Deltabar S
60
Hauser
Influence of the temperature
on the zero point
A temperature change results in a volume change of the filling oil. The volume change is dependent on the
coefficient of thermal expansion of the filling oil and on the volume of the filling oil at calibration temperature
(constant in the range: +21 to +33°C (+69.8 to 91.4°F)).
59, section "Diaphragm seal filling oils".
For example, the filling oil expands in the event of a temperature increase. The additional volume presses
against the process isolating diaphragm of the diaphragm seal. The stiffer a diaphragm is, the greater its return
force, which counteracts a volume change and acts on the measuring cell together with the operating pressure,
thus shifting the zero point. For the "T
K
Process", see
43 ff, section "Process connections FMD78".
The following diagrams display the temperature coefficient "T
K
Ambient" dependent on the capillary length.
The following application is displayed: capillary temperature and transmitter temperature (ambient
temperature) change, the process temperature corresponds to the calibration temperature.
The temperature coefficients obtained from the diagrams apply to silicone oil and the process isolating
diaphragm material AISI 316L/1.4435. For other filling oils, these temperature coefficients must be multiplied
by the T
K
correction factor of the corresponding filling oil. For the T
K
correction factors, see
"Diaphragm seal filling oils".
P01-FMD78xxx-05-xx-xx-en-004
Example
for:
– Diaphragm seal versions "B5, EN/DIN Flange DN 80 PN 10-40 B1, AISI 316L"
– Capillary length: 5 m
– Ambient temperature, capillary/transmitter: 45°C
– Filling oil: silicone oil
1.
Select characteristic curve type for the diaphragm seal versions "B5" in accordance with the following
table.
Result: characteristic curve type 2
2.
Obtain value for T
K
Ambient from the diagram.
Result: 1.4 mbar/10 K
3.
T
Ambient
– T
Calibration
= 45°C – 25°C = 20°C
1.4 mbar/10 K x 20 K = 2.8 mbar
Result:
In this application, the zero point is shifted by 2.8 mbar.
Note!
• The influence of temperature on the zero point can be corrected with position calibration.
• The temperature influence can be minimised by using a filling oil with a smaller coefficient of thermal
expansion, shorter capillaries, diaphragm seals with larger diaphragm diameter or by using a smaller capillary
internal diameter.
1
2
3
4
5
6
7
1.4
mbar
10 K
4
0
2
6
10
8
12
0
1
2
3
4
5
6
7
8
9
10
[m]
Capillary length
[mbar/10 K]
T Ambient
K
