Dosing is quite simply a means of transporting a certain quantity of substance from point A to point B. In principle, there are various different ways to go about dosing. These approaches, and how one should implement them from the technical perspective, are described in this blog post.
Indirectly determined dosage
The amount of substance transferred is determined by a controller which must keep a measured variable (e.g., pH) constant by dosing a substance (acid and base). The goal is that the substance transfer results in a measured value equivalent to the setpoint. This takes precedence over other factors, like the amount of substance that is transferred or the time within which it is transferred.
Directly determined dosage
A substance can be transferred in a determined quantity over either an indeterminate time or alternatively, with precise, controlled dosing over a given period.
A pump could add the entire contents of a bottle into a vessel, or a certain quantity of powder into a container, as in the first case. In this situation, the time required to execute this action plays only a secondary role.
On the other hand, there are numerous instances where a substance needs to be transferred with a precisely defined, continuous (or discontinuous) flow. E.g., in Chemostat cultures, the growth rate of the culture is directly impacted by the flow rate of the medium supplied. For continuous dosing, both the rate of dosing – the flow – as well as accuracy, i.e., the deviation of the effective flow rate from the desired/preset flow value are important.
There are two ways to set and maintain a desired flow rate:
The desired flow is adjusted by means of a delivery device like a pump/an implement built into the valve housing, which generates as constant a flow rate as possible. The flow rate is measured once in order to calibrate the system and then functions on the assumption that once set, it will not change within the given period.
Because these systems must generate a flow that is largely independent of external influences over a long time, the most common types are pressure stiff, positive displacement pumps such as peristaltic pumps, piston-metering pumps, diaphragm or even gear pumps, all of which enable precisely controlled dosing. Centrifugal pumps, on the other hand, are not suited to this type of dosing. Of all the above, peristaltic pumps are most typically used for hygienic applications.
The alternative to controlled dosing is when the flow is constantly measured and managed by a sensor and controller, which makes small adjustments to keep the flow rate at a defined setpoint value. For this approach, the delivery device doesn’t need to be calibrated, but the flow meter should be.
Regulated dosing systems always represent a combination of a flow measurement sensor together with a controller and an actuator as the correcting element. With such systems, there are in principle no restrictions in the choice of controlling device, and switching valves, control valves or even centrifugal pumps may be used.
Regulated dosing with gravimetric flow measurement
In order to accurately measure the flow, weighing balances also come into play. The flow is determined by the time derivative of the measured weight, which is advantageous, in that the total quantity added (consumption) can also be directly and accurately measured via the weight value. In addition, the desired flow can be maintained and determined very accurately over a long period of time, especially if the desired flow is set via a weight-time profile.
Disadvantages, on the other hand, are that very low flow rates to be measured in the present or over a short time frame can often result in an inaccurate result. Small disturbances, such as a draught of air or even minimal change in the position of a hose, can lead to strongly fluctuating flow values in the short-term range. It is also relatively more complex to implement than only controlled dosing.
Regulated dosing with other flow measuring devices
Alternatives to gravimetric flow measurement include magnetic-inductive measurement, Coriolis flow measurement, ultrasonic measurement and other measuring principles. It is sometimes possible with such measurement systems to be able to maintain a more accurate flow in the short-term range. In principle, however, the total quantity dosed over a longer period of time can be measured more accurately with gravimetric dosing systems.
Like gravimetric controlled systems, these options too are much more complex to implement than only controlled dosing - and often result in no improvement in accuracy.
In general, regulated dosing has the advantage that disturbance variables, such as strongly changing pressure conditions, a squeezed or worn hose, or pump slippage can be corrected.
In practice, however, the unregulated, controlled dosing has equally important advantages. The flow can be better kept constant in the short-term. Especially with small flows, it fluctuates less and is more cost-effective - above all because it is independent of complex measuring technology.
In any event, all relevant parameters and requirements should be clarified in advance when selecting a suitable dosing method, e.g. flow range, permissible deviation from the set point, permissible deviation of the quantity transferred over a longer period of time, which total volume is to be dosed during the process, which disturbance variables should be considered and planned for, which pressure conditions will be present in the dosing line, which chemical composition as well as the approximate viscosity of the liquid to be dosed.