Motorola MPX4115A pressure sensor

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.

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Tim Wescott
Wescott Design Services
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Tim Wescott
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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.

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Tim Wescott
Wescott Design Services
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Tim Wescott

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

Reply to
Texy

High quality DVM? Most meters are +/- 2%.

Well 2% of 4v is 0.08 so your in the ball park. 2% of 20v is worse. What is the FS of the DVM, how many digits?

Cheers

Reply to
Martin Riddle

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

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Joerg

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

Reply to
Stephan Goldstein

Is that measurement in MSL, ASL, AGL? Perhaps its just a units issue.

Is the sensor absolute or relative pressure? Or is it an altimeter sensor? Pressure depends on specific air parcels (low/high pressure), temperature, and altitude so yes it will vary some even if temperature compensated. I think hangglider instrumentation uses a sensor like this as an altimeter. They work very well.

What is the output impedance of the sensor? If you have a good precision opamp lying around, you might try buffering the output and reading that on your DVM. Failing that, check a 0.01% precision voltage source with the DVM. :)

.

-- "Maybe it's the repetitiveness of daily work which keeps the average person's mind from wandering aimlessly into an ADD-mode?" MCJ

20050119
Reply to
Mark Jones

Hi Rich,

We had one strict ground rule: Everyone resets their altimeter before boarding and everyone checks the altimeter of the others, too, which were supposed to be worn outwards (especially important for formation jumps).

You are right, altimeter corrections were only available for towered airports, not the ones we used. But the main problem wasn't the altimeter since we always zeroed them. It was where to land. Sometimes we were a bit puzzled after opening at 3000 or so. Ah, a bus stop. Whistle to the others and we all flared out just behind that bus stop. Then read the schedule... except on Sundays... oh, drat. We got a lot of hiking exercise in those days.

Regards, Joerg

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Joerg

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. You can see figures ranging from perhaps 950mB (in the middle of a thunderstorm), up to perhaps 1040mB. Currently, where I am sitting, it is

994mB. The unit will be designed to read relative to the _standard atmosphere_ (this should be mentioned in the data sheet). This corresponds to 1013.25mB, which is the value dialed into the altimeter, of aircraft when flying more than a few thousand feet above the surface. This is so that all aircraft are using the same reference, to ensure good clearance between the airways. So currently at my location, the unit would read high by nearly 600 feet. 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...

Best Wishes

Reply to
Roger Hamlett

Hi Roger,

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:

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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

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Joerg

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