From: Brentley C. Stratton
Sent: Thursday, July 14, 2005 1:40 PM
To: Wayne T. Reiersen
Cc: Bradley E. Nelson; David W. Johnson; David E. Williamson; James H.
Chrzanowski; George W. Labik
Subject: Test of Teflon insulation for MC co-wound loops

Wayne,

George Labik and I tested Teflon heat shrink tubing as a possible 
insulation for the mineral insulated cable that will be used for the 
modular coil co-wound loops. The results are positive.

We used a sample of Teflon heat shrink tubing from the TexLoc Corp. and 
a short length of 32 mil diameter MI cable. This is the same diameter 
cable that we have ordered for the co-wound loops.

Our first attempt was to use a heat gun to shrink at 13-1/2" length of 
tubing onto the MI cable. This was not satisfactory because it yielded 
very nonuniform shrinkage of the tubing to the cable. It was possible 
to slide the shrunk tubing along the length of the cable. I immersed it 
in liquid nitrogen several times and kept it immersed for times ranging 
from 5 minutes to over an hour. When removed and allowed to warm up, 
there was no damage at all to the Teflon tubing. I also tried bending 
it while immersed in the LN2 and it bends quite easily and does not 
crack. In this test, it did shrink alot lengthwise while cold, about 
1/4" out of 13-1/2", or 2%. So I don't think there is a problem with 
using this Teflon tubing at LN2 temperatures.

The vendor's datasheet recommends heat shrinking the tubing in an oven 
if possible. We tried this with fresh tubing and got much better 
results. S. Jurczynski used an oven at 650 degrees F and obtained very 
uniform shrinkage of the tubing to the cable and it is very tight: it 
is not possible to slide the shrunk tubing along the length of the 
cable. In this test we did two sections of tubing, one 3-3/4" long and 
the other 4-3/4" long. The measured diameter of the shrunk tubing is 47 
+/-1 mil, based on measurements at half a dozen places. The measured 
diameter of the bare MI cable is 32.5 mil. So the tubing wall thickness 
is 7 mils. I also tested this sample in LN2 and found that it does not 
shrink when cold. This must be due to the better adhesion of the tubing 
to the MI cable caused by the tight shrinkage, which forces the tubing 
to contract the same amount as the cable. Note that these tests are for 
short lengths of tubing so we can not rule out some shrinkage over the 
longer lengths (25-30 feet) that will be needed for the co-wound loops. 
There was no craking or visible damage to the tubing in this case, 
either.

My conclusion is that this Teflon heat shrink tubing is a good material 
for insulating the MC co-wound loops. George is going to contact the 
vendor to get a quote on longer lengths and we will place an order when 
we have this information. Although the standard length is 4 feet, it is 
available as a special order item in lengths up to 100 feet. I am 
allowing 35 feet of MI per co-wound loop so we will try to get lengths 
at least this long.

Brent