Problem:
Varying temperatures of the medium and the surroundings, combined with high humidity, can lead to condensation within the pressure transmitter. This in its turn will cause corrosion damage to the electronics or the sensor.
Condensation within a vessel
Occurs frequently in:
Solution:
Extra code 631 - encapsulated version
The encapsulation of the sensor in silicone gel reduces the volume of air within the sensor so far that it is not possible for sufficient condensation to arise to the extent that it would cause damage.
Why?
Adjusting the zero point is important when high accuracy is required.
The zero point signal of a pressure transmitter can shift:
Long-term stability (the maximum signal drift per year) is a quality parameter which must be specified in the data sheets.
When, how often?
If high accuracy is required, JUMO recommends
For certain installations, such as in power plants, regulations demand a documented check of the zero offset signal.
Which instruments?
What is the zero point?
Multi-way valves for easy zero point adjustment
The use of a multi-way (manifold) valve enables test and adjustment in normal operation, i.e. on site, without any dismantling:
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Problem:
Certain applications, such as CIP processes (CIP = Cleaning In Place) require hygienic process connections with a guaranteed maximum surface roughness RA.
JUMO pressure transmitters with a front-flush connection have an RA less than 0.8 µm. However, special applications require an improved maximum surface roughness RA < 0.4 µm.
JUMO dTRANS p31 with clamp connection
Sectors:
Solution:
Extra code 452 - electrolytically polished
Electrolytically polished process connections have an improved surface roughness RA < 0.4 µm.
Problem:
If, for instance, a valve is closed very quickly, thus shutting off the pipe completely, then the pressure in the pipe falls suddenly to zero. This creates a shock wave that propagates at the speed of sound in the medium against the direction of flow. The shock wave is reflected at the container output, runs back to the valve input, is reflected again, and so on.
The superimposition of the shock wave and its reflections in both directions causes a sharp rise in pressure.
This increased pressure is known as a pressure spike. It can be many times higher than the operating pressure, and thus cause destruction of the sensor.
Pressure sensors can be damaged by pressure spikes.
Pressure spikes are often not noticed.
Solution:
Extra code 591 - restrictor in the pressure channel
The restrictor is welded into the pressure channel.
The restrictor only lets the pressure pass through a restricted orifice, in a zigzag path. This reduces the energy of the pressure peaks arriving at the sensor membrane.
Application tips:
Caution:
Intelligent transmitters with digital electronics can already convert
the measurement signal in the sensor from the original pressure unit
into a unit that is better suited to the application.
This serves not only for easier use but also to reduce the processing that has to be performed by other instruments in the measurement system.
Application examples:
for measuring pressure: display in psi or Torr or kg/cm2
for filling level meaurement: display in cbm or liters or cmH20
for flow measurement: display in cbm/h or liters/minute
JUMO instruments with scaling:
The JUMO dTRANS p02 series only applies scaling to the value displayed by the LCD, while the value of the analog output
(in mA) remains unaltered.
The scaling function in the JUMO CANtrans series directly alters the measurement that is transmitted via the digital CAN bus to the control system.
Definition:
The absolute pressure pabs is the pressure relative to zero pressure in empty space (vacuum).
The relative pressure prel is the pressure relative to the respective atmospheric pressure pamb (air pressure).
Problem:
The air pressure changes constantly depending on the altitude or weather conditions.
Therefore, depending on the application, the pressure must be measured either relatively or absolutely.
Example absolute pressure: Air pressure measurement
Air pressure measurement is an absolute pressure measurement.
It can be used, for example, for weather forecasting or altitude measurement.
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The relative pressure transmitter measures the pressure prel (hydrostatic pressure of the liquid column) and converts it into an electrical signal.
If the density of the liquid is known, this signal is a direct measure of the filling level h.
Applications include
Wells, lakes, bodies of water, swimming pools, containers, tanks, heating systems, etc.
Example: Venting a vessel by means of a vacuum pump
Target: Switch off the pump at residual pressure 5 mbar abs
The aim is to switch off the pump at a residual pressure of 5mbar abs. The table shows how this can be done using absolute pressure measurement or relative pressure measurement.
The switching point is only determined constantly with absolute pressure.
| Condition of the plant |
In fine weather air pressure 960 mbar
|
In bad weather air pressure 940 mbar
|
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|
Measurement absolute
(correct) |
Measurement relative
(wrong) |
Measurement absolute
(correct) |
Measurement relative
(wrong) |
|
| Vessel ventilated (atmosphere) |
960 mbar abs
|
0 mbar rel
|
940 mbar abs
|
0 mbar rel
|
| Vessel empty (residual pressure) |
5 mbar abs
|
-955 mbar rel
|
5 mbar abs
|
-935 mbar rel
|
