Topic: Barometric pressure cal - keeps coming up low.

[dschmidt_2000]

Have a 01035 installation.  Started it up mid January, saw the displayed barometric pressure reading about 0.10" below 'everyone else' (weather stations around me, both personal and 'commercial').  After 4 days it was still reading low.

Read up and saw that if I started up the display when the outside pressure was at 29.92" Hg that things get calibrated much quicker.  Fortunately the pressure soon dropped, it hit 29.92", reset the base station and kept my fingers crossed.

14 days have passed, pressure has gone up and down, yet I'm back to reading ~ 0.10" low.

I upload to Weather Underground using VIS and put in an offset to match nearby stations but I'd sure like to get the base station display to read correct.

What can I do? (or just live with it)

Edit: Another question - if there were no stations local to a person, how does the barometric pressure get calibrated?  For example, I can buy a Bosch BMP085 sensor that gives a calibrated pressure value, but is this valid by itself?

[nincehelser]

Starting the display when the pressure is 29.92 is pure hogwash.  I don't know how that idea got started, but it's nonsense.  It's going to end up applying a long-term running average of your pressure readings, so it really doesn't matter when you start.

Most of Acurite's displays show their adjusted pressure based on long-term pressure averages for your location.  There's no way to change that unless you have a model that specifically allows for it.  I've only heard of one display so far that allows that.

The AcuLink Bridge reports an "absolute" (aka "station") pressure.  You can then adjust that using whatever method you feel appropriate in software.  MBW gives you three choices: absolute, Acurite's averaging method, or absolute with your own adjustment.

I've never checked to see what the LCD displays report to a PC (which then reports to the Internet).  I've always assumed they have absolute pressure in there, but I could be wrong.

As far as properly calibrating a barometer, setting it to the reading of a local station is kind of lazy.  If you really want to do it "right", calibrate it based on your absolute pressure and true elevation through one of the standard formulas or tables. 

[dschmidt_2000]

The AcuLink Bridge reports an "absolute" (aka "station") pressure.  You can then adjust that using whatever method you feel appropriate in software. 
I've never checked to see what the LCD displays report to a PC (which then reports to the Internet).  I've always assumed they have absolute pressure in there, but I could be wrong.

Looks like absolute pressure is reported out the USB port too.  When I fired up the VIS software, the pressure reported was quite a bit lower than that on the LCD display and after I offset it to match neighboring stations, it's tracked them spot on whereas the LCD display has varied in offset due to the calibration routine.

As far as properly calibrating a barometer, setting it to the reading of a local station is kind of lazy.  If you really want to do it "right", calibrate it based on your absolute pressure and true elevation through one of the standard formulas or tables.

Thanks, I'll go look for the formulas and tables.

[dschmidt_2000]

Out of curiousity, does anyone know where the barometric pressure sensor is located?  In the 5 in 1? (so it's really a 6 in 1?)

[nincehelser]

There's no barometric sensor in the 5n1.

There is a barometric sensor in the bridge and in every display.