Crystal nVision Reference Recorder - System.measurement.uncertainties.possible.with.nvision

Page 24 • Crystal Engineering Corporation
System.Measurement.Uncertainties.Possible.with.nVision
To understand the total system measurement uncertainty of the temperature measurement
you must consider both the nVision and the RTD sensing element uncertainties utilized in the
test application. Since the uncertainties of nVision and the sense element are independent of
each other, they must be combined properly with the preferred square root of the sum of the
squares
1
(or “root sum squares”) method.
The proper selection of the RTD sensing element is very important as the error associated with
this device is the majority of the overall system measurement uncertainty. IEC 751 is the stan-
dard that denes the temperature versus resistance for 100Ω, 0.00385 Ω/Ω/°C platinum RTDs.
IEC 751 denes two classes of RTDs: Class A and B. Class A RTDs operate over the -200°C to
650°C range versus -200°C to 800°C for the Class B elements. For example, the Class A uncertain-
ty is about on half that of the Class B elements as illustrated in the following table.
Tolerance Class Temperature Deviation Accuracy at 0°C Standard
Class A ±(0.15 + 0.002*t)°C 100.00 ± 0.06 Ω DIN/IEC751
Class B ±(0.3 + 0.005*t)°C 100.00 ± 0.12 Ω DIN/IEC751
1 Crystal Engineering recommends combining system expanded uncertainties in accordance with recommendations out-
lined in ISO “Guide to Expression of Uncertainty in Measurement (GUM). The uncertainties typically reported by Crystal
Engineering represent expanded uncertainties using a coverage factor k=2 to approximate a 95% condence level. The
typical method of combining uncertainties is the root sum squares of the individual contributing uncertainties and will
be calculated as such for the example shown.
Looking in detail at the use of Class B RTDs the following uncertainties are determined for the
nVision, Class B RTD sensing element, and the System at sample temperatures:
Temperature
°C
nVision
Uncertainty
±Ω
nVision
Uncertainty
±°C
RTD/Class B
Uncertainty
±Ω
RTD/Class B
Uncertainty
±°C
System
Uncertainty
±Ω
System
Uncertainty
±°C
-200 0.02 0.05 0.56 1.30 0.56 1.30
0 0.04 0.09 0.12 0.30 0.12 0.31
200 0.05 0.13 0.48 1.30 0.48 1.31
400 0.06 0.17 0.79 2.30 0.79 2.31
600 0.07 0.21 1.06 3.30 1.06 3.31
800 0.08 0.25 1.28 4.30 1.28 4.31
As mentioned above, the System Uncertainties are calculated using the root sum squares method: