[TCML] Concrete Form Secondary Resistance Measurements

[Jeff Larson <jeff@xxxxxxxxxxxxx>]

I did some resistance measurements on a cardboard concrete form tube 
from Home Depot.  Home Depot's brand is "Quick-Tube".  I used a Megger 
MIT 430 for the measurements.  It uses up to 1100 volts DC to measure 
the resistance.   It was at that voltage for all these measurements.

To do the test I used a caliper set at 0.1 inches lightly scribed 3 
pairs of lines on the inside the tube, 3 pairs on the outside where 
there was no black ink, and 2 pairs on the black ink.  I made sure that 
the caliper did not scrape the ink or surface away as I wanted to be in 
contact with it.

I did the test twice.  First test was done about an hour after the tube 
had been brought in from the garage where the temp was about 45 degrees 
F and 50% humidity. 
The second time I did the test the tube had been sitting in the lab for 
about 5 hours at 75 degrees F with much lower humidity.  I am assuming 
that the tube dried some after sitting in the lab for 5 hours since most 
of the values went up. 
Here are the values.  The letters indicate the test area. 

1st test  after 1 hour inside.
Outside of tube, no ink            Outside of tube on black ink         
  Inside of tube

a)    4.5 gigohm                              d)  14.5 
gigohm                                f)   7    gigohm
b)    8.5 gigohm                              e)   25  gigohm 
                               g)  3.5  gigohm
c)     11 gigohm                                                         
                            h)  2    gigohm   


2nd test  after 5 hours inside.
Outside of tube, no ink            Outside of tube on black ink         
  Inside of tube

a)  20 gigohm                            d)   27 
gigohm                                    f)      9.6    gigohm
b)  60 gigohm                            e)   26  gigohm 
                                 g)     16    gigohm
c)  30 gigohm                                                           
                          h)     30   gigohm   

It seems that the inside has quite low values as compared to the 
outside.  The coating on the inside seems to be
heavier.  Humidity seems to have a pretty big part in the values, but 
there are a few that changed little where as others changed a lot.  The 
ink vs no ink does not show up in this test.  Maybe more data points are 
needed.
The same tests should be done with the "Sono-Tube" and the Menards tube.


Jeff
       

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