The measuring tubes in the flowmeter are oscillated at their natural frequency, which is dependent on the specific meter and the product within it.
The frequency is measured and is directly proportional to the density of the fluid. The electronics convert this into a density value, which can be shown on the display or made available as an output.
A Coriolis flowmeter will typically measure density to an accuracy between ±1kg/m3 and ±20kg/m3 with some specialist meters offering even better performance.
Due to the low density of gases, using Coriolis meters to determine the density is not usually possible. However, this measurement technique offers some excellent benefits when dealing with liquids.
Coriolis meters also measure temperature to provide all the variables needed for density-based concentration measurement.
When two products can be mixed to form a homogeneous liquid, the density and temperature information can be used in a table or formula to determine the concentration. This has many applications such as Brix (sugar in water) or ABV (alcohol by volume) in the food and beverage industry, acid concentrations in the chemical industry and, with advanced meters, can even be applied in the oil and gas industry to determine water cut (oil:water ratio).
Some Coriolis meters on the market have concentration software built in; others are used in conjunction with a separate controller. This makes concentration information directly available over a wide range of industrial protocols including 4–20mA HART, PROFIBUS, PROFINET and Ethernet/IP.
Using digital communication also provides greater diagnostic information, such as flowmeter status, electronic faults and build-up detection. This can help to minimise any unexpected downtime by enabling predictive maintenance
The advantages are simple: accurate in-line measurement concentration measurement that can save time and cost vs typical lab-based techniques. The risk of a lost batch is reduced due to continuous measurement, and human error is removed from the equation. Because the density and concentration information is instantly available, tighter process control can be achieved.
At the same time, the instrument is measuring the flow rate and temperature, so a wealth of process information can be transmitted. It is also possible to use the meter to record the total flow of a specific component of a mixture (e.g. alcohol).
A Coriolis flowmeter has no moving parts in the flow stream, so maintenance costs are dramatically reduced compared to more traditional flow measurement techniques such as positive displacement meters or differential pressure devices.
Depending on the criticality of the measuring point, it is worth considering a calibration schedule which is typically determined by the user or regulatory authority. Some flowmeters can self-verify which can help to qualify the performance between calibrations and, in some cases, even extend the time between calibrations.
Coriolis flowmeters are manufactured in a wide range of pipe diameters so the density, and thus concentration, can be measured in very small pipes DN01 (1/24”) all the way up to large line sizes of DN350 (16”).
To minimise capital expenditure, it is even possible to install a Coriolis meter in a bypass line, providing the product in the bypass is representative of the main line. When using a volumetric flowmeter, such as a positive displacement meter, it is sometimes necessary to compensate for a change in product density (e.g. after a change in temperature).
In this instance, a Coriolis meter can be used to measure the density and convert the volumetric flow reading into a mass flow reading. As above, this could be done in a bypass, which is a technique often used for large hydrocarbon pipelines.
For concentration measurement, it is important to appreciate that including additional components to the mixture increases the error unless these can be accurately compensated for. In the event of three or more changing products, it can become difficult, even impossible, to measure the concentration accurately.
An example of this is using Brix to measure orange juice. Brix is exclusively a measure of sugar (typically sucrose) in water, which is often applied to measure orange juice concentration.
Orange juice contains acids, phenolic compounds and sometimes solids that affect the measurement and make comparison difficult. This also applies to other techniques such as refractometry which is why the two methods may not agree.
It is sometimes possible to configure the Coriolis flowmeter to compensate for the additional components if they are very consistent. If this is not the case, then accuracy will be lost but the measurement will be repeatable.
Another important factor is the consistency of the products. If the density of the components changes, the density for a given concentration will be different, which will impact upon the result. This can often cause a problem in the dairy industry when measuring milk fat concentration.
Different seasons, herds and grass can all have an impact upon milk fat density, so it becomes very challenging to measure accurately. To overcome this, the density of the individual products must be known or measured. This could be done in a main line before the product is diluted or reduced.
A highly accurate and flexible instrument, a Coriolis flowmeter offers precise mass flow measurement of liquids, vapour and gases with very low operating costs, even in tight installations.
Furthermore, the additional measured variables – density and temperature – provide a wealth of process information and facilitate indirect variables such as volume flow and concentration (for liquids). The live concentration measurement can help to drive down costs and increase productivity on plant.