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

3 Planning instructions for isolating systems
Filling oil Permissible
product temper-
atures
Med. white oil -15 ... +260 °C
KN92 (FDA (+5 ... +500 °F)
approved)
Differential pressure
transmitter
Isolating diaphragm,
unilateral
6
Permissible Densi-
product temper- ty in g/
atures at 0.05
cm³
< p < 1
bar
abs
bar
abs
-15 ... +160 °C 0.85
(+5 ... +320 °F)
up to
20 °C
The implemented filling oil also influences the TK
permissible ambient temperature and the response time of an isolating
diaphragm. 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
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 isolating diaphragm. 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 " is listed in chapter "Dimensions and weights".
Process

3.3 Calculation of the temperature error

With unilateral mounting of the isolating diaphragm, the total temper-
ature influence is composed of the following:
Influence of the process temperature on the isolating diaphragm
l
(TK [mbar/10K])
Process
Corrective factor with special materials (with Tantalum, Hastelloy:
l
1.5; with PTFE: 1.8)
Corrective factor for filling oil
l
Influence of the ambient temperature on the capillary (TK
l
[mbar/10K])
Influence of the ambient temperature on the pressure transmitter
l
(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 control volume is the volume that must be shifted in order to utilise the
1)
complete measuring range.
Vis- Expan-
cosity sion co-
2
in m / efficient
-6
1/K
s•10
5.6 0.00065
up to
40 °C
and the response time of the isolating
zero point
Isolating diaphragm CSS • for VEGADIF 65
Cor- Application
rective area
factor
for TK
0.63 suitable for
foodstuffs
, the
zero point
Ambient