Originally Posted by lethalfang
2) The tension bar itself weights a few ounces, does that introduce an error?
In fact, it does, but the mass of the tension bar << mass of the dropweight, thus the effect is negligible.
Also keep in mind that the mass of the tension bar does not change, either. Thus, it is not a variable for your stringing machine. The error can simply be corrected in my equation by adding a small correction to the Mass, by taking into account the mass of not only the dropweight, but also the tension bar.
first, it's amazing how a touch of a 1st yr engineering course can impress so many!!! =) ... nevertheless, your overall analysis looks good (although i didn't read the details). BUT the above highlighted comment is not right.
the effect of the tension rod weight is not
negligible. when i got my drop-weight, being the nerdy engineer i am, i had to verify that the tension scale on the rod was correct. i took all the above measurements and to come up with my own solution to where the weight should be for a desired tension. i KEPT coming up answers different than the tension scale on the machine. i kept thinking about what horrible engineers must have designed my machine. THEN i realize i forgot about the moment produced by the weight of the tension rod. abra-cada-bra all the numbers mached! and thus, the tension scale on my machine WAS correct! it turns out the tension rod (on my machine) contributed about 5-6 lbs of tension! ... definitely, not to be neglected. this is a bias, so no matter where the weight is along the rod, the tension rod itself always adds about 5-6 lbs.
in addition, you need to be very careful when tweaking the mass of your dropweight to account for other error sources. your should NOT do this to account for the tension rod weight. this is b/c the tension rod mass adds a constant bias
to the pulled tension, but the mass of the rod affects the tension via a scale-factor (i.e. the distance R). the bias is a result of the fact that the tension rod mass is fixed, where as the scale factor arises b/c the drop-weight mass moves. different error sources, don't mix 'em.