Topic: Davis VP2 exterior temperature sensor replacement/offset

[gotty1]

I've just replaced a 7346-166 temperature sensor with a 7346-070 (I ordered the 166, which was showing in stock, but got the 070 instead).

I read that it needed a -0.5C offset, but in fact it is reading 4.2C high.

I was very careful to ensure I knew what the temperature actually was before replacing it, including putting an accurate Lascar temperature logger in place. But it is definitely reading 4.2C high

Okay, I can offset this - but is this to be expected (the 166 sensor was reading within 0.1C of the Lascar).

Thanks

[Bobvelle]

Hmm.  How old was the sensor you replaced?

[gotty1]

Well, I bought the VP2 in February 2014.

[Bobvelle]

Well, I bought the VP2 in February 2014.

All I can say is calibrate it to the known CORRECT temp. I can't explain why it would be so off though. Sorry. Maybe someone else can shine some light on the issue.

[mcrossley]

I'd ask for a replacement, that isn't right.

[gotty1]

Okay, so I put a -4.2C offset and it was reading correctly all evening This morning it's reading about 3.9C too low.

So I've done another series of measurements and end up back with a -0.5C offset. It's not reading exactly the same as my other thermometers, but taking into consideration the tolerances involved, I guess it's close enough.

Maybe the 070 needs bedding in?

[mcrossley]

The SHT31 based sensors are normally very accurate in temperature, are you sure your reference temperature is in spec, the environment is the same, and everything stable? Response times may also be coming into play?

[gotty1]

Thanks for the thoughts - I'm comfortable that my reference temperature is as good as I can get.

Although I'm using a Lascar professional temperature logger inside the VP2 ISS, I actually have two other reference sources - a mercury thermometer (±0.5C) in a meteo cage positioned underneath the ISS and a calibrated Fluke platinum thermocouple (±0.2C), both of which I've been checking the Lascar against.

However I've really been using the Lascar (±0.25C with 0.5C internal resolution) as a proxy, as it's the only device which I can actually fit next to the ISS sensor,

Also, main comparisons have been made when temperatures have been reasonably stable for at least an hour, so there shouldn't be any response time issues.

I'm definitely puzzled.

[gotty1]

Single data point "tests" can be less accurate than longer, continuous, comparisons of data.

Indeed, that's why none of my comparisons/checks have been done on single data points. They've been against several hundred samples over several hours, comparing graphs.