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'Barometric Pressure and Humidity Sensors'
1998\02\13@200330 by Adi

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I'll be starting on my weather monitoring system soon. Any
suggestions for what to use for barometric pressure and humidity? I
am using a LM35 for temperature and I like the idea of not having to
do any calibrating. Are such things available for humidity and
pressure?

Thanks,
Adi

1998\02\14@112430 by Steve Turner

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Adi <spam_OUTadiTakeThisOuTspamSL.LAKEHEADU.CA> wrote:

>I'll be starting on my weather monitoring system soon. Any
>suggestions for what to use for barometric pressure and humidity? I
>am using a LM35 for temperature and I like the idea of not having to
>do any calibrating. Are such things available for humidity and
>pressure?

For barometric pressure, I've been using a Motorola MPX5100A
sensor.  This is probably not ideal, since I'm only using about
15% of its range; nevertheless, it has adequate sensitivity for
0.01 inHg resolution.  The unit was within about 0.5 inHg using
data sheet parameters; finer calibration was accomplished by
comparison with a commercial weather station about 10 miles
distant.  The two track quite well, with rarely more than 0.02
inHg difference.

This sensor is available from Newark for about $26.

Steve Turner

1998\02\15@113523 by Tom Handley

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  Adi, Low-cost humidity sensors behave like capacitors that vary with
humidity to change the frequency of an external oscillator. Typical relative
humidity ranges from 10 - 90%. Philips makes a low-cost sensor
(P/N 2322 691 90001). General Eastern also seems to have interesting low-cost
sensors in their G-CAP line. Their sensors cover 1 - 100% RH.

  For my project, I needed better accuracy and I wanted to reduce the
complexity of the support circuitry. I ended up using HyCal sensors which
provide an output within 0 - 5V and require minimal support. The outdoor
sensor is an IH-3602L which comes in a T0-39 can with a slotted cap and for
the indoor sensor, I used the IH-3605 hybrid element. Both sensors operate
from 1 - 100% and provide an output from around 0.8V - 3.9V with a 5V
supply. They should be buffered and require a simple low-pass filter and, as
with most sensors, need to be shielded from sources of bright light. You do
need to factor in temperature compensation so you need to measure ambient
temperature near the sensor. You normally combine both in a package for your
outdoor sensors.

  For barometric sensors, you typically use a 0 - 15psi pressure sensor.
Motorola and Sensym have low cost sensors. I use the Sensym SCX15ANC
temperature-compensated sensor and I've had excellent results over the last
year. I use a 10V reference to supply the bridge excitation voltage and I
connect the output to an Analog Devices ADC620 instrumentation amp with a
gain of 50. That goes to a 12-bit A/D with a span of 0 - 4.096V. I scale and
offset the data in software. You can also use a 5V reference but you should
use some version of an intrumentation amp for the differential output of
the sensor which is normally less than 100mv full-scale.

  For more info, contact:

     Humidity Sensors:
     HyCal          : (818) 444-4000
     Philips        : (817) 325-7871
     General Eastern: (800) 225-3208

     Pressure Sensors:
     Sensym  : (408) 954-1100
     Motorola: http://design-net.com/senseon/pressure.html

  - Tom

At 06:58 PM 2/13/98 +0000, you wrote:
>I'll be starting on my weather monitoring system soon. Any
>suggestions for what to use for barometric pressure and humidity? I
>am using a LM35 for temperature and I like the idea of not having to
>do any calibrating. Are such things available for humidity and
>pressure?
>
>Thanks,
>Adi
>
>

1998\02\16@220911 by Adi

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Thank you for the info!

I think I settled on the MPX5050DP for a pressure sonsor. The
humidity sensor sources you mentioned I'll check out, too.

Regards,
Adi

1998\02\19@015939 by wterreb

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>
> I think I settled on the MPX5050DP for a pressure sonsor. The
> humidity sensor sources you mentioned I'll check out, too.

Adi

 Why the MPX5050DP?  I have not looked at the specs for this
pressure transducer, but I think the DP stands for "Differential
Pressure", which would make it unsuitable to measure absolute pressure.

I make flying instruments for the Paragliding/Hangliding  sport and
in it I use an MPX5100A which measures Absolute Pressure.   As the
matter of fact, I can measure air pressure so precicely with this
that it has enabled me to implement this into an instrument that
shows reading changes for every meter change in altitude.  The only
problem is that the MPX5100A is rather expensive... I pay about $20
for it.  How much do you pay for the MPX5050DP?

Rgds
Werner
--
Werner Terreblanche     users.iafrica.com/w/we/wernerte/index.htm
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