Is there a 0-5v to absolute pressure caculator?

[tryingbe]

Is there a 0-5v to 3 bar map sensor calculator out there?

For example, my map sensor read 1.22v, what is that translate into in absolute pressure number in a 3 bar system?

I've been using this, it works ok, but it's hard to get very accurate on it.

http://hhscott.com/93tmv/images/3BARv2BAR_Map.gif

http://hhscott.com/93tmv/images/3BARv2BAR_Map.gif

[wowzer]

1.22/5 * 44.1 = 10.76 psi (in its simplest form). however, you will also see the 3 bar represented as 43.51. typically you can find the actual formula for a sensor. what sensor are you using? e.g. a gm 3 bar map?

[tryingbe]

1.22/5 * 43.5114 = 10.62 psi (in its simplest form)

Thanks for the response, but I need some clarification.


What number does the absolute pressure of 10.62 psi translate into the -14.7psi to 29.4 psi graph used in MPTuner?

http://www.thelostartof.net/tryingbe/dodge/omniproject/tune/partthrottle.JPG

[wowzer]

using that graph it actually would be 1.22/5 * 44.1 = 10.76. to get it to the graph range you would then subtract 1 atmosphere (14.7). so 1.22 volts = 10.76 (absolute) - 14.7 = -3.94 psi. since it is negative that means you are in vacuum vs boost which is usually indicated in inches of mercury. so the -3.94 psi * 2.036254 = -8.023 inches of vacuum

[tryingbe]

So, let me get this repeat this to you.

If my OTC scanner says my map voltage is 1.22V, it means I'm at the point of -3.94 psi at the graph above, correct?


Thanks.

[wowzer]

yes. 1.22V should be approximately -3.94 on the graph

[wowzer]

btw, for the gm 3 bar sensor i believe the actual calc would be:

Volts *8.94301 - .161, so -->
1.22 * 8.94301 - .161 = 10.7495 psia - 14.7 = -3.9505 psi

so, there is a little bit of error using the simple form but its close enough for our purposes.


****edit****
the documentation i have on the gm 3bar sensor shows a function of:
Vs * (.00318 * kpa - .00353) = Vout

[A.J.]

Thanks for the response, but I need some clarification.


What number does the absolute pressure of 10.62 psi translate into the -14.7psi to 29.4 psi graph used in MPTuner?

http://www.thelostartof.net/tryingbe/dodge/omniproject/tune/partthrottle.JPG

I made a chart for that I can show you/let you copy next time you come by my shop.

A.J.

[RoadWarrior222]

I wouldn't bet the farm on it being more accurate than +/- 5% BTW.

---------- Post added at 07:55 AM ---------- Previous post was at 07:47 AM ----------

Oh, also there will be a quantisation thing, not sure of the resolution of the analog to digital conversion in the SMEC/SBEC but there will only be 64, 128, or 256 discrete values... I think that info is around somewhere, but it would take digging out since it's a bit arcane... but if you want as precise as possible you'd have to be aware of the overlap in the values possible... 256 values = 8 bit resolution means a granularity of 0.02V ... meaning it's about 0.2 PSI either way.. if the resolution is lower it's as bad as .4 or .8 PSI

[tryingbe]

I can deal with +/- .2v. Thanks wowzer!

[wowzer]

I can deal with +/- .2v. Thanks wowzer!

the fact that it's that close is probably more coincidence than accuracy! the gm map sensor has a fair amount of error allowed in the sensor. plus the formula is based on 5.1 volts so if the voltage to your sensor is different then that adds more error. rw222 is probably right.