VEGA VEGADIF 65 Chemical seal CSS - Manual (Page 6)

The implemented filling oil also influences the TK
zero point
, the
permissible ambient temperature and the response time of a chemical
seal. See also chapter "Influence of the temperature to the zero point"
and "Response time".
The differential pressure transmitter also influences the temperature
application range, the TK
zero point
and the response time of the isolating
system through the volume of its lateral flanges and its control volume.
1)
3.2 Influence of temperature changes
The filling oil expands as the temperature increases. The additional
volume presses on the chemical seal. The more rigid the diaphragm,
the more it counteracts a volume change. The additional volume also
adds to the process pressure on the measuring cell and thus shifts the
zero point. The respective temperature coefficient "TK
Process
" is listed
in chapter "Dimensions and weights".
3.3 Calculation of the temperature error
With one-side chemical seals, the total temperature influence is
composed of the following:
l
Influence of the process temperature on the chemical seal
(TK
Process
[mbar/10K])
l
Corrective factor with special materials (with Tantalum, Hastelloy:
1.5; with PTFE: 1.8)
l
Corrective factor for filling oil
l
Influence of the ambient temperature on the capillary (TK
Ambient
[mbar/10K])
l
Influence of the ambient temperature on the pressure transmitter
(thermal modification of zero signal and span)
The calibration temperature of the isolating system is 20 °C. For the
calculation, the temperature must be deducted from the respective
process or ambient temperature.
The TK
Process
chemical seal is listed in the charts in chapter
"Dimensions and weights". The corrective factor for the filling oil is
listed in chapter "Overview". The thermal change of zero signal and
span is specified in chapter "Technical data". The calculated
influencing factors must be summed geometrically.
l Process temperature: 100 °C
l
Flange isolating diaphragm DN 80 PN 10-40 with extension 50 mm
l
Capillary length: 4 m
l Diaphragm material.: Tantalum, correction factor 1.5
l
Ambient temperature TU: 40 °C
TK
chemical seal
= 0.4 mbar/10K
1) The control volume is the volume that must be shifted in order to utilise the
complete measuring range.
Differential pressure
transmitter
Chemical seal, single
Example for isolating di-
aphragm, unilateral
6 Chemical seal CSS • for VEGADIF 65
3 Planning instructions for isolating systems
36134
-EN
-110926