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Home  >>  Products  >>  Surface Tension  >>  SensaDyne Tensiometers Information
SensaDyne Tensiometers Information

Accuracy and Stability

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

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Two probes of dissimilar orifice sizes (most commonly 0.5 mm and 4.0 mm) bubble into a fluid where the differential pressure value of the formed bubbles is measured. This value is directly proportional to the fluid 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 the rate at which the bubbles forms. This determines the Surface Age; the amount of time during which surfactant molecules 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 the QC-Series Tensiometers or automatically, and then automatically repeated at user-programmed time intervals, using the PC500-Series Tensiometers.

SensaDyne Windows ® -compatible software, Version 1.4.1, accurately measures and displays the Surface Age (bubble lifetime), Bubble Interval, and the Bubble Frequency (the inverse of Bubble Interval), at all times, showing the real time results in the “Numerical Results” window of the main screen. This feature was originally added when we developed our Advanced (software) Peak Detection technique (APD), for detecting and monitoring maximum differential bubble pressure, which is exactly proportional to fluid surface tension. Because we can simultaneously track, measure, and time the pressure voltage waveform of the differential maximum bubble pressure signal, we can very accurately measure the valid peaks (maximum bubble pressure), the minimum peaks (capillary action, after the bubble releases from the orifice), the Bubble Interval and the bubble lifetime (Surface Age).



As shown in the screen display, the Bubble Interval has two components: Surface Age (the positive slope of the “saw tooth” pressure waveform), 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 reached 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, which we cannot control, depends on the viscosity of the fluid. The dead time tends to be a relatively constant value, and effectively limits how fast one can bubble with a particular probe before transitioning to the “oscillating jet” mode. This limitation on how fast one can bubble into a fluid, 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 problem area. All Sensadyne Tensiometers have a viscosity compensation software program feature that can be used after applying Stokes Law lied to the initial bubble relationship set up.

As the bubble interval decreases, surface age becomes proportionately smaller. For example, at one bubble per second, an aqueous solution will have a surface age of about 0.95 seconds; at ten bubbles per second the surface age will be in the range of 0.05 seconds; and at thirty five bubbles per second will be on the order of 3 to 5 milliseconds.

 

Differential Pressure Method versus Single Bubble Tensiometers

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It is important to recognize that single bubble tensiometers utilize apparatus and pneumatic techniques for a single probe, that is quite different than one that we employ with the 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 here is the fact that they do not consider the effect on pressure drop ( D P) across the system (between transducer and orifice) which varies. There are also questions regarding other trade-offs used in their technique, such as use of an inclined capillary.

There is a third party article regarding limitations (inaccuracies) of some single bubble tensiometers, that 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”. This was written by the original developers of the Lauda single bubble tensiometer, even though this is not acknowledged by the authors, so this bias needs to be recognized. The article explains why some single bubble tensiometers will read lower than actual dynamic surface tension values. In some cases this can result in thinking 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 in all bubble tensiometer methods, and dynamic curves can differ with differing set-ups. In the SensaDyne scheme of things, we chose to use Mass Flow Controllers (MFCs) which provide us with very stable and linear increases in mass flow at each orifice, unlike the “Lauda” or “Krüss” single bubble tensiometer systems.

The SensaDyne electronics are designed to convert the differential pressure of the resulting bubble formations to a precise parallel electronic voltage signal that exactly duplicates the differential pressure. We know that what we generate as an electrical display signal is a true representation of the differential bubble pressure. In our Windows Ò -compatible software the display is concurrent with sampling and peak detection, In other words, 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 that all signals from the small orifice are “adjusted” by the contribution of the large orifice. For practical considerations we pneumatically dampen the large orifice signal (but only enough) to offset the effect of depth of immersion, specific gravity, and density, so that the resulting differential maximum bubble pressure is directly proportional to surface tension, and accurate to within +/-0.1 Dynes/cm (mN/m).

 

Accuracy of SensaDyne Differential Pressure Transducers

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In the QC-Series tensiometers, we use an MKS differential pressure transducer whose response is very similar to our other transducer (used in our PC500-Series and previously in our traditional PC6000/9000 line) the Validyne DP15. In our opinion, the MKS unit is a performance clone of the DP15. The biggest difference is that the DP15 can be used in pressurized applications to 250 PSI while 40 PSI is the limit for the MKS.

The frequency response of the Validyne DP15 is 80 Hz., 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. On Validyne’s website www.validyne.com under the title ”Estimating the Frequency Response of Variable Reluctance Pressure Sensors in Gas”, there can be found an excellent write-up on necessary ramifications and calculations, and a case study, with respect to this problem. The only difference is that we use 1/8” I.D. tubing instead of the 3/16” that the Validyne case study uses.

We use the same Validyne DP15 demodulator board circuitry as in the referenced study, and we know from extensive testing that the response of the MKS (with built-in demodulator) is very similar. In fact, when we first went to MKS to use their transducer and demodulator combination we initially provided response graphs of the Validyne unit which they then successfully matched. If you make the necessary calculations using the QC-Series configuration, the frequency response will be above our maximum bubble rate for our Tensiometers. Additionally, no signal distortion is evident in the response of the MKS, as used by SensaDyne.

