Dynon sending unit internal wear

Have you ever wondered what happens when the sending unit on your Fuel Press/Oil Press/Oil Temp/ etc goes bad? On our Dynon unit when the resistance on the unit drops, the reading on the engine monitor goes to the highest reading (and so does your blood pressure). We have had a fuel pressure reading that started to be inconsistent, and then would stay at 99PSI when the boost pump was on or the engine was running.
Trouble shooting the unit, I found the readings to be accurate near the low end, but then as the pressure rose to the normal levels, the indication was high. This was confirmed with an ohm meter at the sending unit. The high pressure reading was basically ?open?, the same as if the ring terminal was removed from the post.
A spare sending unit was installed and it appears all is normal. Have you ever wondered what breaks down as the unit wears? Here is the photo of the sending unit with the cover cut off:
http://i54.tinypic.com/ejc3mb.jpg

Take special note of the small wear mark on the resistive material to the right of the ?pointer?. This is evidently where the fuel pressure normally runs at 25PSI on our injected engine. It also is why once the ?pointer? moves to the left of the wear spot, the resistance goes to zero and the engine monitor displays 99psi.
Hope you find this interesting.

Yep;
Good classroom presentation.

Interesting wear mark. How was the sensor mounted?

The Fuel pressure sender was firewall mounted on the manifold block supplied from Vans.

Very interesting

I never knew what was inside those things, thanks for opening it up. I have a couple of spares, now I'm obliged to have a look inside. (Included a smaller version of your pic below)

I have to replace mine every 2 years. The normally steady reading of 28psi starts to become a bit inconsistent such as dropping to 25psi for a second then back up to 28. Gradually this becomes worse and the readings jump from low to high. Mine is also mounted on the firewall with the vans manifold block. I now keep a spare.

Thanks for the picture. Very interesting and also an unbelievably crude mechanical device. With all the elecronic gizomos around these days, isn't there some solid state sensor the size of say, your fingernail, that will sense this pressure and last 2 lifetimes? I built a Rocky Mountain 'micro encoder' on my first airplane (from a kit), that measured temperatures and pressures (airspeed and altitude). They were solid state - no moving parts - and once tested, were very accurate and in theory would last practically forever. Probably not a lot of money in pressure senders.

Scott Hersha said:

Thanks for the picture. Very interesting and also an unbelievably crude mechanical device. With all the elecronic gizomos around these days, isn't there some solid state sensor the size of say, your fingernail, that will sense this pressure and last 2 lifetimes? .

Click to expand...

Yes - Garmin supplies them the G3X system - compact, light, accurate...and they have real connectors on them!

These type of sensors are used all over the place with different brands of EFIS/EMS systems not to mention millions of land based vehicles. Not exclusive to Dynon and I doubt any EFIS/EMS maker manufactures their own sensors.

It would be nice to have solid state alternatives though.

The problem is that the EFIS/EMS needs to either be able to be calibrated to or have the sensor profile embedded in the setup options to be able to make the switch.

I think at least one EFIS/EMS maker is providing SS sensors for their EMS.

My fuel pressure sensor from GRT has a small form factor that is very different than the picture posted, so I suspect it is solid state. Curiously, my oil pressure sensor is like the picture, so not sure what gives there.

Erich

Brantel said:

These type of sensors are used all over the place with different brands of EFIS/EMS systems not to mention millions of land based vehicles. Not exclusive to Dynon and I doubt any EFIS/EMS maker manufactures their own sensors.

It would be nice to have solid state alternatives though.

The problem is that the EFIS/EMS needs to either be able to be calibrated to or have the sensor profile embedded in the setup options to be able to make the switch.

I think at least one EFIS/EMS maker is providing SS sensors for their EMS.

Click to expand...

I believe they all use the VDO style senders. They seem to come in 10-180 ohm ranges or the SW standard of 240-33 ohms - based on a max. pressure rating.

