Modules
34
nVision Operation Manual
RTD100 System Measurement Uncertainties
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 standard that defines the temperature versus resistance for 100Ω, 0.00385 Ω/Ω/°C platinum RTDs. IEC 751 defines two classes of
RTDs: Class A and B. Class A RTDs operate over the -200 to 630°C range versus -200 to 800°C for the Class B elements. For example, the Class A uncertainty is
about 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
We recommend combining system expanded uncertainties in accordance with recommendations
outlined in ISO “Guide to Expression of Uncertainty in Measurement (GUM).
The uncertainties typically reported by us represent expanded uncertainties using a coverage factor
k=2 to approximate a 95% confidence 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.
Class A
Class B
Temperature
nVision
Uncertainty
Class A
Uncertainty
n Class A
Uncertainty
Class B
Uncertainty
n Class B
Uncertainty
°C
±Ω
±°C
±Ω
±°C
±Ω
±°C
±Ω
±°C
±Ω
±°C
-200
0.02
0.05
0.24
0.55
0.24
0.55
0.56
1.30
0.56
1.30
0
0.04
0.09
0.06
0.15
0.07
0.17
0.12
0.30
0.12
0.31
200
0.05
0.13
0.2
0.55
0.21
0.56
0.48
1.30
0.48
1.31
400
0.06
0.17
0.33
0.95
0.33
0.96
0.79
2.30
0.79
2.31
600
0.07
0.21
0.43
1.35
0.44
1.37
1.06
3.30
1.06
3.31
800
0.08
0.25
0.52
1.75
0.53
1.77
1.28
4.30
1.28
4.31
