When measuring surface tension using the SensDyne maximum differential bubble

pressure method, as the viscosity of the fluid increases, the hydrodynamic resistance

against the formation of the bubble also increases. This causes a measurement error

referred to as Viscosity Effect. Though negligible for coatings of small viscosity, it can

be several tens of dynes/cm for highly viscous coatings.

For non-viscous fluids the correction does not exceed normal instrument measurement

error of +/- 0.1 Dynes/cm. For very viscous liquids, however, the value can reach up to

tens of Dynes/cm at small surface age values, and correction factors are needed to obtain

true viscosity compensated values for dynamic surface tension, unless an automatic

viscosity compensating method is used.

For all Sensadyne Tensiometers that have two or more probes there is a method to

eliminate the Viscosity Effect. For manual two probe tensiometers, such as the QC-series,

the Windows® software (version 1.4.1) has an "Individual Data Recording" option as

described in this excerpt from the Software Manual:

"Individual Data Recording: This option should only be used when the tensiometer is

used to measure very viscous fluids. This option collects data from each individual, valid

detected peak when the tensiometer is set up for the viscosity compensation mode using

Stokes Law. [The large orifice is set to a surface age equal to the ratio of the large to

small orifice - for a 4.0 mm. and 1.0 mm. orifice the surface age of the large orifice is set

four times faster than the surface age of the small orifice]. Minimum values will occur

when both orifices release bubbles at the same time and the viscosity effect at each

orifice cancels out. The lowest readings collected in the data file set (after running

perhaps forty or fifty scans) will reflect the true viscosity compensated surface tension."

The SensaDyne PC500-LV uses three probes, one 0.5mm, one 1.0mm, and one 4.0mm.

The 4.0mm acts as a large control probe shared with both small orifices. This allows

simultaneous but separate differential pressure signals to be generated by each of the

smaller orifices. The maximum bubble pressure peak values from each of these

differential signals are electronically substracted by the software to give a final

differential in which the hydrodynamic effect cancels. This patented process makes it

simple for the user to obtain a viscosity compensated value without the use of correction

factors.