Relative Air Pressure

[PennineAcute]

I know the difference between relative and absolute, but am at a loss on how to calculate the relative from the values of the absolute and altitude. Is it something as simple as logging onto wunderground and using the airpressure setting on there?

[steve]

Why is it that you want to do the calculation? Are you wanting to calibrate your station? If so, you don't need to worry about how it's calculated, you just set your relative pressure to the same value as a nearby reliable reference.

If you just want to know what the adjustment is for some other reason, typically a value of 1 hPa per 8 metres is used.

[PennineAcute]

Thank you very much. Just what I needed. Yes, I was trying to set the RP on my WS.

Now reading 1013 instead of 994.8, took the info off the bbc site.

[rampantsmurf]

Hi, Quick question.

i followed the above advice to set my Relative Pressure setting as i saw no change from 1013hpa over the course of two days, not even a slight movement. So i found a site that said its 1006.3 at the time of this post. I have now set the relative pressure setting on the handheld unit to 1006.3. What could i have done wrong if it remains the same reading in lets say 6 hours time.
Many thanks in advance.

Reefy

[steve]

The pressure over the UK isn't changing much at the moment. But 1006.3 is too low for the time you posted, it should have been around 1012 to 1016 or higher, depending on where you are. Did you get the figure from a reliable source, e.g. a Met Office station?

[AllyCat]

... i saw no change from 1013hpa over the course of two days, not even a slight movement.

Hi,

Welcome to the forum. Is that within the Cumulus / Easyweather software or on the LCD Screen of the Console? Certainly a change of +/- 0.1 should be seen quite soon. If it doesn't change within a few hours then a faulty sensor is suggested.

However, could you be reading the High or Low "Alarm" level, or a Max or Min value? (but the Console display normally restores to the current value after a short time). What does the pressure "histogram" which scrolls on the LCD screen show (although that is fairly constant at the moment)?

Cheers, Alan.

[rampantsmurf]

The pressure over the UK isn't changing much at the moment. But 1006.3 is too low for the time you posted, it should have been around 1012 to 1016 or higher, depending on where you are. Did you get the figure from a reliable source, e.g. a Met Office station?

You're right, the pressure should be at 1016, i took the 1006 figure from a nearby weather station in my area, his/hers must be "out" too

i have now set it to 1016.

... i saw no change from 1013hpa over the course of two days, not even a slight movement.

Hi,

Welcome to the forum. Is that within the Cumulus / Easyweather software or on the LCD Screen of the Console? Certainly a change of +/- 0.1 should be seen quite soon. If it doesn't change within a few hours then a faulty sensor is suggested.

However, could you be reading the High or Low "Alarm" level, or a Max or Min value? (but the Console display normally restores to the current value after a short time). What does the pressure "histogram" which scrolls on the LCD screen show (although that is fairly constant at the moment)?

Cheers, Alan.

Thanks for your warm welcome. I'm using Cumulus. Both the Console (handheld smart weather unit) and the pc Cumulus interface read the same, now 1016, and both show no change.
the scrolling Pressure history bars have no change (4bars all the way along).

[rampantsmurf]

Hi again,

After making the changers yesterday, there has still been no change at all in my pressure readings on both the handheld and the pc.

Any ideas for diagnostics before i make the choice to try to replace the faulty unit (if it is that of course) .

kind regards

Reefy

[AllyCat]

Hi Reefy,

As you're probably aware, the anemometer sensor is inside the Console, so there isn't really much (except a fault within the Console) that could be the cause.

If the Console were out of warranty (or unsupported by the supplier) then it's quite easy to open up (just lots of screws) to see if anything "obvious" is wrong. See the two sticky threads " ... insides of ..." at the top of this (FO) forum. However, it would be far better to try to get a replacement (Console) from the supplier.

Cheers, Alan.

[Warrandyte]

I have a number of issues with FO stations, and it seems a common problem in that I see 60hpa pressure drops (that also just happen to coincide with the heating coming on too.

It suggests that the pressure sensors are very slow to compensate for ambient temperature changes - if indeed they do.

Sadly such a key forecasting element is quite critical.

I thought originally that because it was in the master zone (always on) of the house where there is a lot of airflow that pressure may actually change in that room, but checks from other sensors suggested not.

therefore all i can deduce is that the housing for the pressure sensor react to temperature more rapidly.

something fortunately we have sorted out in aviation ! or wed all be flying at funny altitudes!