To the editor:
While the August/September article on subsurface biofilters was very interesting in demonstrating the wide range of geosynthetics used, I have concerns with the description of spark testing.
The method described was: “A probe with a current was passed above the seam with a 25mm [1in.] distance between probe and the seam, and any sparks indicated that a hole was present.” Spark testing is, in fact, a little more precise and controlled than that, as can be read in ASTM D6365.
The conductive element (copper wire or aluminum strip) is placed under the edge of the top sheet under the centerline of the extruded weld bead. This wire should be grounded.
Typically a single-tip probe, but preferably a brass brush positive electrode, is passed along the surface of the bead. A specific electric potential (not a current) of several kilovolts is applied between the survey probe and ground (the copper wire). The potential is set so that a spark discharge (current) will not occur through the insulating polymer material of the geomembrane, but it will occur if there is a hole (lower resistance passageway) through the weld bead.
Thus, the ability to generate a spark is a function of the voltage (potential) setting and the distance of air, including the leak passage way, between the survey probe tip and the conductive wire. The ASTM standard shows that to spark across a 1mm air gap a potential of ~7.9kV is required, while a gap of 10mm requires ~25 kV. A gap of 25mm (1in.), as mentioned above, plus the passageway through the weld bead, perhaps as much as another 12mm would take ~47.5kV. At this very high voltage, if the survey probe were in contact with the bead or geomembrane surface it could punch a hole through 2mm (80mil) of HDPE. The dielectric constant of HDPE is 600V/mil.
Thus, a single tip electrode set at 30kV to spark across a ~15mm air gap (lack of bonding over half the bead width on the lower sheet) would indicate a leak if passed directly over the hole entrance, but would not indicate a leak when passed along the opposite edge of the weld bead from where the distance to spark would be about 45mm (across the bead and through the lack of bond).
On the other hand, a brass brush probe covering the whole surface of the bead would indicate a hole on every pass. Waving a single electrode tip about 25mm over the bead is inaccurate and requires a much larger potential than if it were on the surface of the bead. Thus, the chances of collateral hole-punching damage to the liner is unnecessarily increased.
Keep the voltage as low as possible and maximize the possibility of finding leaks. Calibration is essential.
Note that there are two types of spark testing equipment—AC and DC. The latter is recognized as more accurate. It requires the conductive wire be grounded. The AC does not. It simply requires a highly conductive medium under the weld bead.
So, at the start of spark testing, determine the voltage to be applied that will generate a spark (and audible “zap”) over the maximum distance of interest, typically half the width of the weld bead, but not enough to punch a hole in the adjacent geomembrane. Use a brush survey electrode to ensure the whole bead is surveyed in one pass. If the AC single tip survey probe is used, pass it along both edges of the weld bead and up the center for assured effective coverage.
Using the wire conductor as a ground electrode also improves the performance of water lance geoelectric integrity/leak location surveys.