W8JI compared the readings with EHS and fiberglass guy material in series
and found a large discrepancy. As far as I know, he didn't compare EHS and
Phillystran. My experiment compared the readings of two gauges on the same
material, 1/4" EHS.
The "thought experiment" that with increasing tension the material becomes
more "rod-like", and therefore the differences in tension tend to decrease,
sounds reasonable. After all, my own empirical experiments showed that the
percentage difference between the Loos and Dillon gauges decreases with
increasing tension. Unfortunately, the range of difference was 50%-20%.
But let's try another thought experiment. The PT-2 calibration plate chart
in the Loos manual says that at a reading of 32, the tension of 7/32"
stainless wire rope will be 1060 lbs and the tension of 1/4" stainless wire
rope will be 780 lbs. That's a difference of 36% from a mere 1/32"
difference in diameter of the same material. Most of the other differences
in the series are closer to 50% (they're not very linear, which is what I
found with my Loos gauge.) Given the relatively wide spacing between
markings on the calibration plate, you would think that a 1/32" difference
in the movement of the rollers wouldn't result in that much difference in
the reading. I would conclude that the most of the difference is due to the
difference in stiffness between 7/32" and 1/4" 1x19 stainless steel wire
rope.
My bottom line on this is that either the material really does makes a
difference or my Loos gauge has a bad spring. I'd include the alternative
that my Dillon dynamometer needs calibration, but that wouldn't explain
W8JI's results. Maybe his Loos gauge has a bad spring, too, and we need to
wonder about quality control on these gauges.
Don't get me wrong: the Loos gauge is incredibly useful for setting the
final tension of all the guys to the same value, which is very important.
Also, I suspect the difference between 660 lbs (my Dillon reading) and 840
lbs (my Loos reading) won't have a significant effect on the typical amateur
tower installation. I'm sure a lot of people out there just eyeball the
tension and don't get all that close to the 10%-of-breaking-strength target.
73, Dick WC1M
> -----Original Message-----
> From: Roger (K8RI) [mailto:K8RI-on-TowerTalk@tm.net]
> Sent: Sunday, September 21, 2008 10:03 AM
> To: wc1m@msn.com
> Cc: K7LXC@aol.com; towertalk@contesting.com
> Subject: Re: [TowerTalk] pulling guy lines tightly
>
> Dick Green WC1M wrote:
> > Well, this must be one of those YMMV situations.
> >
> >
> Apparently <:-))
> > All I can say is that on EHS near the 10% target tension, my Loos
> reads
> > about 30% high compared with a Dillon dynamometer. At the low end of
> the
> > scale, it's off by about 50%. At the high end of the scale, it's off
> by
> > about 20%. Undoubtedly one of the gauges is off.
> I'm running Phyllistran with 10 to 15' headers of EHS at the bottom.
> It
> seems to matter little whether I put the gage on the Phyllistran or
> EHS.
>
> Roger (K8RI)
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