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  #1  
Old 08-26-2019, 08:53 AM
MMiller MMiller is offline
 
Join Date: Oct 2010
Location: Babylon NY
Posts: 50
Default High Indicated Temps & Instrument Error

These comments are specific to the RV-12 Rotax/D180 configuration.

There are two main components in the system, a sensor to generate the signal and a detector to measure that signal. A Thermistor is used for the sensor; this is a special type of resistor whose resistance is temperature dependent. A voltage signal is generated by placing a resistor in series with the Thermistor and connecting the assembly across a fixed 5 volt supply, we then measure the voltage drop across the sensor. This generates a sensor voltage from about 1 V at 120°F down to about 0.080V at 300°F. These voltages are measured by the Analog/Digital Converter (A/DC) inside the D180. These voltages are extremely low so even small losses will result in large errors and because the sensor is non-linear, these errors increase dramatically with temperature.

Design issues can prevent the D180’s A/DC from seeing all the voltage from temperature sensor. When this happens the displayed temperature indicates higher then actual temperature. With a signal loss of 100mV, the display may indicate 234°F, well into the yellow arc, while the actual temperature is only 204°F. To understand what’s going on we need to calculate the wiring voltage drops and load them into a big picture. In the drawing below (Vsensor) is the voltage provided by the sensor and (Vinput) in the voltage as seen at the input of the D180.


jpg drawing
https://drive.google.com/open?id=1HQ...ZqwQiFpDve3nak

or the pdf version
https://drive.google.com/open?id=1H2...Vz2bxa68OrnB7O


Looking at the diagram it’s clear where we need to concentrate our attention. The voltage drops are excessive between the D180 Master Ground pin to the airframe grounding point, but only when the master is ON. These are worse case calculated numbers. In the real world the voltage drops are less and will vary between aircraft because the D180 attach screw will shunt some of the ground load. Relying on the D180 attach screw as a grounding point is a poor alternative to a properly designed ground. I was unable to get the voltage drop below 20mV using the attach screw. This caused a 5°F error mid scale and near the top end at 294°F actual, it was indicating 317°. While this was a vast improvement over my initial 100mV losses, we can do better.

In the above drawing I included voltage drops that appear to be irrelevant, these are included to illustrate the big picture. Often with grounding problems the common response is to blindly “add another ground” or “run a wire to the battery.” If you look closely, adding additional engine grounding does nothing as these drops are already near zero. And connecting the engine sensor ground directly to the negative battery post, if that were possible, would only add to the voltage drop.

The maximum wire size a D-sub pin will accept is #20. Using material on hand, I installed D-sub pins in one end of three 22-awg wires 1.5 inches long, spliced to a piece of 12-awg wire 15 inches long. (I would have preferred to use 20-awg, but I didn’t have any mil spec wire that size at the time, I recommend using 20-awg.) The female D-sub pins were inserted into the unused ground pins 5, 16 and 17 of the 37 pin connector on the back of the D180. The 12-awg wire’s ring terminal is attached to an existing AN3 bolt, for a wire harness Adell clamp, on the panel base directly behind the D180. This reduced my voltage drop to 4mV. Now at 294°F actual I am indicating 298°. I expect the error to be less than one 1°F mid-scale. Everything is accessible by removing the D180 from its tray. There is plenty of room to work with no need to open the pitot/static/AOA lines.

It’s a simple procedure to test for ground side losses. Other than a 10 cent resistor, no special tools, meters or skills are needed. The greatest errors are on the high end of the temperature scale, so that’s where we need to test it. A 22 Ω resistor will indicate about 290°F-300°F depending on the tolerance. The actual displayed value doesn’t matter because we aren’t checking calibration, we’re testing for losses.
  1. 1) Temporarily remove the wire from the sensor, connect it to one side of a 22 Ω resistor, ground the other side of the resistor to the engine crankcase.
  2. 2) Turn the Master Switch ON.
  3. 3) Record the value displayed on the D180 for that sensor, call this the “Master ON Value.”
  4. 4) Turn the Maser Switch OFF (the display will remain on for 30 seconds powered by its internal battery.)
  5. 5) Record the value displayed for that sensor, call this the “Master OFF Value.”
  6. 6) Ideally the two readings should be equal. The larger the error, the more ground side losses you have.


It’s an easy fix. With the wire pre fabricated, the job takes about 30 minuets. If you don’t have the wire or the D-sub pin crimping tool, I’m sure SteinAir can make one up for less than the cost of the $30 tool.
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  #2  
Old 08-26-2019, 09:37 AM
Piper J3's Avatar
Piper J3 Piper J3 is offline
 
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If I’m following this correctly… a problem may show up as higher indicated temp on a D-180 Vs. actual temp if there is an electrical grounding problem. I suspect this would happen for either oil or coolant temps.

So, if my D-180 stays in the “Green” – I don’t have a ground problem? I only see oil temp in the low Yellow arc during extended climb in very hot weather. When I push over into cruise the oil temp returns to Green arc.

If poor ground causes higher indicated temp that seems like a fairly safe error. It would be bad if the problem showed temps lower than actual.

Am I looking at this right?
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  #3  
Old 08-26-2019, 11:04 AM
NinerBikes NinerBikes is offline
 
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Would a poor grounding scenario causing micro voltage loss cause a similar scenario in a D1000 SkyView Touch equipped RV-12?

Friend is having some issues on 30 to 33C days with oil temps on climbs at 85 kts climbing into the yellow and running at 5050 - 5100 rpm with throttle about 1.25 to 1.5 " from the wall. He wants to direct more air to the oil sump tank or add alum fins around oil tank for more heat dissipation.
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  #4  
Old 08-26-2019, 11:18 AM
MMiller MMiller is offline
 
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Location: Babylon NY
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Quote:
Originally Posted by Piper J3 View Post
Am I looking at this right?
Correct. If you are content with the operation of your display great.

