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NOTE: To get off this list, send email to majordomo@aqua.ccwr.ac.za with the body of the message containing the line: unsubscribe sowacs Brad, Yes. Ignore the polyethylene cable, set vP to 0.99, and measure the apparent distance from the beginning to end of the waveguides from your signal trace. If you have a Tektronix cable tester, attach your lead cable and TDR probe, use a paper clip or knife blade to short across your TDR rod at the beginning of the rod while watching the screen. You should then be able to pinpoint where the beginning or your TDR rods are by seeing where the trace diverges upon shorting. This is a fixed point as long as you use the same lead cable each time. The distance shown on the screen will NOT be the actual distance from the meter to the probe since the vP is set to 0.99, but that is not important as long as you know the apparent distance to the beginning of the probe. When you insert the probe into the soil, you can see the endpoint of the rod by the inflection in the signal shortly following the beginning of the rod. The difference between these two points on your screen indicate the travel time or apparent distance down the TDR waveguides. This is what you use for soil moisture estimation. Hope this helps. Jason Jason Smesrud CH2M HILL 825 NE Multnomah, Suite 1300 Portland, OR 97232-2146 -----Original Message----- From: owner-sowacs@aqua.ccwr.ac.za [mailto:owner-sowacs@aqua.ccwr.ac.za] Sent: January 31, 2001 11:28 PM Subject: vp equation for TDR NOTE: To get off this list, send email to majordomo@aqua.ccwr.ac.za with the body of the message containing the line: unsubscribe sowacs Jason, Thank you for the information. What I am confused by is this: our cable is 6ft and is polyethylene insulated. However, it connects to a pair of unisulated waveguides buried in the ground. Do I just ignore the cable and use the vP setting of 0.99 for the un-insulated rods??? thanks, Brad Shankwiler