64
9 Supplement
WIKA Operating Instructions - Process pressure transmitter CPT-2x
Altitude above sea level
Ʋ
by default
up to 2000 m (6562 ft)
Ʋ
with connected overvoltage protection
on the master sensor
up to 5000 m (16404 ft)
Pollution degree
16)
4
Protection rating (IEC 61010-1)
II
9.2
Calculation of the total deviation
The total deviation of a pressure transmitter indicates the maximum measurement error to be ex-
pected in practice. It is also called maximum practical deviation or operational error.
According to DIN 16086, the total deviation F
total
is the sum of the basic deviation F
perf
and the long-
term stability F
stab
:
F
total
= F
perf
+ F
stab
The basic deviation F
perf
consists of the thermal change of the zero signal and the output span F
T
as
well as the deviation F
Kl
:
F
perf
= √((F
T
)
2
+ (F
Kl
)
2
)
The thermal change of zero signal and output span F
T
is specified in chapter "
Technical data
". The
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
Calculation of the total deviation - Practical example
Data
Pressure measurement in the pipeline 4 bar (400 KPa)
Product temperature up to 50 °C
CPT-2x with measuring range 10 bar, deviation < 0.2 %, process fitting G1½ (measuring cell
ø 28 mm)
1. Calculation of the Turn down
TD = 10 bar/4 bar, TD = 2.5 : 1
16)
When used with fulfilled housing protection.
