B

To: "'[email protected]'" <[email protected]>

Subject: RE: SENSOR: Accuracy (was Global Warming)

(now we're getting into some measurement-related stuff!).

Yes, the uncertainties of a single system made up of components must be added to give the overall uncertainty of the resulting measurement. But the addition is not linear - it is on an RMS basis. So if I have an RTD with an uncertainty of +/- 2 deg, and it is connected to an indicating gauge with an uncertainty of +/- 2 deg, the overall uncertainty of the reading is SQRT(2^2 + 2^2) = SQRT(8) = 2.83. THis is needed to take into account the possibility that a+ error in one device is compensated for by a - error in the other.

When talking a large number of devices, each accurate to the same specified uncertainty, the "accurate" value is taken (pretty well by definition) as the mean of the indicated values of each individual element. Taking the mean of a small number from a large population gives a result which is more accurate than any of the individual readings - this is of course the basis of the SPC method. The uncertainty of the mean of readings from 5 RTDs will be the uncertainty of any individual value divided by SQRT(5).

So in your example, if you take the means of the readings of A and B, the result will be nearer the true temperature than either reading alone, (and certainly not worse, as your argument would indicate).

Perhaps another question ... How much have temperature readings over the last 100 years been affected by changes in the definition of the

International Temperature Scale?

Bruce.