FAQ on Pressure Measurement

Basics

Avoid condensation

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:

cooled piping or tanks (e.g. in breweries, dairies and the beverage industry)
exhaust systems (automobiles)

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.

Zeroing of pressure instruments

Why?
Adjusting the zero point is important when high accuracy is required.

The zero point signal of a pressure transmitter can shift:

when it is installed, because of the mounting position or mechanical strain on the pressure connection,
in operation, because of temperature changes,
over a long time, because of aging.

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

adjusting the zero offset signal during commissioning, after the instrument has been installed
and subsequently according to requirements (e.g. every 12 months).

For certain installations, such as in power plants, regulations demand a documented check of the zero offset signal.

Which instruments?

Zero offset adjustment is only possible on the more sophisticated instruments:

JUMO dTRANS p02 series: digital, via keypad or the HART interface
JUMO CANtrans series: digital, via CAN interface
JUMO dTRANS p30 series: analog, by potientiometer
JUMO MIDAS series: no zero adjustment possible

What is the zero point?

for absolute pressure: absolute vacuum (0 bar absolute)
for relative pressure: atmospheric pressure (0 bar relative)
for differential pressure: both pressures are equal

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:

for absolute pressure: 2-way valve (for connection to a vacuum pump)
for relative pressure: 2-way valve (open to theatmopheric pressure)
for differantial pressure: 3-way or 5-way valve

pressure_t1q5_3

Improve surface smoothness

Problem:
Certain applications, such as CIP processes (CIP = Cleaning In Place) require hygienic process connections with a guaranteed maximum surface roughness R_A.
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:

Pharmaceutical industry
Medicine
Food industry

Solution:
Extra code 452 - electrolytically polished
Electrolytically polished process connections have an improved surface roughness RA < 0.4 µm.

Protection against pressure spikes

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:

This problem does not arise with gases, only with liquid media, such as water, hydraulic fluids, beverages
It is usually associated with valves and pumps

Caution:

The pressure channel may become blocked by viscous media, calcification or deposits
The measuring signal response will become somewhat slower (milliseconds)

Scaling of pressure measurement instruments

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.

Zeroing of pressure instruments

Why?
Adjusting the zero point is important when high accuracy is required.

The zero point signal of a pressure transmitter can shift:

when it is installed, because of the mounting position or mechanical strain on the pressure connection,
in operation, because of temperature changes,
over a long time, because of aging.

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

adjusting the zero offset signal during commissioning, after the instrument has been installed
and subsequently according to requirements (e.g. every 12 months).

For certain installations, such as in power plants, regulations demand a documented check of the zero offset signal.

Which instruments?
Zero offset adjustment is only possible on the more sophisticated instruments:

JUMO dTRANS p02 series: digital, via keypad or the HART interface
JUMO CANtrans series: digital, via CAN interface
JUMO dTRANS p30 series: analog, by potientiometer
JUMO MIDAS series: no zero adjustment possible

What is the zero point?
for absolute pressure: absolute vacuum (0 bar absolute)
for relative pressure: atmospheric pressure (0 bar relative)
for differential pressure: both pressures are equal

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:

for absolute pressure: 2-way valve (for connection to a vacuum pump)
for relative pressure: 2-way valve (open to theatmopheric pressure)
for differential pressure: 3-way or 5-way valve

pressure_t1q5_3

Process pressure transmitter JUMO dTRANS p02

What is the current source function used for?

The current source function (parameter P8) can be used for manual setting of the mA value at the output of the pressure transmitter to a defined value, regardless of the actual pressure prevailing at the moment.
This saves having a separate current source.

This makes it easy to test the 2-wire current loop circuit.

For example:

Is the supply voltage still adequate at 21.5 mA?
Is the display on the PLC functioning properly?
Does a contact (if used) switch when the limit value is reached?
etc.

The current source function is standard for all HART instruments.

Possible settings:

3,85 mA	-	Underrange
4,00 mA	-	Measurement range 0%
12,00 mA	-	Measurement range 50%
20,00 mA	-	Measurement range 100%
21,50 mA	-	Overrange

What is damping used for?

The damping (parameter P4) is used to stabilize the output signal for measurements that are fluctating too rapidly.
This function is similar to the filter in our electronic controllers.

Example: Filling level measurement
Waves on the surface of the liquid can cause fluctuations of the measured value.
Damping can be used to stabilize the measurement.

What is density correction used for?

For measuring the filling level of liquids with a density that is other than 1.0 kg/dm³ (i.e. not the same as water).
For instance: 0.85 kg/dm³ (benzine) can be selected in two different ways:

by adjusting the span (parameter P6)
by adjusting the density correction (parameter P0)

The final result is the same in both cases.

The second method, using the density adjustment parameter, is easier for the user if the density changes from time to time, as it can then be manually corrected by using the keys on the instrument.

Medium	Density	Filling level	Measured pressure
Water	1,00 kg/dm³	10 m	1,00 bar
Benzine	0,85 kg/dm³	10 m	0,85 bar

Example for Filling level measurement Benzine
Density: 0,85 kg/dm³
Filling level: 10 m
Nominal range: 4 bar (=100%)

Configuration (1): by setting the measurement span (parameter P6)

Medium	Density	Measurement span P6	Density correction P0
Water	1,00 kg/dm³	25,00 %	1.00
Benzine	0,85 kg/dm³	21,25 %	1.00

Configuration (2): by setting the density correction (parameter P0)

Medium	Density	Measurement span P6	Density correction P0
Water	1,00 kg/dm³	25,00 %	1.00
Benzine	0,85 kg/dm³	25,00 %	0.85

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