78
11 Supplement
VEGABAR 82 • Profibus PA
45031-EN-220624
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
in turn consists of the thermal change of the zero signal and the output
span F
T
(temperature error) 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 initially to the digital signal output through HART, Profibus PA, Foundation Fieldbus or
Modbus.
With 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
11.4 Calculation of the total deviation - Practical example
Data
Pressure measurement in the pipeline 4 bar (400 KPa)
Product temperature up to 50 °C
VEGABAR 82 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