The temperature sender curves are here, including accuracy % - I presume there is a pressure sender document around too - it's in German but easy enough to follow -

http://tinymicros.com/mediawiki/images/0/0c/VDO_Datasheet.pdf

This is on a VDO web site -

Unlike many companies today, we manufacturer our senders in our own ISO and TSO registered facilities, using the same quality materials and engineering processes, to meet the same quality standards as our OE products.

Perhaps we are just seeing the difference between genuine VDO parts and cheaper (Chinese?) senders.

inquiring mind wants to know

Originally Posted by Scott Hersha
Thanks for the picture. Very interesting and also an unbelievably crude mechanical device. With all the elecronic gizomos around these days, isn't there some solid state sensor the size of say, your fingernail, that will sense this pressure and last 2 lifetimes? .

Yes - Garmin supplies them the G3X system - compact, light, accurate...and they have real connectors on them! - Ironflight

Click to expand...

Ok, if no one else is going to ask, I would like some details about the Garmin hardware. Cost? manufacturer, or supplier? npt threads? pressure vs. resistance values? size?

Lux Wrangler said:

Ok, if no one else is going to ask, I would like some details about the Garmin hardware. Cost? manufacturer, or supplier? npt threads? pressure vs. resistance values? size?

Click to expand...

The G3X system primarily uses Kavlico P4055 series pressure senders. You can get an idea of the end-user cost by looking at the Aircraft Spruce price list. Roughly speaking, they're about a hundred bucks apiece.

mcb

I stand corrected.... I guess there is a lot of money in senders!

Pressure transducers

The sensor that Garmin uses a "load cell" type, or piezoelectric. It has no moving parts but instead generates an extreamly small amount of current when compressed. VDO happens to also make this type of sensor, as do several other manufacturers. Here's a link to the VDO website page which shows the two different types of pressure sensors:

http://www.vdo.com/generator/www/co...or/pressure_sensors_en.html?activetab=3863446
Hope this helps to clear up some of the mystery.

Unreliable fuel pressure sensors!

My "Van's" fuel pressure sensor keeps going back and forth from approx. 1,5 to approx. 1,9 bar, at an approx. 1 sec. freq., sometimes and sometimes it is rock sollid. I attached a ground wire from the unit to the ground bus, to make sure I have good ground, but this does not make a difference.

I guess I will have to order a new one.

Regards, Tonny.

yeah, but

a piezo type sensor isn't going to work as a direct substitute for the resistance type mechanical sensor being discussed

Lux Wrangler said:

a piezo type sensor isn't going to work as a direct substitute for the resistance type mechanical sensor being discussed

Click to expand...

It would on a Skyview if you use an "enhanced" input pin and use the sender manufacturer's calibration curve to set up the Dynon to read correctly. It's all programable.

Interestingly, the Mitchell STC sender and Dynon's instructions talk about using an Adel clamp to support the VDO-type sender. The Adel rubber may provide a little high frequency damping.

I wonder if the sender gets too much vibration hanging forward on the vans manifold - it may be affected by the amount of stiffening in the location selected.

Originally Posted by Lux Wrangler
a piezo type sensor isn't going to work as a direct substitute for the resistance type mechanical sensor being discussed

It would on a Skyview if you use an "enhanced" input pin and use the sender manufacturer's calibration curve to set up the Dynon to read correctly. It's all programable. - az_gila

Click to expand...

"...an enhanced input pin... the sender manufacturer's calibration curve... It's all programable." , obviously we have a different definition of "direct replacement"; not to mention that not everyone has the Skyview.

Lux Wrangler said:

"...an enhanced input pin... the sender manufacturer's calibration curve... It's all programable." , obviously we have a different definition of "direct replacement"; not to mention that not everyone has the Skyview.

Click to expand...

The original post was about a Dynon unit - being a RV-10, I assumed a later Skyview unit...

It is replaceable, and I mentioned what is needed. A lot of other EMS units cannot handle the current input created by these solid state units, Dynon's Skyview can on three specific pins. Those pins can then be programmed to any of the usual (or not so usual) EMS functions.