Hi,
I could not find or understand which is the resolution in the measurement bridge with 3 and 4-wire PT100 on CR1000.
thanks
Smile
Hi,
Following the notes of the CR1000rev7/11 manual, section 11.18.2 Measuring PT100s (100 ohm PRTs), measurements with PT100, I want to calculate the resolution in Celsius degrees, theoretical in the absence of noise.
If 25 mV of scale equivalent to a scale of measurement of 50 ° C (-10 to +40 ° C), it means that every mv equals 2 Celsius degrees ,
then since the resolution in range 25 mV is of 3.33 microV, I think that I can say that the resolution R
is 0.00666 ° C (2 ° C: R = 1000microV: 3.33).
It is correct?
I'm not sure, since the result of the measurement is a ratio Rs/R0 and in any case I have other concerns.
Thanks to everybody that can give me some suggestions.
thanks
Smile
I am afraid one limitation of PT100 sensors is you are looking at a small change of resistance on relatively large base resistance, e.g. about 0.385 ohms (per deg C) on 100 ohms at zero deg C. This means the 25 mV measured by the logger does not represent 50 deg C full scale range. Without going through the mathematics, at around zero deg C one mV equates roughly to about 13 deg C. With a CR1000, this puts the resolution of a single measurement at 0.04 deg C for a differential measurement (used in 4-wire configurations), 0.08 deg C for a single-ended measurement (3-wire) (again for temperatures around zero deg C).
However, the 3 and 4 wire measurements are not based on single measurements but a series of measurements (up to 8 for the 4WHB with all reversal options), so if there is any noise in the system, the apparent steps of resolution will be smaller than the figures given above.
If you want higher resolution than the numbers above for single measurement you would either need to use full bridge measurement or switch to a higher resolution logger, such as the CR3000. You might also consider though that due to noise and natural signal variation simply oversampling and using the average can also increase the resolution of the measurement to some degree.