Reading the Barometer

TimCole

New Member
Greetings all,
I'm just starting to fiddle with 1-wire sensors, and I'm planning on using a Rabbit Semiconductor processor to run it all. I'll be writing some of the software, and I prefer to know how to go about interrogating the various devices. Even if I can count on just using pre-canned software, I'd like to know how to do the basic tasks.

I've started working with a DS18S20 temperature sensor, and it seems straight-forward. But I have no experience with the more complicated sensors.

So, my question: how do you read the barometer?

Do you get a raw A/D conversion output from the DS2438 and scale it to the specified pressure limits? If that's the case, what's the pressure-voltage transfer function? Do you assume it's linear or do you need to obtain or create a function or a look-up table?

Cheers,
Tim Cole
Ottawa ON
 

ericvic

Active Member
Tim,

Reading the Barometer is pretty easy. You need to get the VAD reading from the DS2438Z then you can use this formula to generate the pressure.

currentPressure = (VADvalue * slope) + intercept;

You get the slope and intercept values from our Barometer Calibration Calculator

That should be all there is to it.

Let me know if you need for information.

Eric
 

KBevo

Member
ericvic said:
currentPressure = (VADvalue * slope) + intercept;

You get the slope and intercept values from our Barometer Calibration Calculator

That should be all there is to it.
Eric, I must be doing something wrong. When I go to the referenced page, it shows four text boxes (name, altitude-644, pressure low-28 & pressure high-32). No submit or enter button. How do these values give the slope & intercept values?

Thanks
Gary
 

ericvic

Active Member
Gary,

Sorry about that. That is a bug when viewing with IE that I haven't had time to fix. If you view it with FireFox it shows up fine.

Not very many people use that page since we calibrate the Barometers before they leave but I will get around to fixing it sometime.

Eric
 

KBevo

Member
ericvic said:
... since we calibrate the Barometers before they leave but I will get around to fixing it sometime.
Well that may be the source of my confusion regarding getting a meaningful pressure reading from my Barometer. I know you calibrated my barometer before it shipped, so do I need a software slope (i.e. altitude) adjustment? (Up to this point I have assued a slope of 1.) But what about the offset?

Currently using Michaels 1Wire Xap program I am getting 3 voltage readings, the temperature reading and a "other" reading that when adjusted by the slope of 1 provides readings of values like 5.95. While the pressure recorded at a weather station a couple miles from me would be something like 30.11. Any suggestions on what course I should take to get more meaningful pressure readings?

Thanks
Gary
 

ericvic

Active Member
Gary,

You should have gotten a printout with your order of the Barometer calibration values that would have slope and intercept on it.

If you don't have it anymore just email [email protected] with your altitude and I will send you the info you need.

Eric
 

KBevo

Member
ericvic said:
You should have gotten a printout with your order of the Barometer calibration values that would have slope and intercept on it.
Thanks Eric, problem solved. Yes, your right I did get that printout with my barometer. Unfortunately, I didnt pay enough attention to what it was telling me. Too much time passed for my feeble mind between the time I put that sheet away, and the time I needed to enter the parameters into Michaels program. Complete disconnect. Sorry for the trouble.

Gary
 
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