Humidity Error

S

sdcohe

Member
I have a 1-wire weather station with an AAG anemomemter, Rainwise rain gauge converted to 1-wire, and hobby boards temperature/humidity, solar, lightning, and barometer boards. Everything was working fine and my data uploaded to CWOP was looking good. One day recently my software was unable to see any of the 1-wire devices in my weather station. It appeared that the bus may have been shorted. I think that may have been due to moisture. After everything came back to life it appeared to be working OK, with all the readings being reasonably close to CWOP and no 1-wire errors being reported by the software. But this evening after sunset the humidity spiked up to 100% and has stayed there for an extended period of time. I have another humidity gauge that reads 70%, which is probably about right. I'm not certain if this is related to temperature or not (it was above freezing today, but dropped below freezing shortly after sunset). I will observe the behavior for another day or two and watch for any pattern.

Do I need to replace the humidity sensor or is there something else I should be looking at?

Thanks,
Seth
 
Seth,

I don't think it has anything to do with the freezing temperatures.

Can you give me some more information about how your network is setup.
Do you run external power to your devices?
How are they wired (daisy chain, serial, etc...)?
Where is your Humidity sensor located?
Any other information that may be special about how your 1-Wire network is setup.

Thanks,
Eric
 
Eric,

Everything is daisy chained, and I run external power from the barometer board. I have a DS9097U, followed by the barometer located indoors. Outside I have the humidity/temp board located in a homemade radiation shield, along with a separate solar board located on top of the radiation shield. Following that I have a lightning gauge, anemometer, and finally a rain gauge. They are all located together on a pole in a shady area in my back yard. The humidity seemed to be fairly accurate until everything stopped working and then recovered. I also have a Lacrosse WS2310 located close by. The temperature readings are very close to the 1-wire readings. When my 1-wire humidity readings aren't 100%, the Lacrosse and 1-wire humidity are very close as well. The accuracy appears to be pretty good according to CWOP, except for when the humidity goes to 100%.

Some additional info: Here are some of the lower level voltage readings being returned by the board when it spikes to 100%.

Fri Dec 21 08:58:11 EST 2007: Vdd 4.88 Vad 4.11 TempC 3.40625 RH (uncomp) 110.03437334743523 RH (cal) 100.0
Fri Dec 21 08:58:39 EST 2007: Vdd 4.88 Vad 4.22 TempC 3.21875 RH (uncomp) 113.67001586462189 RH (cal) 100.0
Fri Dec 21 08:59:40 EST 2007: Vdd 4.88 Vad 4.29 TempC 3.25 RH (uncomp) 115.98360655737706 RH (cal) 100.0
Fri Dec 21 09:00:43 EST 2007: Vdd 4.88 Vad 4.49 TempC 3.34375 RH (uncomp) 122.59386567953466 RH (cal) 100.0
Fri Dec 21 09:01:43 EST 2007: Vdd 4.88 Vad 4.4 TempC 3.59375 RH (uncomp) 119.61924907456374 RH (cal) 100.0

So it appears to be actually reading quite a bit higher than 100%. The software limits readings to a max of 100% as part of the temperature compensation.

Thanks for any advice you can give me. Let me know if there's any other info I can provide.
Seth
 
sdcohe said:
Fri Dec 21 08:58:11 EST 2007: Vdd 4.88 Vad 4.11 TempC 3.40625 RH (uncomp) 110.03437334743523 RH (cal) 100.0
Click to expand...

If it really is close to 100%RH out there, then looking at the operating range chart you're right on the borderline. 0C and 100%RH.

Assuming RH is being calculated using the straight line formula, this is correct.
There's no temperature compensation used.

Vout =Vsupply (0.0062(sensor RH)+0.16)

or

RH = ((vout/vsupply)-.16)/.0062

or

(4.11/4.88)-.16)/.0062 = 110.0343733

If VDD is correct, then for VDD=4.88v, VAD s/b about 3.5V for 90%RH.


Can you compare the readings from the LaCrosse to the 1wire and determine what the conditions are when it goes whacky?


