Tensiometer Accuracy

 
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 Freqeuncy (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).
 

The Bubble Interval has two components: Surface Age and Dead Time (negative slope of the waveform). The limitations to how fast one can bubble in a specific fluid is determined primarily by the “Dead Time”; the time it takes for the bubble to break down after it reaches maximum bubble pressure (an ideal hemispherical shape at the orifice tip), depart from the orifice, the time for the fluid to flow into the area vacated by the departing bubble, and the capillary action (based on the orifice size and physical configuration). A significant portion of this cannot be controlled, and depends on the viscosity of the fluid. The dead time, therefore, tends to be a relatively constant value and effectively limits how fast one can bubble, with any particular probe, before transitioning to the “oscillating jet” mode. This limitation holds true regardless of what bubble tensiometer is used. If the fluid contains a high level of solids, then the “apparent viscosity” will be higher than the real viscosity and will add to this limitation. All Sensadyne Tensiometers have a viscosity compensation feature used, in conjunction with applying Stokes Law to the bubble relationship set up, to accurately measure highly viscous fluids.

 

As the bubble interval decreases, surface age becomes proportionately smaller due to a relatively fixed dead time. For example, at one bubble per second, an aqueous solution will have a surface age of around 0.95 seconds; at ten bubbles per second the surface age will be around 0.05 seconds; and at thirty five bubbles per second surface age will be in the range of 3 to 5 milliseconds, so it is critical to accurately measure the Surface Age, rather than estimate it as some Tensiometers manufacturers do.  

 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.

 

Some single bubble tensiometers have a “reservoir” with “dampening” chamber, used to dampen the pressure oscillations in the system. This technique allows a dynamic curve to be generated using a “burst” (varying frequency) technique, but the problem with this technique is that it ignores the pressure drop (DP) across the system (between transducer and orifice), which can vary considerably. There are also questions regarding other trade-offs used in these techniques, such as use or result of an inclined capillary.

 

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.

 

Physical orifice configurations and the materials of the probe tips are quite critical in surface tension results obtained by all bubble tensiometer methods, and dynamic curves can differ with differing set-ups. SensaDyne's technology uses Mass Flow Controllers (MFCs) which provide stable and linear increases in mass flow at each orifice, unlike most single bubble tensiometers. SensaDyne also prefers to use compressed inert gases, such as Nitrogen or Dry Air for bubble generation, rather than air pumps which often are not robust enough to overcome higher test sample viscosities at faster bubble rates. 

 

SensaDyne electronics converts the differential pressure of the generated bubbles to an electronic DC voltage signal that exactly replicates the differential pressure waveform. We output an electrical signal that is a true representation of the differential bubble pressure. In our WindowsÒ-compatible software the display is concurrent with sampling and peak detection. SensaDyne Tensiometers use real-time sampling, calculation, and display.

 

SensaDyne Tensiometers output a differential pressure signal rather than an absolute pressure signal, as used by single bubble pressure tensiometers. This means small orifice signals are pressure adjusted by the pressure contribution of the large orifice. This eliminates errors due to depth of immersion, specific gravity, and density, so that the resulting differential maximum bubble pressure is directly, and accurately, proportional to surface tension to within +/-0.1 Dynes/Cm. (mN/m).

 

 Accuracy of SensaDyne Differential Pressure Transducers

 

In QC-Series tensiometers, SensaDyne uses a differential pressure transducer whose response is similar to the transducer used in the PC500-Series. The QC-Series unit is a performance clone of the PC500-Series transducer. The biggest difference is that the PC500-Series transducer can be used in pressurized applications up to 225 PSI, while 40 PSI is the limit for the QC-Series.

 

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. 

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