Gauge oddities part 2

[joe_db]

Some may remember my fuel pressure issues that I now suspect were caused by the expansion and contraction of the fluid in the gauge. I studied the little rubber plug on top of the gauge used to fill it with glycerine and cut off just enough of it to make a small hole. Voila! Instant gain from 2 PSI to 4 PSI on the gauge Note this only works if the gauge is near vertical unless you want glycerine all over the place

Second thing was my Isspro vacuum gauge. It was always kind of erratic and I assumed it was because the hose is a loose fit on the barb. I got some tiny size wire ties to tighten the hose on and it did not cure the issue. I could reach around behind the gauge and make the needle jump still by jiggling the hose. Hmm.... Jiggling the WIRES also made the needle move. Seems the light bulb sticks in just far enough to sometimes touch the gauge mechanism. I switched to a red LED bulb awhile ago and can't recall if the original bulb did this too. The LED doesn't look any bigger. I'll be modding the light holder to not stick in quite so far this weekend. 1/8" will do it.

[hanleyclifford]

The best way to solve an erratic gauge issue is the use of a "snubber", available at McMaster. http://www.mcmaster.com/#catalog/120/589/=rciej8

[joe_db]

True if the issue is in the vacuum itself jumping around.
Not gonna help the light hitting the gauge mechanism though. Depending on how the light was twisted the reading was going from 3 to 20 inches.
It will very much help the fuel pressure gauge if you get a dry gauge to avoid the errors I found and don't want it jumping around.

The best way to solve an erratic gauge issue is the use of a "snubber", available at McMaster. http://www.mcmaster.com/#catalog/120/589/=rciej8

[hanleyclifford]

I'm grateful that you pioneered the user of liquid filled gauges in the engine room. Clearly the differentials in temperature are not friendly to such gauges and I for one was about to order some. Thanks again.

[joe_db]

Here is what I did. We will see if it fixes the issue.

[Dave Neptune]

Gage snubbing

Joe, snubbing the vac gage is easy, just use about 4 feet of 1/8" tubing. The gage will be quite steady at idle and smooth as you throttle up. I suggest copper and the poly is fine if you keep it away from heat. You light should not touch the needle or the vacuum bladder it works off of.

Also go for cheaper gages and don't worry about the needle moving a bit. It isn't the amount of pressure that is important just that it is present and enough. My cheap fuel pressure gage a 2" jumps around between 1 1/2~3 1/2 psi with the mechanical pump~who cares. You won't really get a steady flow unless your really pushing an A-4.

Dave Neptune

[ndutton]

Some may remember my fuel pressure issues that I now suspect were caused by the expansion and contraction of the fluid in the gauge. I studied the little rubber plug on top of the gauge used to fill it with glycerine and cut off just enough of it to make a small hole. Voila! Instant gain from 2 PSI to 4 PSI on the gauge

OK, gauge used to read 2 PSI, you modified it and now it reads 4 PSI. Got it. What is the actual fuel pressure? I think the modified gauge needs its accuracy confirmed before I'd trust the reading.

Analogy:
The temp gauge was running a little hotter than I thought it should. After I added a resistor in series with the sender the engine is running much cooler now.

[hanleyclifford]

I always thought the fluid in the gauge was meant to modulate the movement of the pointer itself whereas the snubber modulates the pressure input to the gauge. If the fluid is somehow affecting the reading doesn't that indicate a defective (internally leaking?) gauge?

[joe_db]

The fluid damps the movement of the gauge. It keeps the needle from jumping around quickly. What I did not realize at first is it was also changing the reading of the gauge. Since the fluid was trapped inside the gauge, as it expanded and contracted it changed the pressure on the outside of the mechanism. As it got hotter, the pressure on the outside would rise and the fuel pressure reading would fall to zero. I found numerous reports of this online after I knew what to google for. This winter was so cold it did the opposite and "gained" pressure while everything was shut down.

When I fired up the engine last week I decided to cut the top off the rubber nipple and leave it open to atmosphere. Since I was planning on replacing it anyway couldn't hurt, right? This changed the reading from 2 to 4 PSI. I can always get a second gauge to check it. I was planning on getting the electric fuel gauge from Chief Aircraft anyway, so no harm if it leaks a little glycerine out for my test. I am sure some gauge engineer somewhere is thinking EVERYONE knows you have to cut the ends off the plug, but I sure didn't

I always thought the fluid in the gauge was meant to modulate the movement of the pointer itself whereas the snubber modulates the pressure input to the gauge. If the fluid is somehow affecting the reading doesn't that indicate a defective (internally leaking?) gauge?

[hanleyclifford]

The fluid damps the movement of the gauge. It keeps the needle from jumping around quickly. What I did not realize at first is it was also changing the reading of the gauge. Since the fluid was trapped inside the gauge, as it expanded and contracted it changed the pressure on the outside of the mechanism. As it got hotter, the pressure on the outside would rise and the fuel pressure reading would fall to zero. I found numerous reports of this online after I knew what to google for. This winter was so cold it did the opposite and "gained" pressure while everything was shut down.

When I fired up the engine last week I decided to cut the top off the rubber nipple and leave it open to atmosphere. Since I was planning on replacing it anyway couldn't hurt, right? This changed the reading from 2 to 4 PSI. I can always get a second gauge to check it. I was planning on getting the electric fuel gauge from Chief Aircraft anyway, so no harm if it leaks a little glycerine out for my test. I am sure some gauge engineer somewhere is thinking EVERYONE knows you have to cut the ends off the plug, but I sure didn't

That makes two of us. If the foregoing is all true then glycerine filled gauges are only useful in controlled environments and should not be relied on in an A4 engine room. Leaving the top of a gauge open is a non-starter for me.

[joe_db]

Quite so. I would use a snubber instead for the same effect.

That makes two of us. If the foregoing is all true then glycerine filled gauges are only useful in controlled environments and should not be relied on in an A4 engine room. Leaving the top of a gauge open is a non-starter for me.

[joe_db]

Wish I had found this BEFORE I went gauge shopping
My liquid filled gauge went from showing 6 PSI to 0 PSI, do I have a faulty gauge?

The gauge is fine, you are most likely seeing the effect of temperature increase on the filled gauge offsetting real pressure.
All mechanical liquid filled gauge cases are sealed (to keep the liquid in) so as they heat up pressure will build up in the case (approx 1psi for every 30-40F temperature change). This case pressure exerts a force on the mechanical movement and offsets the actual process pressure. This effect is typically only noticable when measuring low pressures (i.e. on a 0-15 psi gauge).

For an accurate pressure reading on a liquid filled gauge you should vent the gauge at a specific temperature before taking a pressure reading. You should revent the gauge whenever the temperature of the gauge has changed significantly (either heated up or cooled down).

To vent the gauge, turn the gauge so that the air bubble is next to the fill plug and gently pull the vent plug to equalize the gauge case pressure with the atmosphere (reference pressure). Then carefully reinsert the fill plug so no liquid can leak out.

If you are taking readings on a frequent basis we recommend remote mounting the gauge with the plug in the upright position to make venting the gauge easy. You can even use a needle to put a very small hole in the plug to permanently vent the gauge. If you do this however, make sure the plug is in the upright position so no liquid fill leaks out.
from http://www.marshallinstruments.com/f...tail.cfm?id=26