There is a temperature compensation formula. See spec sheet. When I tried figuring RH using it the results were even worse. 133%.
Here's the formula, taken from the spec sheet, if you want to try your luck. Maybe my equation solving is off.

' Vout = (0.0305 + 0.000044T - 0.0000011T**2) (Sensor RH) + (0.9237 - 0.0041T + 0.000040T**2)
' T=Temperature in ºC
' Vout / (0.0305 + 0.000044T - 0.0000011T**2) - (0.9237 - 0.0041T + 0.000040T**2) = (Sensor RH)
vout = 4.11
t = 3.40625
rh = vout / (0.0305 + 0.000044 * t + 0.0000011 * t * t) - (0.9237 - 0.0041 * t + 0.00004 * t * t)
Debug.Print rh
133.129150249739
 
The actual relative humidity when those readings were taken was around 70%. Later it went up to 80% and I was reading 127%. My readings have stayed at or above 100% for two days now with it reading 127% for about 24 hours. The highest the true humidity reached was around 80-85%. So I am reading much too high. While i did have readings slightly higher then 100% occasionally in the past, I never seem to drop below that now. If I were to just subtract 40 from the raw RH (no temp compensation) it would be about right. In the past it was very close even before applying the temperature compensation formula. But compensating that much in software isn't the right answer. Especially when it always worked fine in the past.

I am using the formula from the spec sheet (the same one you posted) to calculate the RH like so:
double rh = (Vad/Vdd - (0.8/Vdd)) / 0.0062;

This is slightly different, as the formula in the spec sheet assumes Vdd is 5 volts to arrive at the .16 that gets subtracted. (.8/5=.16). In reality, it doesn't make enough of a difference to matter one way or the other.

For temperature compensation here is the formula I'm using:
double humidity = (float)(rh / (1.0546 - 0.00216 * temp));

I then check for values > 100.0 or < 0.0 and limit the result to that range. Otherwise I would get an occasional reading greater than 100%. It only happened once, but I didn't like to see that 101% reading on my WEB page.

One thing that I did change after moisture shorted out the 1-wire bus. I used electricians putty around all of the RJ-45 connectors to waterproof them. Could that somehow be causing a problem? I realize it sounds a little far-fetched, but I'm trying everything I can think of. I have seen where some people use petroleum jelly to waterproof the connections, but I was afraid it would melt and run when it got hot.

Thanks for your suggestions. Please keep them coming.

Seth
 
Seth,

One thing you may want to do is to bring the Humidity sensor inside for a few days to let it dry out (make sure to leave it unconnected). I have heard of a couple of examples of HIH-4000 humidity sensors not working quite right after being subjected to direct rainfall or very very heavy fog (too much water on the sensor).

Eric
 
Since the electrician's putty was the only change I had made to the station, I tried removing it this morning. The humidity reading dropped like a stone. It went down from 127% to 80% in just a couple of hours. It was raining outside and the humidty was around 80-85%. I'm now reading at 60%. The Lacrossse station reads 66%, so I'm a whole lot closer now. Apparently the putty was outgassing or in some way affecting the humidity sensor. So now I need to find something else to weatherproof the connections. I'm using petroleum jelly right now. Maybe some dielectric grease they sell at the auto parts store?

Anyway, thanks for all the suggestions.

Seth
 
sdcohe said:
Since the electrician's putty was the only change I had made to the station, I tried removing it this morning. The humidity reading dropped like a stone. It went down from 127% to 80% in just a couple of hours. It was raining outside and the humidty was around 80-85%. I'm now reading at 60%. The Lacrossse station reads 66%, so I'm a whole lot closer now. Apparently the putty was outgassing or in some way affecting the humidity sensor. So now I need to find something else to weatherproof the connections. I'm using petroleum jelly right now. Maybe some dielectric grease they sell at the auto parts store?

Anyway, thanks for all the suggestions.

Seth
Click to expand...

Seth,

Glad to see that you may have found the culprit. We sell dielectric compound here but by the time you add shipping you may be able to get it cheaper somewhere local.

Merry Christmas.

Eric
 
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