Frequently Asked Questions

How is Dynamic Surface Tension measured?
The SensaDyne surface tension method is based on a patented modification of the maximum bubble pressure method. Two or more probes with different sized orifices are inserted into a fluid. The instrument determines the differential pressure between the bubbles generated at the orifices and calculates an accurate surface tension value so that effects of liquid density, gravitational constant, and depth of immersion are resolved.

Classical methods such as the DuNoüy Ring or Wilhelmy Plate measure only equilibrium (static) surface tension, while the SensaDyne method can measure both equilibrium and dynamic surface tension since the user can choose the rate at which the bubble forms. The rate of bubble formation determines the surface age; the amount of time during which any surfactant molecules in the tested fluid can migrate to the newly created air/fluid interface.

Why is Surface Age important?
Surface age, as measured in dynamic surface tension, is directly related to the process time in coating, spraying, dipping, or other time-dependent processes; the amount of time the process allows surfactant molecules to migrate to newly formed interfaces where they lower the surface tension. As process speeds increase, surface ages decrease and fewer surfactant molecules migrate to the interface, resulting in higher dynamic surface tensions.

What are the main advantages of the multiple probe method?
Both dynamic and equilibrium (static) surface tension measurements can be made with the multi-probe SensaDyne method, where surface tension is directly proportional to the maximum differential bubble pressure. This method is immune to surface contaminants and surface foam. Since bubbling is continuous, this method is applicable to real time continuous on-line measurement and control. This method also allows surface tension to be measured under pressure; in polymerization reactors, and for liquefied gases.

When will my readings match those of the DuNoüy Ring or Wilhelmy Plate?
Classical surface tension methods such as the DuNoüy Ring or Wilhelmy Plate work with a dead or static surface; where the gas/fluid interface has very likely reached equilibrium. If you take progressive readings at slower rates with a SensaDyne tensiometer, allowing more time for the surfactant in solution to reach equilibrium conditions, you will eventually match the DuNoüy Ring or Wilhelmy Plate readings, when readings do not go any lower. At this point, you will know the equilibrium surface tension value, and also how much time is required to achieve equilibrium, which a DuNoüy Ring or Wilhelmy Plate will not tell you.

There are several conditions where classical methods are not accurate. If the surfactant floats on the surface of the bulk fluid, then DuNoüy Ring and Wilhelmy Plate Tensiometers measure only the surfactant-rich surface layer. If there are trace contaminants, then these contaminants can, on occasion, lower the fluid surface tension to an erroneous value below the equilibrium value.

How do I generate a Dynamic Surface Tension curve?
A Dynamic Surface Tension curve can have as little as three points (two points will show if there is a dynamic effect and three or more points will characterize the general shape of the curve), or as many as ninety nine points (for the PC500-series Tensiometers).

With the manual instruments (QC-series and IP6000) the user changes the flow rate prior to each calibration. When the surface tension reading is displayed, the correct surface age will also be displayed. This process is repeated to expand the dynamic curve to the number of measurement points desired, from milliseconds to several minutes. When points are saved in a Quicklog file the resultant graph can be displayed on the computer screen using the standard "View" software graphing program.

Examples of dynamic curves for two different formulations
 
With automatic Tensiometers (PC500-series) the user selects the desired maximum and minimum flow rates (ranging from milliseconds to minutes) and the number of desired points for the dynamic curve. The instrument automatically cycles through these preselected points for calibration (DynaCal) and measurement (DynaCurv). At each measurement point the instrument applies the calibration values stored in memory and calculates an accurate surface tension value. Dynamic curves and data tables are displayed using the "View" option.

How fast can I generate a dynamic curve?
For manual tensiometer models (QC-series) the time is a function of how many points are desired for the curve and at what bubble rate (or surface age) the tensiometer is set up. Typically, a dynamic curve of five points can be generated in about fifteen minutes. This includes time needed to calibrate.

The PC500-series Tensiometers reduce the time for this example dynamic curve to about five minutes with its automatic DynaCal and DynaCurv operating features.

How are SensaDyne Tensiometers calibrated?
Two fluid standards of known surface tension values, such as Deionized water and one of three alcohols, preselected by the user, are used for an accurate two point calibration of the instrument. In all except the QC3000 Tensiometers, calibration can be performed automatically in Deionized water and the selected alcohol by clicking on "AutoCal High" and "AutoCal Low" buttons, respectively. The system automatically reads the fluid temperature and selects the correct calibration standard value for the specific calibration fluid into which the probes are inserted and bubbling. Any other two standard fluids, with known surface tension values, can also be used.

How often do I have to recalibrate?
The instrument has to be recalibrated if the flow rate or probes are changed (except for normal DynaCal and DynaCurv operations in the PC500-series). The Tensiometer contains pneumatic spring/diaphragm devices that are subject to normal thermal hysteresis. If there are large ambient temperature swings at the test facility, then calibration may be more frequent than in a more temperature controlled environment.

What kinds of SensaDyne Tensiometers are there?
SensaDyne Tensiometers fall into three categories; Manual (QC-series) and automatic (PC-series) that need to interface with a Windows computer, and in-process (IP6000); those that have their own embedded computer system. Each system uses differential pressure transducers to sense pressure from the bubbles that are generated, and a demodulator circuit to convert the resulting fluctuating pressure signal (sawtooth waveform) to a fluctuating voltage signal. Advanced software peak detection software calculates surface tension and displays values for surface tension, temperature, surface age, bubble interval and bubble frequency.

