Accuracy and Stability of SensaDyne Tensiometers

Accurate Measurement of Surface Age, Bubble Interval, and Bubble Frequency

Two probes of dissimilar orifice sizes (most commonly 0.5 mm. and 4.0 mm.) bubble into a fluid where the differential pressure of the formed bubbles is measured. This value is directly proportional to the fluid's surface tension. Since the method allows continuous bubbling, it also allows continuous, in-process, measurement. While classical methods measure only equilibrium (static) surface tension, Maximum Bubble Pressure Tensiometers can measure both equilibrium and dynamic surface tension, since the user can choose, and accurately control, the rate at which the bubbles form. This determines Surface Age; the amount of time during which surfactant molecules in the formulation can migrate to the gas/fluid interface. Additionally, by varying the bubble rate and therefore the surface age, in a pre-selected sequence, a complete dynamic curve can be generated either manually using QC-Series Tensiometers, or automatically using PC500-Series Tensiometers.

SensaDyne Windows®-compatible software accurately measures and displays Surface Tension, Temperature, Surface Age (bubble lifetime), Bubble Interval, and the Bubble Frequency (the inverse of Bubble Interval) at all times, showing real-time results in a “Numerical Results” display. This feature was added when SensaDyne developed its patented Advanced (software) Peak Detection technique (APD), for detecting and monitoring maximum differential bubble pressures, while rejecting false peaks due to electrical and/or pneumatic "noise". We track the differential maximum bubble pressure waveform at high sampling frequencies and very accurately measure the valid peaks (maximum bubble pressure), the minimum peaks (capillary action, after the bubble releases from the orifice), the resulting Bubble Interval and bubble lifetime (Surface Age).

Differential Pressure Method versus Single Bubble Tensiometers

It is important to recognize that single bubble tensiometers utilize apparatus and pneumatic techniques for a single probe, that are quite different than what SensaDyne employs with its differential maximum bubble pressure method. First of all, we use mass flow controllers (MFCs) at each orifice to provide constant mass (volumetric) flow.

An article, regarding limitations (inaccuracies) of some single bubble tensiometers, was published several years ago in the SöFW Journal, International Journal of Applied Science, Issue ½-2004, Page 41-46. “Comparative Studies of Dynamic Surface Tensions of C12EO6 Solutions Measured by Different Maximum Bubble Pressure Tensiometers”. The article explains why some single bubble tensiometers will read lower than actual dynamic surface tension values. In many cases, this can falsely suggest that the surface tension of a formulation is lower than the surface energy of the substrate, and there is a positive wetting coefficient, when in fact there is not.

Accuracy of SensaDyne Differential Pressure Transducers

The frequency response of SensaDyne Tensiometer transducers is in the 80 Hz. range, more than twice the practical limit of bubble frequency for most dynamic surface tension applications. This response is at the input ports and the “plumbing effect" (length of tubing between the probe tips and the differential pressure transducer) must be applied to this to obtain an “effective” frequency response. Contact us and we can refer you to a web site that will give you these calculation methods and related case studies.