Cerabar M
Hauser
61
Influence of the temperature
on the zero point
A temperature change results in a volume change of the filling oil. The volume change depends on the
expansion coefficient of the filling oil and the volume of the filling oil at calibration temperature (constant in
the range: +21 to +33°C (+69.8 to 91.4°F)).
→
See also Page 60, "Diaphragm seal filling oils" section.
For example, the filling oil expands in the event of a temperature increase. The additional volume presses
against the diaphragm seal membrane. 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 temperature coefficients "T
K
Process" and "T
K
Ambient (for devices without capillaries)" see
Page 45 ff, "Process connections PMP46 and PMP48" section.
The following diagrams illustrate the temperature coefficient "T
K
Ambient" as a function of the capillary length.
The following situation is illustrated: capillary and transmitter temperature (ambient temperature) change, the
process temperature corresponds to the calibration temperature.
The temperature coefficients derived from the diagrams apply to silicone oil and 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 this Page, "Diaphragm seal filling oils"
section.
With regard to the temperature coefficient "T
K
Ambient", devices with temperature isolators behave like
devices with the same process connection with 1 m capillary.
Diagrams for diaphragm seal PMP46 with sample calculation
P01-PMP46xxx-05-xx-xx-xx-000
Diagram T
K
Ambient as a function of the capillary length for PMP46
Example
for:
– Diaphragm seal version "AL, DIN 11851 DN 50 PN 25, AISI 316L"
– Capillary length: 5 m
– Ambient temperature capillary/transmitter: 45 °C
– Filling oil: silicone oil
1.
Select characteristic type for diaphragm seal version "AL" in accordance with the following table.
Result: characteristic type 4
2.
Determine value for T
K
Ambient from the diagram.
Result: 8.4 mbar/10 K
3.
T
Ambient
– T
Calibration
= 45 °C – 25 °C = 20 °C
⇒
8.4 mbar/10 K x 20 K = 16.8 mbar
Result:
In this example, the zero point is shifted 16.8 mbar.
3
4
5
8.4
mbar
10 K
0
1
2
3
4
5
6
7
8
9
10
6
0
2
4
8
10
12
14
16
1
2
5
4
[mbar/10 K]
T Ambient
K
[m]
Capillary length
Summary of Contents for Cerabar M PMC41
Page 83: ...Cerabar M Endress Hauser 83...
