Clay, you're the perfect example of the saying, "Give a man a watch and he knows what time it is. Give him two and he has no idea."
On Tue, Jul 7, 2026 at 8:10 PM Clay Jackson <[email protected]> wrote: > Pardon me for jumping in here; but I've done a lot of fiddling and > research on just this topic. > > Some background, I currently have THREE pressure sensors within a 600' > radius, and Acurite 5-n-1, the pressure sensor that comes with an > "original" PurpleAire Outdoor unit and the sensor in my Tempest > WeatherFlow. At one time or another I've had as many as five pressure > sensors. Being in the US, I typically display pressure as inHg; but when > fiddling, I use millibars. Right now, my Tempest WeatherFlow is reporting > 29.736 inHG, my purpleair is reporting 29.08 inHG and the Accurite is > reporting 29.63 inHG. I've NEVER seen agreement on any 2 sensors, let > alone 3 or 4 and I've seen differences as much as .5 inHG > > I'm 3 miles and about 100 feet lower than S40 (our nearest airport) which > is reporting 29.81. But I'm also reporting a CORRECTED (to a NIST > traceable sling psychrometer) temp of 94.2 as opposed to 86 at S40. For > what it's worth, the purpleaire is reporting a temp of 104, because the > sensors are inside a small non-ventilated case. > > I also participate in the NOAA Citizen's Weather Observer Program, and > here's what they say over the last 30 days. > Average barometer error: 0.1 milliBars > Error standard deviation: 0.4 milliBars > > When I first started w/CWOP, I went crazy trying to correct stuff and > actually did correct my Acurite to SLP; but Based on the current CWOP data, > which is taken from the Tempest, when I installed that 3 years ago and > observed the differences daily for a month, I gave up trying to "calibrate" > it. I do check once or twice/week and have never seen anything but two > "thumbs up" out of CWOP except during frontal passes. It's also been my > experience that even 3 miles and 100 ft can make a large difference, a few > tenths inHG, especially when you consider temperature as well. > > Bottom line: > 1) With "consumer grade" equipment, you're never gonna get NIST reliabilty > or traceabilty > 2) Different sensors will report different results. Pick one, get it to > approximate either Sea Level Pressure or "altitude corrected" pressure and > then observe it for a while. > > And as far as what WeeWx does, "What Tom and the documentation say". > > On Tuesday, July 7, 2026 at 2:11:16 AM UTC-7 Tomasz Lewicki wrote: > >> Please help me understand the workflow for pressure/barometer >> calculations in Weewx. >> >> Here’s the situation. Let’s say my barometer reads 970 hPa. Setting aside >> how the reduced pressure is calculated, I know that it underestimates the >> pressure by 6 hPa. I’d like to inform Weewx of this without using the >> weather station console—that is, without physically entering corrections >> into the console. In other words, I want to do this in weewx.conf. >> >> I’ve made the following changes: >> [[[Units]]] >> [[[[Groups]]]] >> group_pressure = hPa >> >> [StdCalibrate] >> [[Corrections]] >> pressure = pressure + 6.0 >> >> [StdWXCalculate] >> [[Calculations]] >> barometer = WHAT SHOULD BE HERE? >> pressure = AND HERE? > > -- > You received this message because you are subscribed to the Google Groups > "weewx-user" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To view this discussion visit > https://groups.google.com/d/msgid/weewx-user/1fcc87ca-6e24-48f5-bf3a-358fc983c915n%40googlegroups.com > <https://groups.google.com/d/msgid/weewx-user/1fcc87ca-6e24-48f5-bf3a-358fc983c915n%40googlegroups.com?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "weewx-user" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion visit https://groups.google.com/d/msgid/weewx-user/CAPq0zEBXE%2BcSB_8V4%3D9rF_16dUkWSPbbEBDsHiMGvq3715tzEg%40mail.gmail.com.
