46% error in pressure gauge?

[pat123]

Me and Drex have been working on a hybrid and we couldn't get the fueling right. I think i found out why.

I found the volume of the meter and the volume of the chamber + meter by filling it with water. the chamber + the meter was 759mL and the meter was 74mL.

I then pressurized the meter to 100 psi and opened the valve letting it into the entire gun and the pressure dropped to 18 psi. so if the meter was 74mL then 74ml x 100psi/18psi = 411.1mL. so the chamber + the meter would be 411mL.

% error = (|Result - Accepted Value| / Accepted Value) x 100
% error = (l 411.1 - 759l / 759) x 100
%error = 45.8%

I bought the pressure gauge at lowes or homedepot. I just bought 3 new gauges that should be better. has anyone else had this problem with a pressure gauge?

[Gun Freak]

Sounds like a defective product, try returning it.

[Technician1002]

Use a welding regulator gauge. Use a gauge that will read the target pressure in the upper 2/3rds of the scale.

[pat123]

I don't think i can return it, I've had it for over a year. anyways I'm getting some that actually have at least every 5 psi increment marked. I'm getting a 0-30psi and 0-60psi which are in 1 psi increments for propane fueling and another 0-160 for air. do you think I need a gauge with more the 1 psi increments to accurately meter the propane? I used one with 5 psi increments for my last hybrid and it actually fired most of the time.

[jimmy101]

Since you have measured and calculated the apparent error of the gauge just figure out what it will say when it is at the pressure you want.

Basically, correct the gauge reading using the numbers you've already measured.

Is the gauge supposed to read absolute pressure or gauge pressure?
For absolute pressure your equation;
74ml x 100psi/18psi = 411.1mL
is correct.

For gauge pressure, I think you need to add in the 1 ATM of pressure;
74ml x (100+15)psi/(18+15)psi = 257mL

That's one toasted gauge you got there.

(Of course, the calc's assume there is no heat lost or gain, which probably isn't true, but that shouldn't be enough to give an error this big.)

Edit: is that math correct.

The pressure times volume at the start should equal the pressure times volume at the end. (ignoring heat loss ... and not worrying about proper units )

(meter vol)(meter pressure) + (chamber vol)(chamber pressure)
= (meter + chamber vol)(final pressure)

(74mL)(115PSIA) +(759ml)(15PSIA) = (74mL + 759mL)(33PSIA)
19895 = 27489

Or, rearragning and prediciting what the final pressure reading should be gives
{(74mL)(115PSIA) +(759ml)(15PSIA)}/(74mL + 759mL) = final pressure in PSIA = 23.9 PSIA = 9 PSIG

(playing fast and loose with the sig. figs.)