How to get accurate pressure in single pascals? Pressure shown by RuuviTag Scanner rounded to nearest hPa

#1

Hi. I’m using the example app on the tags (two of them). Since I was interested in very high accuracy (at least within the unit, ie if I move it from place to place it shows the change), I installed RuuviTag Scanner from https://lab.ruuvi.com/android-app/ since the screenshot shows decimals in use, unlike the default weather page.

However, in practice the app seems to show ,00 values for both of my tags(eg 995,00 and 996,00), any idea why? I found that from some API description that the tag itself should output the pressure in single pascals instead of hehtopascals.

Any ideas besides RuuviTag Scanner of how I could read the values more directly.

Or is the rounding just indicative that even though RuuviTag technically can output single pascal differences in numbers, the sensor itself only works in bigger steps and has roughly accuracy of 100 pascals or so?

Update (since I didn’t post this right away). Now both show 950,56 hPa (=95056 pascals) which should be wrong because one is outside the house and one is inside the house, and there should be at least a couple pascals difference. Again, is this indicative that the real sensitivity of the sensor is more like around 100 pascals than 1 pascal?

#2

You need to enter the RAW mode by pressing “B” once. The URL mode rounds the value to avoid spamming phones with notifications with constantly changing URLs

#3

That was an excellent tip, thank you!

So it turns out my units differ around 0.9 hPa if side-by-side, sometimes seems more than 1.2hPa though. If the delta itself wouldn’t fluctuate, I could take it into account and get rather accurate delta readings, but now it’s a bit shaky still. It still helps to know these values however.

Also moving the single unit around gives me some info, although since it takes unknown amount of time to accomodiate to the new environment, the environment itself has time to change meanwhile. I think it’s best to assume some limits how much error there can be between the units, and then calculate with assumption of eg 40 pascal accuracy.

Thank you again!

#4

The pressure sensor is specified to 1 hPa absolute accuracy, so in extreme case you could see 2 units differing by 2 hPa. The sensor is sensitive enough to notice a difference in height, so if one is at floor level and another is on table you’ll see difference between sensors.

#5

BME280 docs are at

Absolute accuracy pressure ±1.0 hPa
Relative accuracy pressure ±0.12 hPa

#6

Thank you again, very useful to know the limits and indeed I need to re-check my former measurements and keep book of what I actually measured and where. Relative accurary of 0.12hPa is quite nice and useful.