In my HLT, it's a big 185 litre pot with a 4500 watt heater element. Obviously the amount of water varies on brew-size.
Generally the more water, the slower it crawls over the finish line. With 120 litres, I guess it takes about an hour to cover the last 2 oC. This is my real problem, that lousy 2oC. It's quite frustrating, you can watch it and see the algorithm only flashes the heater on for a split-second, yet it still significantly below the correct temp. It's been suggested to not start using PID until it's within a degree of set-temp, but this removes the whole point of using PID.
BTW, and FWIW: The pot is insulated (yoga mats!).
But the HX ~ Ah, ahem, geeze.
Ok, so it's Nev's s.steel "HERM-IT" coil. - https://onlinebrewin...rm-itr-coil-kit
It doesn't say exactly how long it is, but 9x coils, with 12mm tube, ~90mm OD. So that's: 9 x Pi x 90 => 2544.7 -> so say 2.5 metres.
It's in a about a 6-litre pot, with a 2500 watt element. With the coil submerged, there's just about exactly 5 litres of water around it.
It doesn't really have a whole lot of trouble getting to the target temperature.
The way Arduino PID library is used is that the PID-control governs the amount of a 5 second window used for heating each iteration.
So for "full power" that whole 5000ms is used, but for "half power" only 2500ms of it used, etc. The output of the PID computation is how much of that 5000ms should be used for heating in the upcoming 5000ms. It has to be like this, because a heating element can only be on or off.
I think this 5 second time amount is a bit arbitrary, if it's shorter or longer, the PID algorithm would still turn the heater on for a calculated percentage of the time.
With a smaller time, it would evaluate the should-I-apply-heat-now question more often, but I expect it would heat much the same (probably).
I'm not at all familiar with any PID stuff other than this Arduino Library (I did not do any control systems courses at uni), so I can't really say what the units represent - if anything.
I have, for example, used the same PID library to hover a quad-copter test-balance-arm by changing the motor speed on feed-back from a gyro' (here P=2, I=0.95, D=0.011) - so the library works well, and is pretty flexible. But I don't think it has the concept of the units meaning anything in the non-mathematical sense. I guess that's not much help but.
 circumference of a circle is (Pi x diameter), a coil is roughly /N/ diameters, thus length is (/N-coils/ x Pi x diameter).
 http://playground.ar...Code/PIDLibrary and an intro to it http://brettbeaurega...d-introduction/
Edited by Mr Wibble, 22 January 2017 - 08:47 AM.