Topic: Hygrometer stuck

[WXman]

90 days in service, and the dewpoint reading is stuck on 55 degrees. Ugh... been showing 99% humidity all week.

Any advice? Is this a serviceable part?

[nincehelser]

90 days in service, and the dewpoint reading is stuck on 55 degrees. Ugh... been showing 99% humidity all week.

Any advice? Is this a serviceable part?

Should be.  On the Atlas the module can be removed by hand without tools.

[WXman]

They had me do a hard reset, which didn't fix the issue.  It's stuck on 55 degree dewpoint, which of course throws off the RH also.  Thankfully it's under warranty so hopefully they'll do something for me soon.

[havtrail]

I thought it was the other way around - that dew point is calculated using measured RH as a factor.

Rich K.

[WXman]

Yep, in a strange twist the Acurite stations actually use RH to calculate dewpoint, which technically is backward.  But, that is in fact how it's done....somehow.

So I took the 5-n-1 apart to take photos of the sensors for an Acurite customer service rep. and when I put it all back together again and reinserted the batteries, things started working normally again.  I guess having it powered down that long must have reset it somehow.  Hmm...

Well, we'll see how long it stays functional this time.

[miraculon]

I have had all kinds of problems where electronics stops working, then the act of taking things apart and reassembling magically fixes them.

Usually, this is contact oxidation. You can't necessarily even see it. The act of unplugging and reinserting the connectors wipes the contacts clean and it works again for a while.

I would recommend a good contact cleaner like DeoxIT or CRC QD. Then coat the contacts with a dielectric grease to prevent further oxidation.

I have done this on several items and never had a recurrence of a problem afterwards. The dielectric grease is "key" otherwise it will just happen again.

Hopefully, you can identify the suspect connector.

Greg H.

[nincehelser]

Yep, in a strange twist the Acurite stations actually use RH to calculate dewpoint, which technically is backward.  But, that is in fact how it's done....somehow.

Would you please tell us what PWS calculate dewpoint directly?

What, exactly, is the dewpoint sensor in a Davis or a Ambient, for example?

[miraculon]

Davis measures RH and calculates Dew Point.

See their AN-28 pg. 6.

Greg H.

[WXman]

RH, expressed as a percentage, is simply a calculation of dewpoint temperature vs. air temperature. So RH is the derivative of the two, not the opposite.

I'll reverse the question and toss it back to you... how can these consoles accurately display RH before they even know what the dewpoint is?

That's like saying the answer is 4 before you're given the 2s to add together.

[nincehelser]

RH, expressed as a percentage, is simply a calculation of dewpoint temperature vs. air temperature. So RH is the derivative of the two, not the opposite.

I'll reverse the question and toss it back to you... how can these consoles accurately display RH before they even know what the dewpoint is?

That's like saying the answer is 4 before you're given the 2s to add together.

No PWS that I'm aware of has the ability to measure dewpoint directly.  You act like it is unusual for Acurite to derive the dewpoint from RH and temperature.

So I ask you again, what PWS measures the dew point directly?

[Jim_S]

RH, expressed as a percentage, is simply a calculation of dewpoint temperature vs. air temperature. So RH is the derivative of the two, not the opposite.

I'll reverse the question and toss it back to you... how can these consoles accurately display RH before they even know what the dewpoint is?

That's like saying the answer is 4 before you're given the 2s to add together.

I don't believe there is any device that measures dewpoint temperature directly.

In the olden days people used a sling psychrometer to get wet bulb and dry bulb temperatures. With that information it was posible to directly calculate relative humidity and or dewpoint.

In modern times our weather stations have an electronic sensor that measures relative humidity directly. Using relative humidity and air temperature it is possible for the station to calculate dewpoint temp, but it is not something that can be measured directly.

