The measuring cup, an ally in the manual measurement of viscosity
A detailed comparison between advantages and disadvantages of the use of a simple yet essential instrument for the measurement of viscosity in the world of printing.
The most widespread and commonly accepted unit of measurement of viscosity is the measuring cup (Ford-4, Din-4, Zahn-2, etc). Some say that up to this day printers have mistakenly used the measuring cup to measure the viscosity of inks, glues and varnishes. The measuring cup may not be the ideal instrument for the whole range of viscosity measurements, but nevertheless, if used correctly in the measuring frame for which it was designed, the measuring cup is a very precise instrument.
Technical sheet SELECTRA viscosity control systems - Sensors 
Measurement range and precision
Every measuring cup is accurate if used in the measurement range considered reliable. The working range of each type of measuring cup is, however, limited. To say that the measuring cup is unreliable and hardly repetitive is not exact, and such a statement could be misleading for printers. It is however fundamental to take into consideration the fact that it is necessary to use this instrument correctly, which means it needs to be cleaned thoroughly, completely filled and the measurement must be taken in the exact instant the flow from the metering orifice stops.
Unfortunately, the measurement range of viscosity of each cup is very limited. When, for example, we try measuring a value lower than 11 seconds with a Ford-4 cup, we make an error of assessment. Measuring water or solvent with a Ford-4 cup is wrong. Within 10 seconds the Ford-4 cup is not working anymore within the range for which it was certified and therefore is no longer measuring the viscosity correctly.
Consequently, it is possible generally to find a correlation curve that would allow us to align the measurement obtained with the cup and the scientific unit of measurement of viscosity, the centipoise (cP). This means that in turn it is possible to find a curve that well interpolates the data obtained by using the measuring cup. Since the range of the measuring cup is limited, outside of said range obviously it is necessary to extrapolate values to define the ideal measuring cup with a virtually unlimited measurement range. Regardless, the correlation curve between measuring cup and cP is not linear and so 0,1 cP never corresponds exactly to the same number in whole the working field
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Measuring cups and viscosity control systems
Selectra has been producing viscosity control systems for over 40 years. Our technicians oversaw the production of thousands of systems and in our experience it is pointless to question the use of the measuring cup only because the proposed viscosity control system does not manage to correlate well with the measuring cup. What may be inaccurate is the repeatability and proportionality of the vibrating measuring instrument if the comparison provided is not clear.
The viscosity control systems need to be able to maintain absolute precision without resorting to continuous recalibrations. This means that any instrument that requires daily calibrations to preserve the correlation with the measuring cup is imprecise and yields a measurement that drifts over time. The challenge for a viscosity control systems producer is to avoid continuous recalibrations and to allow the measurement of any type of fluid and any range of viscosity all with a single instrument. Uncorrelating the measurement of a viscosity control system from the measuring cup is absolutely wrong. Furthermore, each measuring instrument must avoid depending on a single operator, for whom performing a perfect daily recalibration of every instrument is for all intents and purposes a waste of time.
The advantages of the combined use of the viscosity control system and the measuring cup
The viscosity control system is an extremely useful instrument if it is well used. It allows to drastically reduce ink consumption, while also maintaining an optimal repeatability of the printing quality thus simplifying the working process. Using a single measuring cup implies an excessive employment of workforce and does not guarantee the actual maintenance of the correct viscosities.
Using an instrument that is perfectly correlated to the measuring cup to perform the necessary corrections is a much more efficient solution.
With a view of Industry 4.0, the viscosity control system becomes an additional instrument to control the printing process and allow the measurement of the production performance. Integration is the strong suit of new systems. Thanks to modern communication interfaces it is possible to have a continuous exchange of information between viscosity control system and printing machine. The viscosity control system correlated to the measuring cup is the key to obtaining the improvements for which printers are looking. Abolishing the use of the measuring cup could be, on the other hand, a risky decision.
Selectra viscosity control system sheet
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