Siemens SITRANS P - Manual (Page 156)

SITRANS P measuring instruments for pressure
Remote seals for transmitters and pressure gages
Technical description
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Calculation of the temperature error
The following equation is used to calculate the temperature er-
ror:
Example of temperature error calculation
Dependence of temperature error on diaphragm material
The temperature errors listed in the previous table are based on
the use of stainless steel as the diaphragm material. If other dia-
phragm materials are used, the temperature errors change as
follows:
Maximum temperature of medium
The following maximum temperatures of the medium apply de-
pending on the material of the wetted parts:
Maximum capillary length for diaphragm seals (guidance
values)
dp = ( ϑ
RS
– ϑ
Cal
) ⋅ f
RS
+ ( ϑ
Cap
– ϑ
Cal
) ⋅ l
Cap
⋅f
Cap
+ ( ϑ
TR
– ϑ
Cal
) ⋅ f
PF
dp Additional temperature error (mbar)
ϑ
RS
Temperature on remote seal diaphragm (generally
corresponds to temperature of medium)
ϑ
Cal
Calibration (reference) temperature (20 °C (68 °F))
f
RS
Temperature error of remote seal
ϑ
Cap
Ambient temperature on the capillaries
l
Cap
Capillary length
f
Cap
Temperature error of capillaries
ϑ
TR
Ambient temperature on pressure transmitter
f
PF
Temperature error of the oil filling in the process
flanges of the pressure transmitter
Existing conditions
SITRANS P pressure transmitter for
differential pressure, 250 mbar, set
to 0 ... 100 mbar, with DN 80 remote
seal diaphragms without tube, dia-
phragm made of stainless steel,
mat. No. 1.4404/316L f
RS
= 0.1 mbar/10 K
(0.0014 psi/10 K)
Capillary length l
Cap
= 6 m (19.7 ft)
Capillaries fitted on both sides f
Cap
= 0.07 mbar/(10 K ⋅ m
Cap
)
(0.001 psi/(10 K ⋅ m
Cap
))
Filling liquid silicone M5 f
PF
= 0.07 mbar/10 K
(0.001 psi/10 K)
Process temperature ϑ
RS
= 100 °C (212 °F)
Temperature on the capillaries ϑ
Cap
= 50 °C (122 °F)
Temperature on pressure transmit-
ter ϑ
TR
= 50 °C (122 °F)
Calibration temperature ϑ
Cal
= 20 °C (68 °F)
Required
Additional temperature error of
remote seals dp
Calculation
in mbar
dp = (100 °C – 20 °C) ⋅ 0.1 mbar/10 K + (50 °C – 20 °C) ⋅ 6 m ⋅
0.07 mbar/(10 K ⋅ m) + (50 °C – 20 °C) ⋅ 0.07 mbar/10 K
dp = 0.8 mbar + 1.26 mbar + 0.21 mbar
in psi
dp = (212 °F – 68 °F) ⋅ 0.0014 psi/10 K + (112 °F – 68 °F) ⋅ 19.7 ft ⋅
0.001 psi/(10 K ⋅ 3.28 ft) + (112 °F – 68 °F) ⋅ (0.001 psi/10 K)
dp = 0.012 psi + 0.018 psi + 0.003 psi
Result
dp = 2.27 mbar (0.033 psi) (corresponds to 2.27% of set span)
Note
The determined temperature error only applies to the error resulting
from connection of the remote seal.
The transmission response of the respective transmitter is not included
in this consideration.
It must be calculated separately, and the resulting error added to the
error determined above from connection of the remote seal.
Diaphragm material Change in temperature error of
remote seal
Increase in values by
Stainless steel See previous tables
Hastelloy C4, mat. No. 2.4610 50%
Hastelloy C276, mat. No. 2.4819 50%
Monel 400, mat. No. 2.4360 60%
Tantalum 50%
Titanium 50%
PTFE coating on stainless steel dia-
phragm 80%
ECTFE coating or PFA coating on
stainless steel diaphragm 100%
Gold coating on stainless steel dia-
phragm 40%
Material p
abs
< 1 bar
(14.5 psi)
p
abs
> 1 bar
(14.5 psi)
°C (°F) °C (°F)
Stainless steel, 316L 200 (392) 400 (662)
PTFE coating 200 (392) 260 (500)
ECTFE coating 100 (212) 150 (302)
PFA coating 200 (392) 260 (500)
Hastelloy C4, mat. No. 2.4610 200 (392) 260 (500)
Hastelloy C276, mat. No.
2.4819 200 (392) 400 (662)
Monel 400, mat. No. 2.4360 200 (392) 400 (662)
Tantalum 200 (392) 300 (572)
Nom. diam. Max. length of capillary
Diaphragm seal Clamp-on seal
m (ft) m (ft)
DN 25 (1 inch) 2.5 (8.2) 2.5 (8.2)
DN 32 (1¼ inch) 2.5 (8.2) 2.5 (8.2)
DN 40 (1½ inch) 4 (13.1) 6 (19.7)
DN 50 (2 inch) 6 (19.7) 10 (32.8)
DN 65 (2½ inch) 8 (26.2) 10 (32.8)
DN 80 (3 inch) 10 (32.8) 10 (32.8)
DN 100 (4 inch) 10 (32.8) 10 (32.8)
DN 125 (5 inch) 10 (32.8) - -
© Siemens AG 2008