Motorola MPX4115A pressure sensor

[Texy]

Has anyone had experience of this device?

Although I don't know exactly my height above sea level, I estimate that it
is around 200-400feet.

According to the datasheet, I should expect Vout to be around 4.03 volts
with a 5volt supply,

but I,m getting 4.09 volts, which equates to -36 feet. This is reading
directly from the device with a high

quality DVM. Also some days it reads slightly less and other days slight
more, but never anywhere

near what I expect it to be. I can only assume that the atmospheric pressure
also plays a part in its reading.

I read in the datasheet that temperature is compensated for (over a large
range), but no mention of the

weather is mentioned.

What am I not taking into account?

(or does Motorola have a help forum).

TIA

Texy

 

[Joerg]

Hi Texy,

As Martin and Tim hinted, this may be pushing a bit in regards to device
precision and your DVM. It's been a long time and I'd have to dig out
the math but in my parachuting days the altimeter moved a lot when a low
pressure system rolled in and we had to call it a day. So weather can
make a big difference here.

Regards, Joerg

http://www.analogconsultants.com

 

[Tim Wescott]

Joerg wrote:

Hi Texy,

As Martin and Tim hinted, this may be pushing a bit in regards to device
precision and your DVM. It's been a long time and I'd have to dig out
the math but in my parachuting days the altimeter moved a lot when a low
pressure system rolled in and we had to call it a day. So weather can
make a big difference here.

Regards, Joerg

http://www.analogconsultants.com

Come to think of it you can get altimeter corrections from the local
weather service or nearest airport. Even a podunk landing strip will
tell you the correction to apply.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Rich Grise]

On Wed, 19 Jan 2005 19:47:48 -0800, Tim Wescott wrote:

Joerg wrote:

Hi Texy,

As Martin and Tim hinted, this may be pushing a bit in regards to device
precision and your DVM. It's been a long time and I'd have to dig out
the math but in my parachuting days the altimeter moved a lot when a low
pressure system rolled in and we had to call it a day. So weather can
make a big difference here.

Regards, Joerg

http://www.analogconsultants.com

Come to think of it you can get altimeter corrections from the local
weather service or nearest airport. Even a podunk landing strip will
tell you the correction to apply.

The drop zones I've been to don't even have that, although usually the
pilot has presence of mind to set the altimeter to "0" while he's still on
the ground. ;-)

Cheers!
Rich

 

[Stephan Goldstein]

In article <7XEHd.12696$wZ2.7507@newssvr13.news.prodigy.com>, Joerg <notthisjoergsch@removethispacbell.net> wrote:

Hi Texy,

As Martin and Tim hinted, this may be pushing a bit in regards to device
precision and your DVM. It's been a long time and I'd have to dig out
the math but in my parachuting days the altimeter moved a lot when a low
pressure system rolled in and we had to call it a day. So weather can
make a big difference here.

Regards, Joerg

http://www.analogconsultants.com

I haven't followed the whole thread, but recall that the OP was seeing
a peculiar reading - somewhat below sea-level or something.

IIRC many of these sensors are ratiometric. Are you certain of
your reference voltage? Have you done the maths correctly?

Stupid questions, yes, but I like to start with the easy ones.

Steve

 

[Joerg]

Hi Roger,

Look on the web, for a 'METAR' for whatever airfield is closest to you.
This will give the current QnH, which is the normalised barometric
pressure, needing to be dialed into an aircraft altimeter to make it read
right in this area. The amount of variation, at times can be massive. ...

Indeed it can be. Sometimes when a storm front rolls in I can almost see
the barometer needle turn. We live right next to a runway but as far as
I know the closest METAR is KMHR, about 17 miles away, and the weather
there can be grossly different from ours. For folks not living close to
an airport location here is where the barometric pressure can also be
obtained. Just key in city and state and it will lead to the closest
'official' weather data:

http://www.nws.noaa.gov/

The sensor, is no longer made by Motorola (they 'split off' this part of
their operation some years ago), it is now made by Freescale, and a few of
the specs on some models did change with the split (on the MPX7100 for
example, the recommended excitation voltage to get accurate operation was
changed). The unit is only warranted to 1.5% accuracy, so the figure you
are seeing, is well inside spec, even if the atmospheric pressure was not
changing...

Pressure sensors are no small feat when it comes to production. I had
been slightly involved in one and the process control itself is a huge
task. Producing a high-voltage integrated circuit seemed like a cake
walk compared to that.

Regards, Joerg

http://www.analogconsultants.com

 

[Joerg]

Hi Rich,

It sounds like you needed someone to figure out a spot. I've never done
anything as advanced as RW; for some reason I have never been able to hang
around a given DZ long enough to get past a 10-second free-fall; but they
always threw the streamer, and about my seventh jump, I was learning to
spot myself.

It was an old Dornier 27 and with the pilot and five guys in there it
took a while to get above 10,000 feet. Sometimes the weather wasn't so
great and then, well, you got out. The streamer wouldn't have been
visible all the time but on weekends the area was clear of other
traffic. RW wasn't my forte either, the largest I ever participated in
was a five-star and we held it quite long. I think from 5,000 all the
way to 3,000.

Sometimes I jumped last and opened really high, at 12,000 or so when
jumping out of a Shorts Skyvan. I had a blast, sailing around up there
all by myself. With a stiff wind that could send you way off course
though but usually we found somebody who drove us back.

Regards, Joerg

http://www.analogconsultants.com

 

[Tim Wescott]

Texy wrote:

Has anyone had experience of this device?

Although I don't know exactly my height above sea level, I estimate that it
is around 200-400feet.

According to the datasheet, I should expect Vout to be around 4.03 volts
with a 5volt supply,

but I,m getting 4.09 volts, which equates to -36 feet. This is reading
directly from the device with a high

quality DVM. Also some days it reads slightly less and other days slight
more, but never anywhere

near what I expect it to be. I can only assume that the atmospheric pressure
also plays a part in its reading.

I read in the datasheet that temperature is compensated for (over a large
range), but no mention of the

weather is mentioned.

What am I not taking into account?

(or does Motorola have a help forum).

TIA

Texy


What's the advertised precision of the device? You're complaining about

a 1% variation. These things are used for absolute manifold pressure
measurements in engine controllers, so you're talking about needing
lbs/sq-in kind of accuracy, not Pascals.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com