Marauder Posted August 15, 2022 Report Share Posted August 15, 2022 Speaking about the location of the probes has me wondering for those with multiple probes/instruments, what kind of temperature differences are you seeing. On my plane, I have a probe under the left wing for the JPI 900, I have another probe stuck in the left window and finally one in the Aspen RMS. When I look at the 3 values in flight, I see the JPI is usually 3° or 4° warmer than the window gauge and the Aspen could be 5° or 6° cooler than the window gauge. I haven’t checked the temperatures on the ground to see if it is an accuracy thing or if the placement is affecting them (the window gauge was always my go to gauge). Anyone else experience similar discrepancies between gauges?I think I read in the JPI manual that the OAT can be adjusted. Not sure about the Aspen yet. Sent from my iPhone using Tapatalk Pro Quote Link to comment Share on other sites More sharing options...
jamesm Posted August 15, 2022 Report Share Posted August 15, 2022 I did that with my Insight G3 Engine monitor but temperature ( uses a J type thermocouple ) and Sandia STX 165 Transponder which used RTD (resistive thermo Device), both mounted in the NACA Duct(s) one in the left side and one in the right. I found the readings to be inconsistent and erratic in some cases. when compare to OEM windshield Temp probe and left under wing GAD 13 & GTP59 temp probe on the G5. I expected it to be like a compass deviation card you could just add or subtract the deference and get the temperature correction to get true OAT temperature. It wasn't true in my case. I suspect the Avionics guy didn't want to run the wiring. Just my 2 cent. James '67C 1 Quote Link to comment Share on other sites More sharing options...
Ragsf15e Posted August 15, 2022 Report Share Posted August 15, 2022 1 hour ago, Marauder said: Speaking about the location of the probes has me wondering for those with multiple probes/instruments, what kind of temperature differences are you seeing. On my plane, I have a probe under the left wing for the JPI 900, I have another probe stuck in the left window and finally one in the Aspen RMS. When I look at the 3 values in flight, I see the JPI is usually 3° or 4° warmer than the window gauge and the Aspen could be 5° or 6° cooler than the window gauge. I haven’t checked the temperatures on the ground to see if it is an accuracy thing or if the placement is affecting them (the window gauge was always my go to gauge). Anyone else experience similar discrepancies between gauges? I think I read in the JPI manual that the OAT can be adjusted. Not sure about the Aspen yet. Sent from my iPhone using Tapatalk Pro I think you’d be worse off with one in the naca duct due to the engine heat being so close. My windshield one and my underwing jpi were always similar. The rsm is probably designed to work in its position. I have inadvertently ops tested my jpi probe on ifr flight… 0 degree indication = no ice, -1, ice just barely started forming ice on leading edge. Got a descent and ice melted off 0-1 degree. Close enough for me! 2 Quote Link to comment Share on other sites More sharing options...
N201MKTurbo Posted August 16, 2022 Report Share Posted August 16, 2022 14 hours ago, jamesm said: RTD (resistive thermo Device) Resistance Temperature Detector I'm up to my eyeballs with these things. Designing a new PCR machine. 1 Quote Link to comment Share on other sites More sharing options...
jamesm Posted August 16, 2022 Report Share Posted August 16, 2022 5 hours ago, N201MKTurbo said: Resistance Temperature Detector As kid I had built radio shack indoor / outdoor thermometer kit. one day the wire for the outdoor thermocouple had gotten cut, thinking I could just solder the broken wire to fix it didn't work. so I tried stick the thermocouple in the freezer while I soldered the wire that didn't work either. When I was talking to Sandia Rep about the thermocouple for their transponder they told me it could crimp or solder the connection. I thought I had misheard them. the cable wasn't long enough I solder it and it worked. I couldn't figure out how I could solder wire and effect not thermocouple reading. Later at work a guy called it a RTD device in the context of Highlands V420 and V470 (VME Analog Output/Thermocouple Simulator). Quote Link to comment Share on other sites More sharing options...
EricJ Posted August 16, 2022 Report Share Posted August 16, 2022 6 hours ago, N201MKTurbo said: Resistance Temperature Detector I'm up to my eyeballs with these things. Designing a new PCR machine. Sounds like a fancy name for a thermistor, but is apparently not quite the same? 1 Quote Link to comment Share on other sites More sharing options...
flyboy0681 Posted August 16, 2022 Report Share Posted August 16, 2022 While on the subject, my JPI OAT does not agree with the CHT and oil temperature probes when I walk into the hanger first thing, as it's off by 3 degrees. The other probes are within 1 degree of each other,. Would it be acceptable to adjust the OAT downward to match the others? Quote Link to comment Share on other sites More sharing options...
PT20J Posted August 16, 2022 Report Share Posted August 16, 2022 (edited) One thing to consider when comparing in-flight temperatures measured at different probe locations is that they are usually indicating total air temperature which is static temperature plus the ram air temperature rise due to compressibility. At Mooney speeds, the ram rise is around 4 deg C. So, a variation of a degree or two between different locations might be reasonable. Then there is the possibility of rounding/truncating difference between instruments. Also, the Garmin G3X corrects for ram effect and actually displays calculated static air temp as part of the air data computations. Maybe other systems do that also. Skip Edited August 17, 2022 by PT20J Clarified that ram air rise is in deg C. 4 Quote Link to comment Share on other sites More sharing options...
Ragsf15e Posted August 17, 2022 Report Share Posted August 17, 2022 19 hours ago, flyboy0681 said: While on the subject, my JPI OAT does not agree with the CHT and oil temperature probes when I walk into the hanger first thing, as it's off by 3 degrees. The other probes are within 1 degree of each other,. Would it be acceptable to adjust the OAT downward to match the others? I would probably leave it alone too. There’s a reasonable amount of thermal inertia in the engine block and oil that those temps are likely actually different than the air temp in your hangar. You’d just be guessing where/how much to adjust it. Quote Link to comment Share on other sites More sharing options...
flyboy0681 Posted August 17, 2022 Report Share Posted August 17, 2022 1 hour ago, Ragsf15e said: I would probably leave it alone too. There’s a reasonable amount of thermal inertia in the engine block and oil that those temps are likely actually different than the air temp in your hangar. You’d just be guessing where/how much to adjust it. All excellent points. Quote Link to comment Share on other sites More sharing options...
carusoam Posted August 18, 2022 Report Share Posted August 18, 2022 Location is everything… I wouldn’t try to adjust the reading until that was the last thing available…. The accuracy is super important… when you are using it to escape icing. Mounting in the windscreen is suboptimal… it was great when the sensor was an inch away from the display, and no electricity was needed… the Naca vents on the side of the fuselage are next best, but often are reading a mixture of warm air caused by the giant heat producer up front…. Somewhere around 1990… airplanes have been installing the OAT sensors out under the wing… in the shade, away from the engine… the sensors are a surface mounted tiny nub… that probably minimizes air compression found on larger sensors that stick out into the air stream… For accuracy tests… compare readings from the various sensors after they have been sitting in the hangar… for days… Even there… the air temp in the hangar is going to vary some too…. So consider having a fan moving air around inside to help level the playing field… Then… boiling water and ice baths are the next steps for calibration… Also know that thermocouples are really good for not needing calibration… they work or don’t work…. Thermistors are known for drifting off towards zero over time… when their resistor fails… PP thoughts only, not a mechanic… Best regards, -a- Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.