On 8/29/2012 3:29 PM, jon kimmel wrote: > Look at wikipedia under thermoelectric cooling...I think we're both right. > On Aug 29, 2012 12:57 PM, "Nerobro"<nero...@gmail.com> wrote:
Anywho.... The point is that thermocouples have a hot junction and a cold junction. The hot junction is where the temp is probed (the twisted or welded part where the wires meet.) The cold junction is the other end of those wires. There are two parts to most stories like this - THEORY and REALITY In theory, whatever you do to one side (one wire) on the cold junction, you are supposed to do to the other in order to not affect the reading. This means that when you extend one wire (talking a single thermocouple here), you should extend the other one the same length. In reality, for the type of TC we normally use (J - iron constantan for CHT, and K - chromel alumel for EGT), unless you are flying in a lab environment and care that your reading is 0.1 deg off on 1100, you don't need to worry about it. Last I checked, the resolution on a steam gauge type EGT/CHT was a whole lot more than 0.1 deg. Even if you have a digital readout, you won't notice. Probe proximity to the exhaust port (with EGT) makes a WHOLE lot more difference than a couple inches of wire!... Some gauges will have "cold junction compensation" or a note saying they were calibrate at a cold junction temp of xx deg. Any variation from that calibration temp will swing the gauge reading as well. Again, it doesn't matter because you aren't going to change the temp of your cockpit 50 deg (hopefully anyway) during your flight. What you are really looking for with an EGT is a trend, not an absolute temp. If you think that because your EGT says 1250 deg that that's what it is, I'm sorry. It may be what it is at that POINT (maybe) but without knowing a whole lot more information, that data is preptty much meaningless, and it's a WHOLE lot hotter than that just 1" further in. What you're hunting is when you lean, that you see the EGT peak, and then you can richen up x deg from that - that is to assume you run your engine rich of peak. Some people do different things and this isn't the email for that. CHT is a little different because we don't want to run over some value, usually a nice even 25 or 10 deg increment - meaning again, we don't care about 1 deg error. BOTTOM LINE - - You CAN extend (or shorten) thermocouples. You can use either TC wire (of the same type!) or standard wire (my vote). It matters not. - Try to keep any changes you make consistent across the cold junction of the TC (do to one wire what you do to the other... joint for joint, not necessarily length for length). - If cylinder 1 is 18" long and cylinder 2 is 72" long, you'll get the same reading within the limits of the measuring devices ASSUMING the gas/surface you are measuring is the same temp (they won't be) and the probe locations are identical (again, they won't be). - There is more to keeping up with how well your engine is operating than depending solely on probe data. Feel, sound, smell, plug color, etc.. Matt
