AEM Boost gauge analog output formula help

vacuum vs boost

When I checked your equation before I wondered if there might be a problem during boost. The AEM documentation listed the range of the meter but did not describe the analog output. The meter probably uses a solid state strain gauge that outputs a voltage dependent on the delta pressure between the sample port and a standard. The standard is probably absolute pressure.

We are accustomed to gauge pressure which is the measured pressure relative to atmospheric pressure (14.7 psi). Gauge pressure for vacuum (negative) is normally measured in inches of Mercury and gauge pressure for boost (positive) is measured in pounds per square inch. The AEM 30-4406 has an analog and digital display. The analog display scale has a range from 30 inches of mercury for vacuum to 50 psi for boost which would be easy to calibrate. The digital display range is specified as from -30 to 35 psi.

1 psi = 2.036025 inHg, 1 inHg = 0.491153 psi

That seems unreasonable since -30 psi would be 61 in-Hg of vacuum. Does the digital display match the analog scale. Is the digital display negative for vacuum? If so, then the digital display accommodates the change in measurement units when changing from vacuum to boost.

The analog output may match the range of the analog display in psi.

30 in-hg of vacuum = -14.73 psi gauge pressure
Adding that to the 50 psi gives a range of about 65 psi which matches the range of the equation you're using that isn't accurate.

There's mention in the documentation of a 50 psi sensor. I suspect the range of the meter is actually -30 in-Hg (-15 psi) to +35 psi or 50 psi.

The multiplier for that would be 50 psi/5 volts or 10 psi/volt
Assuming that 0 volts corresponds to -15 psi gives:
Psi = (10 * volts) - 15

To check the above I can solve your equation for the voltage measured at 10 psi of boost:
-1.4 = (v*13)-32.0
30.6 = (v*13)
v = 30.6/13 = 2.35

Substituting 2.35 into my equation gives 23.5 - 15 = 8.5 psi (still not 10)

If I assume the range is the same as the analog display:
-15 to +50 psi = 65 or 13 psi/v and -15 psi = 0 volts then the equation is:
Psi = (v*13)-15 substituting 2.35 volts yields 15.6 psi (still not 10)

I suggest that you datalog the voltage with no equation to determine what the output is vs displayed boost (analog and digital). Points of interest would be engine off (no vacuum or boost), engine idling (max vacuum), and max boost (10 psi).

According to my internet search you're not the only one having this problem. I suspect it's because the available documentation for the meter is incomplete and possibly inaccurate.

I didn't think of logging just 'V'. Excellent idea!!
I can easily log '0' pressure by just unplugging the boost/vac sensor. Then plugging it back in with the engine running gives me the -20 that shows on my gauge.

That would take all guess work out of the voltage coming out of the analog signal.

I'd prefer not to pull 10lbs boost just yet as there is a lean spot in the tuning causing some detonation. Its close, but I don't want to push my luck till I get an updated file.

Quick start guide equation

I found a copy of the AEM 30-4406 Quick Start Guide. It states:
"*Equation*: PSIA=12.5*(volts)-6.25* scale: .5v-4.5v = 0-50psia"
Using the voltage solved from your equation (2.35) for 10 psi in AEM's first equation yields 23.125 psi. If you assume that is absolute pressure and subtract 14.7 to convert to gauge pressure gives 8.425 psi.

Using the scale equation: 50 psia/4 volts = 12.5 psia/volt
If 4.5v = 50 psia then the equation would be psia = (12.5*v)-6.25
Substituting .5v into the equation yields 0 psia

gauge pressure = absolute pressure - atmospheric pressure
If the altitude of the meter is above see level the atmospheric pressure will be less than 14.7 psi so the gauge pressure will be greater.
10 psi - 8.425 psi = 1.575 psi , 14.7 - 1.575 = 13.125 which is equivalent to an altitude of about 2600 ft/790 m. What is your altitude?

