Why Cheap Pressure Gauges Fail

[rossiz]

ok, wow...

that's really cool - where did you pull the vacuum? would this be a good tool to cross-check for vacuum leaks?

what if i have crappy vacuum in my system (even w/no leaks) would that screw things up and give false fuel pressure readings?

and, of course - cost and source must be shared, please...

[Captain Obvious]

The gauge is from Orange Research and I got it off ebay for $40. So it's not disposable and lives in the toolbox as a diagnostic aid. It's a little too valuable and bulky to be a permanent installation. I got Orange Research 1203-PGS series.

I pulled the vacuum off the intake manifold off the carbon canister purge source which is the nipple near the cold start injector. I don't see any reason why you couldn't use that gauge as a standard vacuum gauge by hooking up the LO side and leaving the HI side open to atmosphere. Here's how that gauge works...

Inside there is a floating piston and cup seal that is spring loaded to one direction. The pressure ports are connected to a cavity on either side of that piston and push/pull it to one side or the other depending on the magnitudes of the pressures. The spring pushes toward the HI side, so pressure on the HI side compresses the spring and pushes the piston towards the LO side. Vacuum on the LO side will also compress the spring and pull the piston towards the LO side.

One of the neatest parts to the whole thing is that there is no mechanical connection between the piston and the needle movement... It's MAGNETICALLY COUPLED!! There are no springs to wear out, no gears to strip, nothing like that. The only link between the wetware and the needle is the magnet.

Here's a sketch of the guts design:

They also make a diaphragm version for gaseous applications where there is a diaphragm between HI and LO instead or a cup seal. The diaphragm does a better job of sealing between the two sides, but for liquids you don't need that.

So it's true real time differential.

Don't have to do the math in your head at real time. It does it for you.

No springs, no gears, no linkages.

Cup seal between HI and LO sides. (I'm not positive what seal composition I got, but it's either Buna-N or Teflon, either of which is suitable.)

It's also got a reed switch built into it that you can adjust to close at a desired pressure if you want to set off an indication or something. Red warning lamp if the differential gets too high or too low. Of course, I'm not connecting to that feature, but it's there.

[Captain Obvious]

what if i have crappy vacuum in my system (even w/no leaks) would that screw things up and give false fuel pressure readings?

Forgot about this part...

No. It will read true under all conditions and that's part of the beauty of it. The trick is that if you've got vacuum problems, your fuel pressure regulator will be adjusting the pressure WRT to the same leaky system.

In other words, even if you've got a vacuum leak, your fuel pressure should be OK because the regulator is referenced to the same leaky source. And this kind of gauge, being also referenced to the same leaky source should show proper fuel pressure even if there is a vacuum leak.

[rossiz]

way cool - i like the way that works.

i can see a setup with the ability to shut off the pressure and/or vacuum independently via a couple of simple valves it would be a nice diagnostic tool - especially with a dual-scale dial that would also read in-hg.

love it!

[Captain Obvious]

I'd be happy to send mine on loan if you cover shipping. It's a big brass chunk so it's not the lightest device in the world, but I bet it could make it round trip for less than $20. I might be able to take the reed switch housing off the back, and if so, it might even fit into the smallest flat rate priority box which is cheaper.

Shoot me a PM if you want.

[FastWoman]

Very cool! I checked ebay, and there was a nice 50 psi one for about $20 used. Unfortunately you may have temporarily set that market on fire. The gauge sold in the few minutes between when I did the search and when I looked at that listing! The rest are considerably more expensive or have inappropriate scales. Grrrrrr.... But I'll definitely keep my eye out for one. Thanks for passing along the tip to those of your Neanderthals friends who aspire to be Cro-Magnon!

Edited September 29, 2014 by FastWoman

[Captain Obvious]

Sorry if just created a run on those gauges. Keep your eyes out and I bet you can snag one. I didn't look into it, but I bet I don't even want to know what they go for new from an authorized distributor!!

And here, I made this for you:

[FastWoman]

Mmmmm.... One horsepower! Me like...