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'[EE]: Motorola MPX pressure sensors -- AP vs GP'
2002\03\11@024435 by Pic Dude

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Anyone know what the diffs are between the AP and GP versions
of the motorola MPX4250, MPX5700, etc sensors?  The datasheet
doesn't seem to be too clear on this.  What exactly does "gauge"
mean on the GP versions and how would the output differ from that of the AP versions?

Thanks.

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2002\03\11@100629 by Chris Loiacono

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I believe these designations are for Gauge & Absolute sensing. Gauge refers
to a sensor that will measure the pressure relative to 1 ATM, so a gauge
sensor will report 14.7 psi as zero, and 15.7 psi as 1. The motorola gauge
sensors are differential sensors with the second port open to the ambient
pressure. It will only give predictable results when that port is at a known
pressure, such as at sea level (1 ATM) or 14.7 psi.

The Absolute Sensor will provide an output that is == the absolute pressure.
At 1 ATM the unit should provide output for 14.7 psi. These can be made
several ways: by reducing the pressure on and sealing the second port so
that it offsets the 1 ATM pressure, or by offsetting the output
electronically to obtain the same output.



Chris


> Anyone know what the diffs are between the AP and GP versions
> of the motorola MPX4250, MPX5700, etc sensors?  The datasheet
> doesn't seem to be too clear on this.  What exactly does "gauge"
> mean on the GP versions and how would the output differ from
> that of the AP versions?
>
> Thanks.
>

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2002\03\11@101047 by Paul Hutchinson

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IIRC, AP is absolute pressure measurement and GP is gauge pressure
measurement.

The absolute sensor has an internal vacuum reference and the gauge sensor
uses the ambient atmospheric pressure as the reference. You use gauge
measurement for things like scales and tire pressure and you use absolute
for measurements like barometric pressure.

I think they also make a differential pressure sensor that has a separate
pressure input port for supplying your own reference pressure.

Paul

>
> Anyone know what the diffs are between the AP and GP versions
> of the motorola MPX4250, MPX5700, etc sensors?  The datasheet
> doesn't seem to be too clear on this.  What exactly does "gauge"
> mean on the GP versions and how would the output differ from
> that of the AP versions?

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2002\03\11@105618 by Dwayne Reid

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At 01:51 AM 3/11/02 -0600, Pic Dude wrote:
>Anyone know what the diffs are between the AP and GP versions
>of the motorola MPX4250, MPX5700, etc sensors?  The datasheet
>doesn't seem to be too clear on this.  What exactly does "gauge"
>mean on the GP versions and how would the output differ from
>that of the AP versions?

Absolute pressure: pressure is referenced to a vacuum.  It will indicate
current atmospheric pressure with no input pressure applied.  A barometer
is an example of an absolute pressure sensor.

Gauge pressure is referenced to ambient pressure.  It will indicate a zero
reading with no input applied.  A tire pressure gage is an example of a
gauge sensor.

dwayne


Dwayne Reid   <.....dwaynerKILLspamspam.....planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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2002\03\11@110003 by Douglas Butler

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I would point out that both of these have very good uses.  If you want
to measure the water level in a tank by measuring the pressure at the
bottom, you want a gauge sensor.  If you are in a submarine and want to
measure how deep you are, you want an absolute sensor (plus a weather
report).

If you used an absolute sensor to measure the tank level, the result
would depend on the local barometric pressure.

If you used a gauge sensor to measure depth of a submarine your results
would be influenced by the pressure of the air in the submarine cabin
which, depending on the ventillation system used and cabin temperature,
could vary widely.  Of course either senser will be affected by the
barometric pressure above so you also need a weather report.

Sherpa Doug

> {Original Message removed}

2002\03\11@200414 by Herbert Graf

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> Anyone know what the diffs are between the AP and GP versions
> of the motorola MPX4250, MPX5700, etc sensors?  The datasheet
> doesn't seem to be too clear on this.  What exactly does "gauge"
> mean on the GP versions and how would the output differ from
> that of the AP versions?

       I believe that refers to the type of sensor. AP is absolute, it reads the
absolute pressure (actually it is differential, it reads the difference in
pressure between what's on it's port and an almost total vaccumm), so if you
had it sitting on a desk it would read about 101kPa (current atmostpheric
pressure). GP is more differential, it gives the DIFFERENCE in pressure
between what's on it's port and the ambient pressure, so if I blew into it
it would show a psoitive reading, sucking it would show a negative reading.

       Minor warning, I'm not familiar with those models but not that the non
temperature compensated models are VERY sensitive to temperature, and must
be compesated by yourself. Even a couple degrees can throw off the reading
quite a bit. Of course if they are temp compensated then ignore my warning.
TTYL

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2002\03\11@201011 by Pic Dude

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Ah.  Gotcha.  Since Mot does not specify the output for press < 1atm,
that means that it will not provide a negative voltage I guess.

Bummer.


{Original Message removed}

2002\03\11@201218 by Pic Dude

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But then the GP is the same as the DP version, w/o the second
port.  Why even make the GP version?

Would be nice if they made it with 2 outputs -- one for P1 > P2
and the other for P1 < P2.


{Original Message removed}

2002\03\11@201737 by Pic Dude

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I'm measuring vacuum and boost in my car, and I've used the AP
for this, with a calibration mode to record ambient pressure w/o the
car running.  Vac/boost is determined by using that cal as a reference.

Cheers.


----- Original Message -----
From: "Douglas Butler" <@spam@dbutlerKILLspamspamIMETRIX.COM>
To: <KILLspamPICLISTKILLspamspamMITVMA.MIT.EDU>
Sent: Monday, March 11, 2002 9:52 AM
Subject: Re: [EE]: Motorola MPX pressure sensors -- AP vs GP


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> > {Original Message removed}

2002\03\11@213636 by Paul Hutchinson

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> But then the GP is the same as the DP version, w/o the second
> port.  Why even make the GP version?

GP is significantly less expensive than DP for high volume applications.

Paul

>
> Would be nice if they made it with 2 outputs -- one for P1 > P2
> and the other for P1 < P2.

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