I suspect the AEM equation is accurate for absolute pressure. So the Live Link equation for gauge pressure would be:
psig = [(12.5*v)-6.25]-psiatm
Unfortunately, if the result is negative then you'll have to multiply it by 2.036025 to convert it to in-Hg

This stuff makes my brain hurt.
Winnipeg is about 750 feet above sea level, so the psia = about 14.35
I'll try to find some time to play around with it in livelink tonight.

ok,
I did some measuring for V
-20 = .85v
0 = 1.6v
8.5 = 2.38v

My formula is out to lunch.
More work required

Dono said:

I didn't think of logging just 'V'. Excellent idea!!
I can easily log '0' pressure by just unplugging the boost/vac sensor. Then plugging it back in with the engine running gives me the -20 that shows on my gauge.

That would take all guess work out of the voltage coming out of the analog signal.

I'd prefer not to pull 10lbs boost just yet as there is a lean spot in the tuning causing some detonation. Its close, but I don't want to push my luck till I get an updated file.

Click to expand...

As far as you log for 10 PSI Boost or anything below that. Pull the sensor line and connect it to a small regulator with a known calibrated Gauge on a Tee in the same line. Dial in any pressure you want above zero and observe the voltage output.

I do this all the time in the field when calibrating transmitters.

playing with numbers

Dono said:

ok,
I did some measuring for V
-20 = .85v
0 = 1.6v
8.5 = 2.38v

Click to expand...

using the AEM equation and psiatm = 14.35
psig = (12.5*v)-20.6
psig(.85v) = -9.975*2.036 = -20.3 in-Hg (almost 20)
psig(1.6v) = -.06*2.036 = -1.22 in-Hg (almost 0)
psig(2.38v) = 9.15 (only 7% greater than 8.5)

using psiatm = 14.7
psig = (12.5*v)-20.95
psig(.85v) = -10.325*2.036 = -21.0 in-Hg (5% less than 20)
psig(1.6v) = -.95*2.036 = -1.93 in-Hg (still close to 0)
psig(2.38v) = 8.8 (3% greater than 8.5)

Considering the potential for measurement error I think the AEM provided equation is correct. I suspect the sensor has only one port and measures absolute pressure. Changes in local atmospheric pressure have no effect on the analog output or the digital and analog displays.

2000StreetRod said:

using psiatm = 14.7
psig = (12.5*v)-20.95
psig(.85v) = -10.325*2.036 = -21.0 in-Hg (5% less than 20)
psig(1.6v) = -.95*2.036 = -1.93 in-Hg (still close to 0)
psig(2.38v) = 8.8 (3% greater than 8.5)

Click to expand...

ok, I might be a bit slower than the average bear......

If I'm looking for a result of -20 when I have a voltage of .85 what's the equation?
(.85*12.5)-20.95= -10.325

Help me out here, I'm missing the boat.

vacuum reading

The problem is that the analog output for data recording is always in psia. The equation works fine for boost because its normally measured in psi. However, it doesn't work for vacuum which is normally measured in in-Hg. I'm not aware of an if negative qualifier in Live Link allowing the automatic conversion from psi to in-Hg. So I think these are your options:

psia=12.5*v-6.25 correct but unfamiliar range

psig=(12.5*v)-20.95 correct, unfamiliar range when negative (negative psi)

psig=[(12.5*v)-20.95]*2.036 correct, unfamiliar range when positive (positive in-Hg)

Since you're probably more concerned about boost than vacuum I suggest the second option. If you want to know what the vacuum is (value is negative) then just manually multiply the value by 2.036 to convert to in-Hg

You didn't answer if the digital display matches the analog display when there is boost and when there is vacuum.

The analog display isn't exactly analog. It's led's representing an analog display. Some of them are already burnt out. It's really a digital display only.

Honestly, I'm thinking it's way over priced for what it is. I really thought the analog output would be awesome, but as you are seeing its not so great in practice.

I'll give option 2 a go. I appreciate iate the work you put in to this. It's not straight forward.

Interesting stuff.I run the same gauge but cant offer any help
subscribing though