Author Topic: WeatherJack Barometer  (Read 2868 times)

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Offline aweatherguy

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WeatherJack Barometer
« on: June 22, 2009, 03:17:15 AM »
Has anyone ever used this digital barometer?

I just discovered it and bought one. It's a little over $100 and offers a surprising resolution of 0.004 inHg. There are some problems however. It runs on a 9V battery and turns out to be sensitive to battery voltage. It is also very sensitive to temperature. The last thing: it does not come with absolute calibration -- the user must determine an appropriate offset for their unit.

So far, this doesn't sound too good. But, on the plus side, it comes with a temperature sensor that can be used to "fix" the temperature problem in software. I've hooked this barometer up to a well-regulated 9 volt power source instead of the battery and with temperature correction it appears to be surprisingly accurate. The readings are lining up quite well with the MADIS analysis provided through CWOP -- right now a 3-day analysis is showing an average error of 0.01 inHg and a standard deviation of 0.006 inHg. Not bad for such a cheap little unit!

For those who have a little electronics experience and enjoy science projects, you could get a pretty accurate barometer for $200 or maybe a bit less. I've posted a little more information about this here: http://wmrx00.sourceforge.net/WeatherJack/weatherjack.html

« Last Edit: June 22, 2009, 03:19:12 AM by aweatherguy »

Offline wxtech

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Re: WeatherJack Barometer
« Reply #1 on: June 22, 2009, 09:55:52 AM »
An interesting device.  A challenge maybe but I don't need the baro. 
I recommend using a Mercury 9 volt battery as a voltage reference in a linear regulated power supply.  An op-amp uses the 9v battery only as a reference but does not drain the battery.  The battery lasts almost its shelf life.  An alternate may be to use a precision IC voltage source instead of a zener reference in the power supply.
Al Washington, Lexington, Ga.,  NWS Coop station=LXTG1, Fischer Porter, SRG, MMTS. 
CoCoRaHS=GA-OG-1. CWOP=CW2074.  Davis VP2+ WLIP 5.9.2, VP(original) serial, VWS v15.00 p02. ImageSalsa, Win7 & Win8 all-in-one.

Offline aweatherguy

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Re: WeatherJack Barometer
« Reply #2 on: June 22, 2009, 03:40:32 PM »
I'm using a 12VDC wall wart from CUI Inc. as the main power source. It's only about $7 from DigiKey and uses very little power from the 120V lines. The 9V regulation is provided by an LM431 shunt regulator which has a TC of 50ppm/degC. Everything is such low power that there is not much waste in using a shunt regulator -- it only needs 5mA or so to run. This is coupled with a 9V battery for backup when the power goes out -- there might be a slight bump in pressure readings in this case but the unit should run for a very long time on the battery backup.

I'll post some design files for ExpressPCB and a DigiKey parts list on the WSDL web site at some point. I believe I can fit six copies of the power supply on one of their "MiniBoard" templates. For something like $51 you get 3 MiniBoards which equates to 18 power supplies.