RE: vP equation to convert TDR results required

[Jason Smesrud]

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Assuming you are measuring apparent distance with the TDR rather than travel
time, you can use the following correction.

d(r) = v(r)*t
v(r) = r*c*(epsilon*mu)^-0.5
holding epsilon and mu constant, you can correct as follows,
d(0.67) = (0.67/0.99) * d(0.99)

where
d=distance
v=actual velocity of propagation
t=time
c=speed of light in a vacuum
r=relative velocity of propagation
epsilon=permittivity
mu=magnetic permeability

Note that using an r of 0.67 will give accurate estimations of travel
distance in an polyethylene insulated cable but an r of 0.99 is used for
travel time in an uninsulated cable (i.e. most TDR rods). If you are trying
to measure soil moisture around an uninsulated TDR waveguide, an r of 0.99
is the correct setting for measuring an apparent length of the waveguides.
Same holds for estimating an apparent velocity down the waveguide. Hope this
helps.

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 22, 2001 9:41 PM
Subject: vP equation to convert TDR results required

Hello Bruce,
    I am a research assistant at the University of Georgia in Athens, Ga.
USA. Our lab uses TDR machine to measure soil moisture. Recently, I have
noticed that the velocity of propogation setting was wrong. It was set at
0.99 and should have been set at 0.67. I was wondering if you have an
equation that could transform the old data to the correct measurement???

Thanks
shank
bshank@mindspring.com