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Why Don't I Get 72.8?

Water is often used as a reference for interfacial tension measurements. There are many reasons you may not get the handbook value. Here is a checklist, in order of liklihood of the cause:

  • Magnification may not be set correctly for the system. This is the most common cause of inaccuracy. If you use a stainless steel dispense tip as a reference, measure it with a micrometer to make sure you have an accurate diameter. The micrometer measurement must be made within 5 microns, and preferrably within 2 microns. You must be very careful making this measurement: do not squeeze the needle by applying any force to it with the micrometer -- use the measurement just as the micrometers jaws touch the needle. If you squeeze the needle, you may read 5 microns or so low.

  • Improper camera setup constants. The files supplied by the frame grabber manufacturers are typically not accurate enough to set camera timing. The software installation instructions ask you to install the frame grabber first, then the Fta32 Video software, because the Fta32 Video software overwrites the frame grabber timing files with the corrected ones. If you install the frame grabber after the Fta32 Video, you will use the less accurate files. This is related to the last item in this list.

  • Image focus is important. Bad focus affects where the algorithms find edges and this is the same as changing magnification. Always focus with the aperture open and then partially close the aperture (say 1/3 to 1/2) to make measurements.

  • Water is easily contaminated by things that affect surface tension. Take a measurement within a second after forming a new drop and see if this is materially different than one or two minutes later. If the surface tension decreases in time, something is acting as a surfactant. Water may decrease to the low 60's from simple skin oil contamination.

  • Make sure that gravity has distorted the drop so that it is no longer spherical. The FTA algorithm draws a horizontal line at the height in the drop that would correspond to a spherical drop. Make sure this line appears distinctly below the dispense tip where the drop emerges. If it is too close, add more liquid to the drop.

  • Do not place too much importance on a single SnapShot. Take a movie of 10 or 20 images and look at the average surface tension. Vibration can cause variations of several units; averaging will remove these variations.

  • Too much contrast inhibits the edge finder in the software. As you move the XYZ tool around in the background of the image, you should see some variation, rather than always one number like 255. Lower the contrast setting slightly and take a new image.

  • The IFT of water is affected by temperature to -0.16/C. The handbook value is the value at 20C. A temperature of 25C will therefore cause the IFT to be 5 x .16 = .80 units lower, or a net of 72.0.

  • Last resort: The aspect ratio of the camera may be wrong. This is the magnification in the vertical direction compared to the horizontal. We expect this ratio to be "1", which we call square pixels. Cameras are within a few percent of being square, but this is not good enough for serious IFT work. The standard files are normally correct, but there is always the chance that something is wrong. Here is what to check:

    • USB cameras: You must experimentally determine the aspect ratio for your camera. You can adjust the ratio on the Calibration tab. Take a movie of a pendant drop of water. Adjust the value, say from 1.00 to 1.01 (only a per cent or two) and re-analyze the movie. Continue adjusting until the average value is 72.8, or whatever is desired. The total adjustment should not be more than, say, five percent.

    • MuTech frame grabbers: Open the file "RS170.ini" in the Fta32 application directory with Notepad. Make sure the first entry under [PLL Control] reads H_Period=63.492000. Under the same [Pll control], make sure the second entry reads Pixel_Per_Line=0x30B. This second entry adjusts the number of pixels in a line which affects the aspect ratio.

    • Leutron Vision frame grabbers: Use Camera Editor and open the desired camera settings, say, BW_RS170 or BW_RS170_FTA. Under Timing, select Pixels mode and then check the number of lines in a frame. The number of pixels in the image is HBS - HBE. Typical values are 768 - 128 = 640. The number of lines in an image is VBS - VBE. Typical values are 518 - 38 = 480. See the Camera Editor Help for a diagram of video image timing. The number of lines (480) and number of pixels in a line (640) determine the aspect ratio.