Hmm... I'm not sure I have an answer for you right now. The conversion from kPa to PSIA is all done in C-Com and it's basic math that does it. The numbers you have up here certainly don't line up with the math used for the conversion.
There are a couple possibilities. One is that each of the dash sensors you have in the data log are updated individually. If you have a noisy MAP signal, it's possible that the PSIA reading, although presented in the same frame of data as the kPa reading, was slightly different than the kPa reading by the time it happened. We're talking about pretty small chunks of time, but it is a possibility.
I forgot what the other possibility was while I was writing about the first one. This stuff makes my brain hurt.
Did all the info you posted come from the same data log? Do you see this often?
I can't tell you for certain if all my data logs have a discrepency between KPS & PSIA or not as I haven't always had both measurements on the dashboard until recently.
I'm having a little problem with power drop off and I've been really disecting to find the problem. This is when I noticed the discrepency in the KPS and PSIA
I'm wondering if this is a bug in the program and if so, is it the only bug.
If I have a problem program, it certainly would make diagnostics difficult.
As Craig has said, the discrepency is probably the result of two things.
C-ComWP calculates the the simulated gauge pressure from the last MAP A/D conversion. The data log frame rate is asynchronous with the A/D sampling rate. It's certainly possible for the MAP (KPa) sensor to be sampled at a slightly different time than the MAP (PSIA) sensor and if there was any change in the pressure over this period, it would show up as a difference in the two measurements.
The conversion is straightforward in that it simply multiplies the raw A/D value by the KPa pressure constant for the programmed sensor type, multiplies the result by the Kpa/PSIA conversion constant, then subtracts off one standard atmosphere. It's possible that some integer round-off error could occur and result in small errors in the PSIA value.
Please keep in mind that the PSIA conversion is only used for user reference and not by the ECU for anything. It was added as a benefit to the tuner who may want to reference familiar numbers when evaluating log data. You must also remember that the PSIA number is not the same as the PSIG number you will see on the typical boost gauge. The boost gauge would be referenced to the present atmospheric pressure while the PSIA sensor reading is always referenced to a standard atmosphere.
As far as bugs are concerned, we have identified some minor bugs and there may even be some unidentified bugs in version 1.005. We rely in part on the users of C-ComWP to inform us of possible bugs and we will strive to identify and correct any that we are aware of.
It would be interesting to see if anyone else has experienced this difference in readings between KPS & PSIA . This would ensure that my system is normal.
Has there been other queries on this, prior to mine ??
Is there a PSIA/KPS conversion table anywhere on the net that I may print out ??
Originally posted by oldtimer Thanks Lance .... and Craig !
It would be interesting to see if anyone else has experienced this difference in readings between KPS & PSIA . This would ensure that my system is normal.
Has there been other queries on this, prior to mine ??
Is there a PSIA/KPS conversion table anywhere on the net that I may print out ??
Thought I'd type out a conversion table as a quick reference.
Took the figure ".1450" that you gave me and multiplied it by 235 for kPs.... figured I'd get some # close to 21 representing boost to match my F.A.S.T. data log numbers .. came out to 34.075 lbs.
I must be doing something wrong ... is the difference tied to the PSIA ( atmosphere) vs PSIG (guage)
Is there a conversion table available that converts kPs to PSIG ?
That wasn't me that posted that constant but that will let you calculate PSIA (absolute). To simulate gauge pressure (PSIG), you will need to subtract a standard atmosphere (14.7 PSI) from the absolute pressure.
Another way would be to use this formula: PSIG=(KPa-101.3)*0.145
It is a bit confusing because there is no way to derive true gauge pressure with a MAP sensor.
Um, if you take a MAP reading at key on, before engine cranking, can't you use that as your reference instead of 101.3 and get real psig? At least so long as a big weather front doesn't move in, or you drive up or down a mountain .
Originally posted by ijames Um, if you take a MAP reading at key on, before engine cranking, can't you use that as your reference instead of 101.3 and get real psig? At least so long as a big weather front doesn't move in, or you drive up or down a mountain .
And that's the problem as I see it. When your mobile, it's quite easy for the atmospherics to change dramtically. I think that might be just expecting too much from the MAP sensor and I think it better to leave the expectations as approximate rather than precise.
I forgot what the other possibility was while I was writing about the first one. This stuff makes my brain hurt.
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