Is my boost gauge stuffed?

[Slim]

I've bought an Autometer Procomp boost gauge and i haven't yet installed it. I've been wondering if it's normal for it to be reading 1.25 psi normally. What's atmospheric pressure in psi? Is that what it is or is my gauge broken before i've even used it?

[ra65]

Broken. Atmosphere is 0

[Jin~Jah]

technically atmospheric pressure is 14.7psi, but the guage should read 0psi if it's got no input (i.e. if the engine is off, or if it's not actually connected to anything)

it's possible that it's had a bit of a shock at some point, and the inlay thing with the scale on it has turned slightly, or the needle has been knocked slightly

edit: beaten!!!!

survey says it's dodgy

you may find it still functions properly, but reads 1.25psi higher than it should at all times

[Slim]

Well i did drop it today but it's always read the same. (ie: the fall didn't affect it... plus it was only dropped onto the bench not the ground).

Should i be taking it back? Maybe it needs to be used and it'll settle down.

Edit: the other thing is... i painted the sides of it so it is hidden better in the location i have mounted it. I hope that won't affect taking it back under warranty. See pic.

[rms]

A boost gauge reads absolute pressure. Even an accurate one (which they are not) will only show zero at sea level and will show different readings at different locations.

[Hiro]

A boost gauge reads absolute pressure. Even an accurate one (which they are not) will only show zero at sea level and will show different readings at different locations.

Boost gauges read relative pressure, also known as _gauge_ pressure (for obvious reasons). Gauge pressure is always relative to atmospheric pressure. A gauge reading absolute pressure will read 101.3kPa for atmospheric pressure (approx 1 bar), but a gauge reading gauge pressure will read 0. This also why you can never get a "vacuum" of greater than 101.3kPa (negative atmospheric pressure)

[rms]

So how does a relative gauge know where zero is when atmospheric pressure (the relative part) changes with location and altitude ?

[Jin~Jah]

So how does a relative gauge know where zero is when atmospheric pressure (the relative part) changes with location and altitude ?

unfortunately that's why it's tricky to be perfectly accurate. there are so many variables involved. my cheapo boost guage reads a bee's dick under 0 when the car is off (even less than how far slim's reads over)

it still works fine, nothing wrong with the sweep, and gives me an idea of what's going on, which is why it's there (mind you it is a cheapie)

[Snyper]

If you look at the back of your gauge, take the pight bulb out, you should see a big round brass coloured thing in there, get a small screw driver, push on this and look at the face of your gauge, you'll notice the needle moves, push it the opposite way and the needle moves the other way and so forth, just "adjust" this by pushing it so the needle sits on zero...
How did I find this out?? curiousity.... I just played around with a gauge one day, stuck a screwdriver in the back as you do and realised this was how they worked..
give it a try.

Brendon

[Kyosho]

I have that same boost guage.
Mine reads about 1.5PSi UNDER zero all the time when the cars off.

Give it a few flicks and it gets slightly closer to zero, but never there.

They're a slightly inaccurate guage, but they're useful for identifying spikes etc, really, you should set your boost on a dyno and use there pressure sensor.

[Aust162]

Slim, (edit:this is going to sound bad) try sucking the back of the gauge with your mouth to reset it. see if you can get it read on zero. Alot of people have questioned the quality of autometer guages. They aren't the best. I had a 2" boost guage from them and it would sometimes not set right on zero when i turned the car off, but sometimes it would..

[DEW-016]

Similarly have a pro-comp 2" in the 20psi range that sits slightly high (but always drops back to same spot when off). Was like this from the day i got it at the shop.
Still reads boost responsively, just obviously shows slightly over the true value.

[Slim]

Sounds like the general consensus it that i shouldn't bother taking it back because they're all dodgy... and then there's the opinion of the one pervert that wants me to suck off my boost gauge.



I think i'll just live with it... maybe try the trick that Snyper suggested but even if it doesn't work i'll just read it as though boost is always 1 psi higher than it really is.

[omp]

An old mate of mine has one of the liquid filled Auto meter boost gauges and he has to "reset" (via the screw) it. Told me has to do it regularly.....