61
10 Supplement
VEGABAR 86 • Secondary Device for electronic differential pressure
48048-EN-220817
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
10.3 Practical example
Data
Measurement of the level difference in two water reservoirs, 1,600 mm height corresponds to
0.157 bar (157 kPa), medium temperature 50 °C
VEGABAR 86 with measuring range 0.4 bar, deviation < 0.1 %, meas. cell ø 28 mm
1. Calculation of the Turn down
TD = 0.4 bar/0.157 bar, TD = 2.6 : 1
2. Determination temperature error F
T
The necessary values are taken from the technical data:
50
F
TBasis
(
Fig. 26: Determination of the basic temperature error for the above example: F
TBasis
= 0.15 %
