Error with H08-004-02 Relative Humidity Values

I was measuring air temperature and humidity in a drying facility for medical herbal plants with H08-004-02 dataloggers. Although these dataloggers aren't the latest model, they worked quite well for many months. In my last recordings there is a problem with the relative humidity values. Four dataloggers of the same type in a room of 25 m² / 40 m³ together jump to 100 % humidity at once for about an hour or two and than return to normal measuring. I don't understand why or how this happens neither how to resolve it. I would be very happy about a quick answer because I need those data for a scientific paper. I added a picture of my measurement (https://prnt.sc/h1yvz5). The lower black line shows standard deviation of all four loggers. It equals zero. The four coloured lines show the Humidity values over 48h. I'm puzzled!
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Its important to note that your loggers could be nearly 20 years old! HOBOs do last a long time when taken care of properly. Good job keeping yours working for such a long time.

Please see the following excerpt from the H08-004-02 manual: Relative Humidity - The HOBO's user-replaceable relative humidity sensor is
temperature compensated, and the logger's relative humidity accuracy is ±5% over
the entire +5°C to +50°C (+41°F to +122°F) operating range of the sensor when
used with BoxCar Pro 4.3 or BoxCar 3.7.3 or later version. NOTE: Accuracy
specifications reflect improvements made to BoxCar Pro 4.3 and BoxCar 3.7.3.
Earlier software versions provide ± 5% accuracy except in elevated humidity
environments (60% to 95% non-condensing and non-fogging where accuracy
is ±10%). Upgrading to BoxCar Pro 4.3 or BoxCar 3.7.3 or later version will
bring all current and old data file accuracy to ±5% for the full operating
range. The relative humidity sensor's operating RH range is 25% to 95% at
+25°C (+77°F) for logging intervals of 10 seconds or longer. Full dependance
shown in Plot C. RH sensor drift is < 1% per year. NOTE: The HOBO's RH
sensor will be damaged by condensation. It must not be exposed to
fog, mist or other condensing conditions!

It appears that the sensor is getting saturated, then recovering. It could be failing simply from old-age... Is there spraying, misting or other activity happening at these times that could spike RH? Do you have the correct version of BoxCar? Please be prepared to answer these questions should you contact Tech Support. As the H08 is a discontinued line of products, we do not sell replacement RH sensors for it any longer, and we are unable to repair them due to many of the parts having long since gone EOL.

I would recommend clicking the link below to open a Tech Support case to conduct more detailed troubleshooting with our Tech Support team to see if there is any more light we can shed on the issues you are seeing. I would also strongly recommend that you consider upgrading to one of the newer products. Our sales team (and/or our distribution partners) would be happy to help you choose from several cost-effective options.

Cheers,
Gregg

Please submit a support case by clicking here.

Thanks for this imidiate answer! I'm aware they're old, but they weren't in use that much, explaining why they still worked. Either they really broke because of their age or something happened in the drying room. The herbs start to loose essential oils at 40 °C but this temperature wasn't reached, so mist, fog or spraying shouldn't be the problem. Furthermore I used the latest BoxCar version available via onsetcomp.com! Is this scenario possible: When they reach a "critical" RH value of 96-97% they just jump to 100% until falling under this "limit"? My only worries are why they all at once showed the same problems. I have several other data of the same process where all loggers work fine and show RH of max. 95%. I'll just ask Tech Support for further suggestions concerning malfunctions. But if they failed terminally that won't be a problem, I already have "new" loggers at hand (U12-012), which I also used at this project. Kind Regards

The fact that 4 loggers showed the same data would indicate that there is an environmental factor in the room that is prompting the spike. The RH sensors will saturate if any condensation gets on them. They are rated to 95%, but at +/-5% accuracy, they could be in a higher RH than the sensor can tolerate. Understanding what specifically is happening in the room at the time the data spikes is the key here.
Cheers,
Gregg

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