Topic: OSI / WMRx00 compatible wireless sensors?

[Old Tele man]

Does anybody make a more accurate, wireless TEMP/RH sensor to use with Oregon Scientific Instruments Protocol 3.0 WMRx00 weather stations?

Both the THGN801 (outside) and THGR810 (indoor) sensors exhibit lousey RH-accuracy: their RH responses "flatten-out" terribly below 20-25%RH and above 80%RH.

[FrozenPenguin]

Don't expect astronomical quality and accuracy from a $29 device? 

Although I find my data very accurate when comparing data to more pro stations.
 

[Cynjut]

I've been mulling on this sort of question (i.e., setting up soil sensors or a sun-following UV/overcast sensor for various things) and I think I've about got the thing skulled out.  Actual design is next, of course, but the concept is there.  It starts with a 10-channel temp/RH sensor.  Get the specs on the temp and RH "sensors" (the actual gizmos that report these) and then replace them with the sensor elements that will measure the other things I need.  The raw values will get registered on the console as one of the temperature measurements.  From there, software will be used to convert the fake temperature readings into something meaningful for the rest of the project.

By using a scheme like this, I can build things like ground temp/moisture sensors or (maybe) a leaf sensor.  The console wouldn't know what I was doing, but as long as I do, it should be an interesting challenge.

The next step is to find a temp/RH sensor that's broken so I can take it apart without spending a bunch of money and ruining good sensors.....  Pictures of the circuit board might be enough for the next step.

[FrozenPenguin]

If the offset is unknown and variable, then whats the point of correcting the values...it would be guessing at best..

[Skywatch]

I've taken a small peice of felt, placed it over the openings at the bottom, spent a week dampeing that. After a week the temperature and humidity readings were fine. I read somewere else on the forum where someone used hair gel to correct the readings. Something about moisture retention.

Note; Don't place the sensor over a sprinkler or something that can get water inside the sensor. This will mess the readings up for a while.

[Skywatch]

I'm guessing that you're talking about "reviving" old temp/RH sensors, correct?

Unfortunately, the THGN801 and THGR810 units that I have showing obvious signs of hystersis "flattening" are brand new, I just got them mid-July!

When I overlay the time-line temperature plots of the 'old' (LaCrosse) and 'new' (OSI) systems, the old LaCrosse system plot shows a "pointed-end-of-an-egg" shape at peak heating, while the new OSI system plot shows a "rounded-end-of-an-egg" shape; and, the OSI temp values are TOO high and RH values are TOO low.

The LaCrosse temp/RH sensors were calibrated once and were +1ºF too high at 100ºF on the HIGH-end and -2ºF too low at 32ºF on the LOW-end. That's 'close-enuf' for me (as long as I know what the error amounts are).

• AZ CWOP: DW6988 (inactive)

Mine were new too. The felt over the vents did wonders for it. Spend a week dampening it. If that doesn't work offsets will. Sorry I couldn't be more helpfull. Just the way I had mine setup, and I've been happy reciently. I had to expariment with the offsets and locations to find a situation that was most accurate. MADIS had passed before I moved to Cumulus software when they added support for WMR 100-200. Still have some offset kinks to work out.

[Skywatch]

If someone knows something I don't on making the sensor accurate without offsets, please educate me!

[aweatherguy]

As you've discovered, OS uses resistive humidity sensors. The resistance of the sensor is measured at a low frequency (typically somewhere between a few hundred Hz to a few kHz). The resistance must then be converted to RH. The resistance is a strong function of BOTH temperature AND humidity. This means you must have a stable, repeatable measurement of temperature at the same time. The little cpu in the sensor has some internal tables or a mathematical function that converts a combination of resistance/temperature values into RH.

If you want to see what this 2-dimensional calibration data looks like, have a look at this data sheet for a typical sensor:

http://www.rhopointcomponents.com/images/SYH-2R.pdf

You can also draw some conclusions about the shape of the errors you'll get if the calibration is off here. You can also see that a simple offset or an offset versus RH reading may not be enough. To get a full calibration you could swear by, you would also need measurements at different temperatures. Quite a daunting task.

A given part-number sensor from a given manufacturer will generally have a unique set of calibration curves so without knowing the part number/manufacturer used in OS sensors, it would be a bit of luck if you found a perfect replacement -- not impossible but not easy either.

Notice that the resistance becomes very large at low humidity values (10's of megohms). This makes these sensors a poor choice for measuring low humidity. Capacitive sensors on the other hand work well and low humidity and are not as accurate at high RH.
 
Although the hair gel trick may restore a sensor to the point it appears to work again, without a full calibration over RH and temperature who knows how accurate the result is? It might be very good or it might not -- you just don't know w/o a lot of measurements at different temperature & RH values. For many folks that is good enough however and so this could be considered a valid "repair". The other mitigating factor is that the original sensor is not specified to have very good accuracy to begin with, so a repair with so-so accuracy results may be as good as the original. Again, I'm not saying this trick is a bad idea -- just that it would take a fair bit of work to verify the results over a wide range of temp/RH values.

[Scalphunter]

 Well I have run  these  temps/RH units both under extreme humidity ( 160+ inches of rain /year) to where I am now with less then 12 inches under artic desert conditions. I have yet to see the inaccuracy as is mention here. Hot summer day with 20 hour plus of dayligth can drive humidity down to low 20's and some time into the teens. Checking  with an slingmeter they so they are correct. Also have tracked them against Davis unit inches away and readings are with in tenths.

 If what you are saying about then and the graphing it seems to be more an time problem rather then the unit. more readings make for more accurate spacing on the graph.

[DanS]

As you've discovered, OS uses resistive humidity sensors. The resistance of the sensor is measured at a low frequency (typically somewhere between a few hundred Hz to a few kHz). The resistance must then be converted to RH. The resistance is a strong function of BOTH temperature AND humidity. This means you must have a stable, repeatable measurement of temperature at the same time. The little cpu in the sensor has some internal tables or a mathematical function that converts a combination of resistance/temperature values into RH.

If you want to see what this 2-dimensional calibration data looks like, have a look at this data sheet for a typical sensor:

http://www.rhopointcomponents.com/images/SYH-2R.pdf

You can also draw some conclusions about the shape of the errors you'll get if the calibration is off here. You can also see that a simple offset or an offset versus RH reading may not be enough. To get a full calibration you could swear by, you would also need measurements at different temperatures. Quite a daunting task.

A given part-number sensor from a given manufacturer will generally have a unique set of calibration curves so without knowing the part number/manufacturer used in OS sensors, it would be a bit of luck if you found a perfect replacement -- not impossible but not easy either.

Notice that the resistance becomes very large at low humidity values (10's of megohms). This makes these sensors a poor choice for measuring low humidity. Capacitive sensors on the other hand work well and low humidity and are not as accurate at high RH.
 
Although the hair gel trick may restore a sensor to the point it appears to work again, without a full calibration over RH and temperature who knows how accurate the result is? It might be very good or it might not -- you just don't know w/o a lot of measurements at different temperature & RH values. For many folks that is good enough however and so this could be considered a valid "repair". The other mitigating factor is that the original sensor is not specified to have very good accuracy to begin with, so a repair with so-so accuracy results may be as good as the original. Again, I'm not saying this trick is a bad idea -- just that it would take a fair bit of work to verify the results over a wide range of temp/RH values.

and a reliable source to compare/tweek to. I was lucky to have the Met.'s Metar equipment across the street (VTCC) and also used my other station to run against when restoring my 2310's resistive element sensor. Like you say,  "it might be good or not" but I know it's a heck of a lot closer now than when I started the treatment. It stayed down around 20% all the time.