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 Coriolis-
Mass Flow Meters
If a liquid stream runs trough an oscillating tube, the phase shift
of this oscillation depends on the Coriolis force and finally of
the mass flow of the liquid stream.
Because Coriolis flow meters are optimized
for precise phase shifting determination, they have only poor performance
for the measurement of the oscillation frequency, which is required
for precise density measurement. That´s why concentration
and density measurements with these instruments allows only accuracies
of ± 5% to 10% full scale.
A concentration measurement with this technology
has disadvantages:
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- High sensitivity of gas bubbles and sediments
- In most cases the instruments have temperature compensation for
the device, but not for the density calculation
- Calibration is only possible in the factory
- Difficult installation, especially for larger tube diameters
- High pressure lost because of internally reduced tube diameter
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Please click here to download a comparison between a LiquiSonic system and a Coriolis mass flow meter |
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U-tube, Vibrating
Densitometer
The vibrating U-tube densitometer is a proven and very precise method,
especially for laboratory applications. While using this method for
process applications these instruments will have some limitations: |
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- Maximum diameter of 10mm, therefore a by-pass will be required
- Sensitive against pressure spikes
- No immersion type available
- High sensitivity against gas bubbles and sediments
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Conductivity
The conductivity of a liquid depends on the concentration and activity
of conducting ions in this liquid.
For concentration measurement the (inductive
type) conductivity method is a low cost technology with the following
limitations: |
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- Activity, and therefore conductivity strongly depends
on temperature with up to 3% per °C
- Conductivity measurement is effected by fouling, dirt, generation
of complexes or hydrate covers
- Because of the physical background, the method is only suitable
for inorganic liquids and solutions.
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pH Value
The pH value measurement is a proven laboratory method for
the indirect determination of concentration or density.
The advantage of this method is it is very inexpensive. PH technology
has the following disadvantages: |
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- Direct contact of the sensitive membrane with the process is
necessary
- Time drift behavior requires ongoing calibration effort as well
as expansive sample fittings and retractables
- Not suitable for typical concentration measurement ranges above
1% by weight
- pH sensors consist of glass. Because of this fragile material,
the application in some processes like food or pharmaceutical is
very critical.
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Refractive Index
The determination of the refractive index (critical angle of the
total reflection) is a proven laboratory method for the determination
of concentration or density using various calibration charts.
The refractive index will be determined with
an optical window. For process applications this has several disadvantages: |
typical design of a process refractometer |
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- Coating on the window surface generates a drift of the values
or prevents the application completely.
- The design requires a gasket or glued joint. Both can be corroded
while using such instruments in aggressive processes
- Parts of the electronics (CCD-line) require a Peltier cooling.
Such chillers have a limited life time.
- The refractive index depends on the wave length of the light
source
- Literature based on refractive index charts or calibration charts
of lab devices will not be suitable for the calibration of process
devices
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