Reading the Barometer

[TimCole]

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]

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]

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]

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]

... 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]

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]

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

 

[ericvic]

Gary,

No trouble, glad you got it squared away.

Eric