at 2000 psi I would definitely step in.
Now none of those numbers are translatable: you can’t use those numbers to gauge where your compressor should be running. Different compressor, different environmental conditions, and variation in where you’ve placed the thermal couple!
again, the numbers themselves are meaningless. It’s just knowing that whatever those numbers are, the same as I got the last 20 or 30 times I ran the compressor, so it’s probably running exactly the same way as it was during those 20 or or 30 times.OK, enough. I think I’ve beaten the horse to death.
But: I would love to hear what you eventually decide on and how that looks to you. There’s no doubt in my mind you will substantially over engineer it… Any SCUBA Diver who owns their own compressor at home probably has a fair amount of that over-engineering trait. You will fit right in. 
