Thread: 12V tyre inflator gauge reading?

I got a 12V tyre inflator. Not top end, but not bottom end either. When connected to a tyre and turned on, the gauge reads way higher than when connected to a tyre and turned off. Is this normal?

I never trust the inflator gauge, have a reasonable quality standard gauge I use for obtaining correct pressure, my 12 volt inflator reads about 45psi to obtain 35psi.

Originally Posted by tetawa

I never trust the inflator gauge, have a reasonable quality standard gauge I use for obtaining correct pressure, my 12 volt inflator reads about 45psi to obtain 35psi.

I realise that could also be an issue, but don't understand why the gauge reads higher when the unit is turned on. In fact, it reads about 25 PSI when not connected to anything. Thought I could subtract 25 PSI from the gauge reading, but didn't work out like that. It's usable as it is, but not as convenient as it could be.

The gauge on the inflator for my compressor does the same thing - reads high when inflating, but reads correctly when connected but not inflating.
I assume it's reading the pressure correctly, and the pressure is actually higher when inflating.

Sounds like your gauge is poked. Normally the inbuilt gauges are cheap, nasty and the part that lets down the rest of the machine - basically saving the manufacturer a few cents a unit in production costs but ending up costing them customer satisfaction.

You can usually replace them with a standard gauge from an engineering supplier but they can cost more than the unit is worth in a lot of cases. Probably worthwhile doing with a powered inflator though.

Also, fairly normal to have the gauge reading high when inflating as you aren't reading the static pressure on the tyre but the pressure in the inflator which is required to force the air through the system into the tyre.

Originally Posted by No.3

Sounds like your gauge is poked. .

Gauge with the unit turned off is accurate.

Originally Posted by No.3

Also, fairly normal to have the gauge reading high when inflating as you aren't reading the static pressure on the tyre but the pressure in the inflator which is required to force the air through the system into the tyre.

They should be in equilibrium soon after the valve is opened, but whatever. Guess I'll live with it as can't be bothered explaining to shop staff and getting a replacement which will most likely do the same thing.

Originally Posted by MB

They should be in equilibrium soon after the valve is opened, but whatever.

Yeah nah, the air won't flow into the tyre and pump it up if there isn't higher pressure in the inflator than the tyre.

Originally Posted by Cigar

Yeah nah, the air won't flow into the tyre and pump it up if there isn't higher pressure in the inflator than the tyre.

Doesn't mean it won't be equalised. The entire system should be at the same pressure, that's how it works when filling dive tanks.

It’s higher pressure in the hose while it’s running due to the flow resistance in the Schroeder valve. We used to let a little air out before reading the pressure on our pneumatic inflators to let everything equalise.

Originally Posted by 2post

It’s higher pressure in the hose while it’s running due to the flow resistance in the Schroeder valve. We used to let a little air out before reading the pressure on our pneumatic inflators to let everything equalise.

OK, that makes sense. Thanks.

Originally Posted by MB

Gauge with the unit turned off is accurate.

I took it to mean that your gauge isn't returning to zero when there isn't any pressure on the unit at all? What that means is that the bourdon tube in the gauge has stretched for want of a better description, and it will only be accurate now in one part of the range. Also, it can be variable where that range is. If you have a known good gauge you might be able to dissasemble the thing and reshape the tube so that the pointer reads zero at rest without pushing onto the zero pin, and it will probably be pretty accurate over the range again - you can compare to the known good gauge to make sure it's good. Or not depending how out of shape the tube is...

AS @2post states, the gauge is reading the pressure in the hose, as the tyre inflates with the pump running it creates more back pressure which increases the pressure in the tube giving you higher readings. . When you turn the pump off the pressure equalizes between the tyre and the pump so your gauge reads more true. Pump gauges aren't known for accuracy but can give a reasonable indication. Use a decent gauge to check pressures when done. My reasonably top end comp gauge reads 6-8 PSI lower than my mates identical comp.

Yup, to send the above… Being a 4WDer I am forever changing tyre pressures. It’s the most effective tool you have off-road. Some days in the Outback you’d be deflating / inflating a dozen times a day. On the farms and braided rivers here I deflate routinely. Quads too, lots are run way over-inflated.

I carry a decent hand held gauge. I never trust the random gauge on the compressors at the garage. Even the one on my onboard ARB compressor is wrong. Some of the gauges at the garage are spot on, particularly the self-inflating types like BP, but many regular manual types are way off. It’s not a hassle to use a hand held gauge and its worthwhile to have your tyres are exactly the pressure you intend, not 7 or 8 psi off!