Manifold pressure error

[mhubel]

I just installed a new Dynon Manifold pressure sensor on a D180. First issue is that it reads 27.7 in with the engine off while the actual pressure was 30.4 in. I am used to mechanical gauges that are within a needle width of actual.
Just how accurate should I expect this to be? I would think almost 10% off is a bit much.
Also, even given the error, the wide open throttle it only reads 23 inches add 10 % and that is 25 in, which is still a bit hard to believe. Is there a set of rules about the location for the pressure pickup location? There was no place for so I had to make one.

 

[dynonsupport]

What altitude are you at? The MAP sensor only matches your kollsman window if the aircraft is actually at sea level. The kollsman window is the pressure at zero MSL while the MAP is the pressure at your current altitude.

I'd bet you're at 2,500 FT above MSL.

It should be accurate within about 0.2 inHg.

Same in flight. 23" is full atmospheric pressure at 7,000'.

 

[mhubel]

Everything I am talking about is effectively at sea level (300 feet). That could only explain about 1/4 inch of the error.

Is the voltage that should come out of the sensor published anywhere? Then I can double check the accuracy of the sensor independent of the D180.

 

[dynonsupport]

I don't have access to the voltage chart while remote here at the AOPA expo, unfortunatlely. Are you able to check that the 5V excitation line is actually 5V at(or near) the MAP sensor? That could affect things too.

 

[mhubel]

I am also at a convention (not as interesting) so I will check when I get back. The +5V and Gnd are using the wires directly from the D180 but there could still be an error. Also many electronic senders I have used of this type have internal references so their output is stable over a moderate range of supply voltage.

 

[mhubel]

The problem appears to be solved. The primary issue of the pressure never returning to ambient appears to have been caused by the hose. In an area were it was bent a moderate amount, it collapsed at engine idle. Then if the engine was shut down, there would be no vibration to re-open it and the reading would stay stuck low for long periods of time.

The hose was replaced with a much thicker walled version and that problem went away. I also put in a restriction at the carb end using a #60 drill through a 1/2 inch long plug in the hose fitting. The reading is nice and stable now.