63
9 Supplement
WIKA Operating Instructions - Process pressure transmitter CPT-2x
F
perf
= 0.71 % (result of step 1)
F
stab
= 0.41 % (calculation of above table)
F
total
= 0.71 % + 0.41 % = 1.12 %
The total deviation of each sensor is hence 1.12 %.
5. Calculation of the total deviation of the measuring system
Both sensors are included in the calculation of the total deviation of the measurig system. With
4 … 20 mA master sensors the thermal error of the analogue output is also added:
F
total
= √(F
total-Master
)
2
+ (F
total-Slave
)
2
+ (F
a
)
2
0,3 %
-0,15 %
-40°C
-20°C
20°C
40°C
60°C
80°C
0,15 %
-0,3 %
Fig. 36: F
a
through thermal change current output, in this example = 0.1 %
F
total
= √(1.12 %)
2
+ (1.12 %)
2
+ (0.1 %)
2
)= 1.6 %
The total deviation of the measuring system is hence 1.6 %.
Deviation in mm: 1.6 % of 500 mm = 7.9 mm
The example shows that the measurement error in practice can be considerably higher than the
basic deviation. Reasons are temperature influence and Turn down.
The thermal change of the current output is in this example is negligible.