What do I get when I buy a standard tensiometer system?
Each tensiometer system is a complete instrument except for the computer to which the instrument is interfaced and the Nitrogen or compatible inert gas supply required to generate bubbles. Each tensiometer is tested, prior to shipment as a system, at SensaDyne’s test facilities, with all software, computer interface board, and interface cable. Click on "Products" to see individual Tensiometer component listings.

How difficult is software installation and start up?
Simple, step-by-step, set up and software installation procedures are included in the Operator Manual furnished with each instrument. On-site assistance is not required. Software manuals have pictures of all software screens that the user will use with explanations of all related functions. During operation, on-screen help (text) files explain the operation and guide the user through various instrument functions.

How difficult is hardware installation and start-up?
The Tensiometer system is as close to "plug and play" as is possible. The computer interface card is installed in one of the available personal computer slots. An interface cable connects the computer (interface board) to the tensiometer. The probe assembly stand connects to the tensiometer rear panel fittings. Nitrogen or dry air connects to the pneumatic input using a (furnished) quick connect fitting. Following software installation, the tensiometer is ready for operation.

The IP6000 requires only a low voltage (receptacle) power source, since the computer system is embedded within the tensiometer. An RS232 laptop or remote personal computer (PC) interface, using a special (furnished) interface cable, allows the user to check signals and receive data at the remote laptop or PC, once the probes are immersed and the system is powered up. 

What Operating Systems work with SensaDyne Tensiometers?
Software for all Tensiometers will run on all Windows platforms through Windows 7. Software is installed using easy, self-prompting installation programs, supplemented with detailed written installation instructions.

What types of probes are available and which should I use?
Standard glass probes with 0.5 mm. and 4 mm. orifice sizes are used for most fluids. For fluids that may tend to plug (latexes, for example), a larger 1 mm. probe is used for the small orifice to mitigate coating build-up. Pure Teflon, or Teflon-coated glass probes (not shown below), are used to further mitigate plugging. Inverted probes mitigate the effect of bubble distortion caused by highly viscous fluids. Offset probes allow measurement of small samples in the 3 to 5 ml. range. For industrial environments or where durability is desired, stainless steel probes are available. We can test fluid samples to determine the best choice of probe type and material, at no charge.

Available Probes (left to right): Stainless Steel, Glass, and Teflon

What accessories are available?
Users involved in Concentration or Critical Micelle Concentration (CMC) studies will find the STS Dispensing Accessory a valuable time saver, to automate these processes. STS software allows users to pre-program the dispenser to generate two and three dimensional concentration curves automatically, depending on the tensiometer model used. The tensiometer/dispenser system is programmed by the user to sequentially dispense, stir, measure, and store data automatically while the user is doing other things.

Process control, for all tensiometer models, is implemented using an  optional analog output board.

What bubble rate should I use?
Proper bubble rate depends on your application and fluid. Generally, surface age should mimic real-life process time, to which the fluid is subjected. For example: if your printing press process speed is 200 milliseconds, from the time the ink is picked up by the roller until it is deposited, you adjust the flow rate for a 200 millisecond surface age and the resulting measurement indicates the dynamic surface tension of your ink at the moment the ink contacts whatever you are going to print on. If the press is run at several speeds, or over a range of speeds, the user can generate a dynamic curve for the equivalent surface age range to determine how well the ink will perform under all conditions.
 
How do I know if I need Viscosity Compensation?
     As viscosity of any fluid increases, the hydrodynamic resistance against the formation of a bubble in the fluid also increases. This causes a measurement error, referred to as the Viscosity Effect, when using any maximum bubble pressure technique. Though negligible for coatings of small viscosity, it can be several tens of Dynes/Cm. (mN/M.) for highly viscous fluids.

 

    For low-viscous fluids, up to approximately 1000 Centipoise, the Viscosity Effect does not usually exceed normal SensaDyne Tensiometer measurement stability of +/- 0.1 Dynes/Cm. Low viscous fluids up to this range would, typically, be water-bone or similar to light machine oil.  For moderately to very viscous fluids, such as motor oils or Castor Oil, the Viscosity Effect can begin to reach up to tens of Dynes/Cm. at small Surface Age values. Correction factors are needed to obtain true viscosity-compensated values for dynamic surface tension, unless an automatic, viscosity compensating, method is used. Additionally, solids and other additives can give an "apparent viscosity" that is greater than the real viscosity. If in doubt, contact SensaDyne, or the representative in your area, to arrange feasibility tests on your samples; up to two samples, free of charge.

 

    For all Sensadyne Tensiometers, there is a measurement method to eliminate the Viscosity Effect. A document on how this works can be requested from SensaDyne.

 

    The SensaDyne PC500-LV uses three probes, one 0.5 mm., one 1.0 mm., and one 4.0 mm. The 4.0 mm. acts as a control probe, shared with both small orifices. This allows simultaneous, but separate, differential pressure signals to be generated by each of the small 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 PC500-LV user to  obtain viscosity-compensated surface tensions, automatically, without the use of correction factors.

    
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