Coriolis density measurement
The phase shift of a flowing, oscillating tube is related to the mass flow of the flowing liquid via the Coriolis force. Various manufacturers of Coriolis mass flow meters promote the suitability of their devices for determining density.
However, Coriolis flow meters are designed to capture the phase shift of the oscillation system, while density measurement requires precise measurement of the oscillation frequency. Therefore, the achievable accuracy is in most cases worse than ± 5% to ± 10% of the measuring range.
Density or concentration measurement is therefore only conditionally possible and associated with many disadvantages:
- high sensitivity to gas bubbles and sediments
- implementation of a temperature compensation of the device, but not of the density calculation
- only factory density calibration possible
- high installation effort for larger nominal widths
- internal reduction of nominal width, therefore high pressure drop and sensitivity to contamination
Flexural oscillator density measurement
The flexural oscillator principle is a proven method in the laboratory field for density measurement and uses the dependence of the oscillation frequency of a flowing tube on the density of the flowing liquid.
However, in process applications, this method encounters the following limitations:
- only usable in bypass, maximum nominal width is typically 10 mm
- the flexural oscillator is pressure-sensitive and pressure-shock-sensitive
- no immersion sensors feasible
- high sensitivity to gas bubbles and sediments
pH value measurement
The determination of the pH value is a method adopted from the laboratory for the indirect determination of concentration or density.
However, the advantage of the low price for the sensors used is offset by a number of disadvantages:
- direct contact of the membrane with the process is necessary
- high drift requires ongoing calibration effort as well as complex and expensive fittings and sampling technology
- no longer usable in typical concentration measurement ranges greater than 1 m%
- pH sensors are made of glass; due to their susceptibility to breakage, their use is critical in certain industries (food, pharmaceuticals)
Refractometry
The determination of the critical angle of total reflection (refractive index) is a laboratory-derived method for determining concentration or density using calibration curves.
The refractive index is determined at the optical window. This results in a number of disadvantages for process devices (refractometers):
- Deposits on the window cause a drift in the measurements or prevent measurement.
- Optical windows require a seal or adhesive that can be attacked by corrosive process fluids.
- Parts of the electronics (CCD line) require Peltier cooling, resulting in a limited lifespan.
- The refractive index depends on the wavelength of the light.
- Refractive index values from literature or a handheld or laboratory refractometer cannot be used for process devices.
Radiometry
A radioactive preparation emits its radiation onto the measurement object, which is received by the detector. A scintillator converts the radioactive radiation into light flashes and evaluates their number. Since the penetration of gamma radiation depends on the material, the density is determined from the intensity of the incoming radiation.
1: Emitter with shielding
2: Scintillation counter
3: Clamp-on measurement section on the pipeline
Radiometry is nowadays replaced by modern measurement methods, as the use of radiometric measurement is associated with high effort, regulatory requirements, costs, and potential hazards:
- complex, expensive approval of the devices by TÜV / trade association
- ongoing maintenance effort, e.g., regular leak tests
- training of radiation protection officers
- information and documentation obligation towards the fire department
- very expensive disposal of radiation sources in case of replacement or return of the devices
- Delivery in special vehicles
- High risk potential for employees in accidents