Topic: NIST Temp sensor

[npapadak]

Hi to All

According to Davis, NIST gives accuracy +- 0,2 Celcius?? unless i am reading wrong?

Please check

http://www.davisnet.com/product_documents/weather/spec_sheets/NIST_Traceable_Sensors.pdf

Nikolas

http://www.reaweather.gr
http://www.hydraweather.gr

[johnd]

Not 100% sure what you're asking. How I read it is:

The reference standard is assumed to be accurate to within 0.2degC (presumably right across the temperature range being checked, though not explicitly stated). Then the Davis sensor is checked at 6 different temperature points and verified to be reading to within the accuracy specified by the accuracy graph. So the maximum temperature error is (nominal error + 0.2degC). 

For example, in an extreme case, at -40C as indicated by the reference reading, the temperature could read -43C and it would pass NIST verification. But the true temperature could be -39.8C and so the maximum temperature error could then be 3.2C.

NB In general the errors would be less, probably much less, than the example above, but the example is true (at least, as I'm understanding it) in the extreme case.

[npapadak]

Hi John

So a NIST Davis sensor has an accuracy of 0,2 C in normal conditions and normal temps and on the other hand a Davis sensor without NIST Traceability has an error of 0,5 ?

Nikolas

[johnd]

So a NIST Davis sensor has an accuracy of 0,2 C in normal conditions and normal temps and on the other hand a Davis sensor without NIST Traceability has an error of 0,5 ?

That's not really what I wrote. Two problems (all as I understand it):

1. The temperature accuracy varies with temperature. You can only talk about temperature errors at a specified temperature;

2. The Davis/NIST option is not a calibration but simply a check/verification. The errors will typically be no different between NIST-checked and not NIST-checked. But with NIST-checked you have confirmation that the maximum allowable temperature error has not been exceeded.

[npapadak]

.ok so  with NIST-checked you have confirmation that the maximum allowable temperature error has not been exceeded.

What is the maximum allowable temp ? 0,2c or 0,5 c ?

[johnd]

What is the maximum allowable temp ? 0,2c or 0,5 c ?

Well, as per the PDF you linked to and as per my post above the maximum allowable error is 3.0C (at -40C). But 0.5C from roughly 15-35C - look at the graph.

[geofb13]

Davis instruments are checked against the NIST standard no matter what, there is no Davis NIST temperature sensor or a Davis non-NIST temperature sensor.

All this says is that the Davis Sensors are checked against the Vaisala HMP-233 sensor which has an accuracy of +/- 0,2 C. The actual sensors that Davis uses, are given an accuracy of +/- 0,5 C as shown by the linked pdf from your link and that I've added below.

http://www.davisnet.com/weather/choose_stations_detail.pdf

So all in all your answer is +/- 0,5 C accuracy for the Davis temperature sensors within the operating range of -40C to +65C.

[npapadak]

Great.!

Thanks all

[geofb13]

Well, as per the PDF you linked to and as per my post above the maximum allowable error is 3.0C (at -40C). But 0.5C from roughly 15-35C - look at the graph.

I think you might have misread the graph, the 3.0 on the left side of the graph is for the Fahrenheit scale I believe.

And no problem npapadak! Glad we could get to the bottom of it for you.

[johnd]

I think you might have misread the graph, the 3.0 on the left side of the graph is for the Fahrenheit scale I believe.

Yes, you're right - it's 1.6C for the C scale at -40C. So maximum error vs true temp is potentially 1.8C at -40C. It's only 0.5C vs nominal reference sensor over (roughly) 15-35C.

[npapadak]

Hi to All

I just got n email from Davis about NIST temp accuracy

Hi Nikolas,

That is correct….NIST calibration for temp sensor standard accuracy is +/- 0.2 C


Gerry Thrash
Technical Support
Davis Instruments, Inc.
  510-732-7814
  gerryt@davisnet.com