 

SensaDyne Software for Windows Ò 95/98/NT4.0/2000/XP/Vista – Version 1.4 for all SensaDyne Computer-Interfaced Tensiometers

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Program Features

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  • On-Screen “AutoCal” Buttons for Faster Surface Tension Calibrations with expanded choice of Ethyl, Isopropyl, or Methyl Alcohol, for low standard calibration fluid*.
  • Continuous Real Time Acquisition and Data Display for Surface Tension, Temperature, Surface Age, Bubble Interval, and Bubble Frequency, with Continuous Maximum Bubble Pressure Waveform and Temperature Signals.
  • Faster Test Results with User-Selected Options for Initial Analyzation Time.
  • Automatic Surface Age Measurement for Surfactant Diffusion Rate Studies.
  • Expanded Capabilities for Viscous Fluids with Advanced Software Peak Detection .
  • Graphic User Interface for direct Graphing of Data on Linear or Logarithmic axis. All Data Files convertible to ASCII Format for importing to other graphing programs.
  • On-Screen Status Indicators for Analyzation, Peak Detection, and Detected Peaks.
  • Easy Access to Help Text Files for On-Screen Tensiometer Operation Assistance.

 

Minimum Computer Requirements

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Minimum Computer Requirements
Computer Processor: Intel Pentium or compatible 32 bit processor
Memory: 8 MB used by the program, 16 MB for data acquisition, (32 MB recommended)
Display: VGA with screen resolution set to 800x600 or higher

 

Software Info

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True Windows® Point and Click Operation: 32 bit version for Windows® 95, 98, NT 4.0, 2000, and XP Operating Systems. True point and click mouse operation with pull-down menus for fast program navigation, operation, data file generation, and graphic display.

Advanced Software Peak Detection: Positive triggering only on maximum differential bubble pressure peaks. Eliminates false triggering on electrical and/or pneumatic noise and from viscous fluid effects. Adjustable analyzation time for faster readings when dealing with long surface age (equilibrium/static) measurements.

Automatic Surface Age Determination: Calculates (does not estimate) precise values for Surface Age (surfactant migration time), under all operating conditions from milliseconds to several minutes for all SensaDyne Tensiometers.

Real Time Bubble Pressure Display: Concurrent viewing of surface tension maximum differential bubble pressure waveform while surface tension is being measured in real time.

Individual Peak Data Recording Option for Viscosity Compensation: Allows the instrument, using a special data file collection software option, to be set up and used for true viscosity compensated surface tension measurements, using standard QC and PC tensiometers, when occasional viscous fluids need to be measured.

Expanded Graphing Capabilities: Graph and print results of Surface Tension, Temperature, Surface Age, Bubble Interval and Bubble Frequency directly, with ASCII data conversion for all files. Linear to logarithmic axis. Full compatibility with earlier SensaDyne Windows® software files.

Surface Tension versus Surfactant Concentration Curves: Generate and plot surface tension versus surfactant concentration, both manually and automatically (using the STS dispenser accessory) to determine both equilibrium and dynamic Critical Micelle Concentration (CMC) curves.

One Button “AutoCal” Calibrations*: On-screen high and low AutoCal buttons allow more direct and faster calibration when using D.I. Water and Ethyl, Isopropyl, or Methyl Alcohol calibration standards. All standard calibration value tables are stored in memory and accessed for calibration based on the sample temperature. *except QC3000Tensiometers

On-Screen Help Menu & E-Mail Access: On-screen help menu features access set-up, operation, and calibration text help files in program memory. Direct e-mail messaging and web site access allows users to obtain prompt answers to any technical or operational questions.

Comprehensive Data Acquisition: User-selected “scan” or “time” options for programmed data collection (Data File). On-demand (Quicklog) data collection of Surface Tension, Temperature, Surface Age, Bubble Interval, and Bubble Frequency.

 

PC500-Series Automatic Tensiometer Features

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Improved Mass Flow Controller Operation: Mass Flow Controller (MFC) operation is improved by unloading (purging) the MFCs before each DynaCal and DynaCurv run. This sets the MFCs at the same physical setting and assures that the characteristics of the MFCs and mass flow is the same for every calibration and test run, increasing stability and repeatability.

Single, Multiple, and Continuous DynaCurv Operation Options: Users can now choose among single, multiple, and continuous DynaCurv ( Dynamic Curve generation) options. The tensiometer can be programmed for single or multiple tests runs with user-programmed time delay between test runs. This allows multiple dynamic curves to be run on single test samples to reveal dynamic surface tension changes that may occur over time for the tested sample. Each distinct DynaCurv test result is automatically stored in a separate data file.

Strip Chart Voltage Values Display Option: Analog output channel maximum voltage values for both Surface Tension and Temperature can be displayed on the screen to verify instrument settings.

Easier MFC Online Checking Capabilities: Easier verification of computer COM port operation with the MFC controller board. More positive COM port and controller communication.

Simplified Mass Flow Controller Software Editor: The MFC Control Panel now has an editor option that allows the software settings of the MFCs to be changed and more precisely matched to the actual physical MFC settings in the event of, and following, a computer COM port malfunction. Users no longer have to physically reset the MFCs or “jog” the MFCs back to their required positions.




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