I’m just offering my experience. I had unacceptable indicated temperatures and this is what I found. I consider it a design issue.



Quote:
Originally Posted by NinerBikes View Post
Would a poor grounding scenario causing micro voltage loss cause a similar scenario in a D1000 SkyView Touch equipped RV-12?.
I don’t know. The test I described above could be used to test for ground side losses in the shared ground conductors, but the fix won’t be the same.
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  #5  
Old 08-26-2019, 12:37 PM
Mich48041 Mich48041 is offline
 
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The D-180 Installation Guide agrees with Mike M. It states,
Quote:
. . . even very small resistances between battery ground and instrument
ground can cause voltage differences which adversely affect engine sensor
readings. . . .With the FlightDEK-D180 powered on, connect one lead of your
voltmeter to a free ground lead coming from the FlightDEK-D180. Connect the
other lead of your voltmeter to the ground terminal of your battery. The voltage
between these two points should measure very close to 0 mV (within 5 mV).
The test is best performed while ALL electrical loads are turned on.
This is a valid test no matter what the brand or model of avionics.
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  #6  
Old 08-26-2019, 10:16 PM
RFSchaller RFSchaller is offline
 
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I had spiking oil and coolant temps. I sent my D180 in to Dynon, and they replaced the EMS board. Problem solved.
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  #7  
Old 08-27-2019, 09:23 AM
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AllThumbs AllThumbs is offline
 
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Hi Mike,

We ran the test on a friend's 12. All I had was 10 ohm so it read 340F with just the D180 running on internal battery. When we turned on everything it jumped to 402F. Wow.

Do you have a theory as to why this happens when the D180 is powered externally and under load? As I turned on each additional load item the temp went up. Does that imply that the supply voltage running through the thermistor is decreasing? How would the ground change due to load?
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  #8  
Old 08-27-2019, 09:45 AM
NinerBikes NinerBikes is offline
 
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Quote:
Originally Posted by AllThumbs View Post
Hi Mike,

We ran the test on a friend's 12. All I had was 10 ohm so it read 340F with just the D180 running on internal battery. When we turned on everything it jumped to 402F. Wow.

Do you have a theory as to why this happens when the D180 is powered externally and under load? As I turned on each additional load item the temp went up. Does that imply that the supply voltage running through the thermistor is decreasing? How would the ground change due to load?
All it take is one resistive point in the ground connections wiring from battery to frame points to indeed start dropping the voltage as more load is placed on either the battery or the alternator.

Which means you need to go in and clean up all the grounding points where ground wire meets the frame or panel.
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  #9  
Old 08-27-2019, 06:52 PM
MMiller MMiller is offline
 
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Location: Babylon NY
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Quote:
Originally Posted by RFSchaller View Post
I had spiking oil and coolant temps. I sent my D180 in to Dynon, and they replaced the EMS board. Problem solved.
Rich, are you sure the unit was broken, this is the exact problem I had. I suspect re-seating the D180 in the tray improved the ground. You aren’t the first to send the D180 in for repair because of temp issues.




Quote:
Originally Posted by AllThumbs View Post
Hi Mike,

We ran the test on a friend's 12. All I had was 10 ohm so it read 340F with just the D180 running on internal battery. When we turned on everything it jumped to 402F. Wow.

Do you have a theory as to why this happens when the D180 is powered externally and under load? As I turned on each additional load item the temp went up. Does that imply that the supply voltage running through the thermistor is decreasing? How would the ground change due to load?
Neil, those numbers doesn’t surprise me. Dynon tops off the indicator at 402°F . Based on your numbers I would estimate you have at lease a 41mV loss (maybe more because the Dynon is over scale.) As a guess I would estimate, at 230°F indicated you would be 217°F actual.

Let me try and explain again what’s happening.
Let’s say the engine is at 181°F, the sensor has a voltage of 0.335 volts. The display knows if it sees .0335 volts it should display 181°F. But the ground wire into the display has power current and sensor current running through it. On battery power all the sensor voltage makes it to the display input, so it reads correctly. But with the master ON we pull power through that ground wire, this causes a voltage drop in the wire, say 0.114 volts. Now the display only sees 0.241 volts of that of 0.335 volts so it indicates 209°F.

Here’s a chart that may help.

https://drive.google.com/open?id=1Im...9yp7aUo9P7-gIi

Note: these are sample numbers for this example, not actual Rotax values.

Turning on additional items makes it worse because the D180 is grounded to the Vans PCB along with all the avionics and then it connects to the airframe ground. so part of your ground wire is shared.

The fix is to use all the available ground pins on the Dynon, Use the largest wire possible and keep the runs as short as possible. Maybe move Dynon "Master Ground" from the Vans PCB and connect it directly to the airframe, (with wire lagrer than the 22-awg that there now.)
Here is a drawing of sample wire that can be made up, it shows a worksheet of resistance and calculated voltage drops for various wire sizes and combinations. I would use 3 pins at 20-awg spliced to 12-awg.

https://drive.google.com/open?id=1Yi...Bku4jqbB-KqCH1

Last edited by MMiller : 08-29-2019 at 06:48 AM.
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  #10  
Old 08-30-2019, 10:43 PM
RFSchaller RFSchaller is offline
 
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Mike,

I reseated all the connectors and used electrical cleaner on each connector before I sent my D180 in for repair. Dirty connectors are an easy assumption, but not necessarily the right one.

Rich
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