Calculation Of The Temperature Error - Vega VEGADIF 65 Operating Instructions Manual

3 Planning instructions for isolating systems
Actuating variables
Example of double-sided
chemical seal:
16
3.6

Calculation of the temperature error

With double-sided mounting of chemical seals, the total temperature
influence is composed of the following:
•
Influence of the process temperature on the chemical seal
(TK )
• Process
Corrective factor with special materials (with Tantalum, Alloy: 1.5;
with PTFE coating: 1.8)
•
Corrective factor for filling oil
•
Influence of the ambient temperature TK
mitter (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 chemical seal is listed in the tables in chapter "Dimen-
Process
sions and weights" of this operating instructions. The correction factor
for the filling oil is listed in chapter "Influence of the components".
The thermal change of zero signal and span is specified in chapter
"Technical data" of the differential pressure transmitter.
Information:
In case of double-sided, idential chemical seal mounting, the
temperature influences should compensate. It shows that there is nev-
ertheless an error through the temperature influence. In practice we
calculate with 20 % of the sum of the invidual errors of the two chemi-
cal seals. This is also taken into account in the following example.
Finally, the calculated temperature errors of pressure transmitter and
chemical seal must be added up geometrically.
•
Process temperature: 100 °C
•
Flange isolating diaphragm DN 80 PN 10-40
•
TK Process flange isolating diaphragm: 1.34 mbar/10K (see chap-
ter "Supplement" of this manual)
•
Capillary length: 4 m
•
Filing oil silicone: correction factor 1
•
Diaphragm material.: Tantalum, correction factor 1.5
•
Ambient temperature TU: 40 °C
•
TK = 0.3 mbar/10K (see diagram in chapter "Influence of
Capillaries
temperature changes" or delivery documents
ΔT Process temperature-Reference temperature chemical seal
= 100 °C - 20 °C = 80 K
ΔT Ambient temperature-Reference temperature capillaries
= 40 °C - 20 °C = 20 K
Number of chemical seals = 2
Error calculation
Δp = (1.34 mbar/10K) • 80K • 2 = 21.44 mbar
chemical seal
Corrective factor diaphragm material = 21.44 mbar • 1.5 = 32.16 mbar
Δp = (0.3 mbar/10K) • 20K • 4 m • 2 = 4.8 mbar
Capillaries
on the pressure trans-
Amb.
Chemical seal CSB • for VEGADIF 65