61
9 Supplement
WIKA Operating Instructions - Process pressure transmitter CPT-2x
basic temperature error F
T
is shown in a graphic. Depending on the measuring cell version and Turn
down, this value must be multiplied with the additional factors FMZ and FTD:
F
T
x FMZ x FTD
Also these values are specified in chapter "
Technical data
".
This applies for a digital signal output through HART, Profibus PA or Foundation Fieldbus.
With a 4 … 20 mA output, the thermal change of the current output F
a
must be added:
F
perf
= √((F
T
)
2
+ (F
Kl
)
2
+ (F
a
)
2
)
To provide a better overview, the formula symbols are listed together below:
•
F
total
: Total deviation
•
F
perf
: Basic deviation
•
F
stab
: Long-term stability
•
F
T
: Thermal change of zero signal and output span (temperature error)
•
F
Kl
: Deviation
•
F
a
: Thermal change of the current output
•
FMZ: Additional factor measuring cell version
•
FTD: Additional factor Turn down
9.3 Practical example
Data
Level measurement in a small vessel, 500 mm height corresponds to
0.049 bar
(4.9 KPa), superim-
posed pressure 0.35 bar (35 KPa), medium temperature 40 °C
CPT-2x Master and slave sensor each with nominal measuring range
0.4 bar
(40 KPa), deviation
< 0.1 %, process fitting G1½ (measuring cell ø 28 mm)
The required values for the temperature error F
T
, deviation F
Kl
and long-term stability F
stab
are avail
-
able in the technical data.
1. Calculation of the Turn down
TD = 0.4 bar/0.049 bar, TD = 8.2 : 1
2. Determination temperature error F
T
The temperature error F
T
consists of the basic temperature error F
TBasis
, the additional factor meas-
uring cell F
MZ
and the additional factor Turn Down F
TD
.
50
(
F
TBasis
Fig. 35: Determination of the basic temperature error for the above example: F
TBasis
= 0.15 %