Davis is the only company I'm aware of that explains this in any detail. Here is a link to their Application note on "Derived Variables (a derived variable being one that is calculated from other, measured, values): https://www.davisinstruments.com/product_documents/weather/app_notes/AN_28-derived-weather-variables.pdf
and a quote:

DEWPOINT
Parameters Used: Outside Air Temperature and Outside Humidity

[CW2274]

RH, expressed as a percentage, is simply a calculation of dewpoint temperature vs. air temperature. So RH is the derivative of the two, not the opposite.
I'll reverse the question and toss it back to you... how can these consoles accurately display RH before they even know what the dewpoint is?

Because they do. All PWS sensors that I'm aware of are combo temp/humidity sensors, not separate, like an ASOS is for example, which indeed does calculate the dew. However it's done, they measure humidity and derive the dew point from that.

[WXman]

So, the point is that these stations must be using extremely complex formulas (along with a few assumptions) to calculate RH.  Which probably explains why every residential grade station on the market, Davis included, has imperfect RH readings that users notice and comment on frequently.

Regardless, I never said that Acurite is the only company doing it this way.  All I said is that it's a bass-ackward way of doing it.

Bottom line is this... my station was showing accurate temperature, wind, pressure, etc. but my dewpoint was wacky, and therefore my RH was wacky.  Actually, it was pegged on 99% at both the console and WU.  Since powering it all the way down for an extended period of time, disassembling, reassembling, and remounting it outside, it has begun to work better today.

Odd, but I'm glad it's doing better.  I hope that it will just keep on working, because so far, Acurite customer care has been more like going through a police investigation than a warranty claim.

[CW2274]

All I said is that it's a bass-ackward way of doing it.

When I first got my VP2, I thought the exact same thing. I think, without a doubt, it's because to derive the dew point is a much more expensive and difficult task (think condensation mirrors and separate shield, no aspiration) than using a combo sensor. Is it perfect, no, but just a touch cheaper than buying for instance, a Vaisala.

[nincehelser]

So, the point is that these stations must be using extremely complex formulas (along with a few assumptions) to calculate RH.  Which probably explains why every residential grade station on the market, Davis included, has imperfect RH readings that users notice and comment on frequently.

Quite the reverse.  RH is simple to measure electronically and can be very accurate.

The Magnus equation is likely the most popular formula for approximating dewpoint from RH and temperature.  Its uncertainty is less than 1%.

[Jim_S]

So, the point is that these stations must be using extremely complex formulas (along with a few assumptions) to calculate RH.  Which probably explains why every residential grade station on the market, Davis included, has imperfect RH readings that users notice and comment on frequently.

According to Davis your Davis Pro uses a lookup table to calculate dewpoint from temp and humidity. Here is a quote from the article I linked to above:

The Vantage Pro and Vantage Pro2 console uses a lookup table and it only differs from the
formula in that whole degrees and less resolution in the table are used for code and memory
space conservation.

Regardless, I never said that Acurite is the only company doing it this way.  All I said is that it's a bass-ackward way of doing it.

And what I'm saying is that you are mistaken, and are the one that has it backwards. I've been wrong before, convince me, provide some examples or documentation.

Odd, but I'm glad it's doing better.  I hope that it will just keep on working, because so far, Acurite customer care has been more like going through a police investigation than a warranty claim.

I'm glad you got it working.

[havtrail]

Yes, glad it's working now. Keep an eye on it...

Jim_S, Onset HOBO also indicates in their documentation that RH is a measured quantity and Dew Point is a calculated quantity. That's why I asked.

Rich K.

[galfert]

Ambient also uses the Magnus formula.

https://ambientweather.net/help/dew-point-calculation/

Being that Ambient, Davis and Acurite all use the same series of Sensirion SHT3x temperature/humidity sensors the process is the same. Ambient uses the SHT30 and Acurite and Davis use the SHT31.

[John Z]

Maybe someday we'll see tiny low cost MEMS-based dew point sensors in PWS devices. Plenty of research papers out there, but I don't see any sensor products yet. Running time in a battery operated system could be a challenge. Finding a good reason to displace the established T&H sensor technology in PWS  could be tough, too.