Adjusted Humidity Readings

We are now adding 6% when humidity readings reach 94% since that is the maximum our sensor will read. For many years Davis Instruments’ sensors have struggled to achieve 100% humidity. Unfortunately, the problem seems to worsen with time. When we placed our sensor in service last November 28th, it would reach 98%.

Also, we are adding 1% to 92% (to make 93%) and 3% to 93% (to make 96%) to reduce the jump between 91% and 100%. Tests demonstrate these adjustments achieve our accuracy goal of +/- 3%.

You may occasionally see other readings besides 93%, 96%, and 100% due to 5 minute averaging of humidity values.

Raw values are available on the stats page.

Here’s some more info on this problem if you’re interested:

We have a backup weather station for emergencies. It also contains a Davis VP2 station with an SHT31 sensor inside that is less than 2 years old. It maxes out at 96% humidity, but it also has an average +8% bias below 80% which worsens as temperatures climb into the 80s and 90s. So it has a wet bias in the middle and a dry bias at the top. Keep in mind that the maximum error for this sensor is supposed to be 3.5% according to the manufacturer.

We are not the only ones to experience these issues. Many other Davis VP2 weather station owners have reported them (in a thread approaching 1200 posts). Davis says they are “looking into it”, although that was almost 2 months ago now. Some particularly brave souls have attempted to informally wire up older and/or unsupported sensors (SHT75) with varying degrees of luck. We’re not keen on performing ad hoc experiments with our primary temperature/humidity sensor. Some combination of Murphy’s Law and common sense would argue that such slapdash “solutions” will inevitably break at the worst time. Indeed our various attempts with other sensors on the test bench have not gone well. Therefore, we await a viable, proven fix preferably from Davis Instruments.

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