How to weigh someone over 350 pounds (or the upper limit of your scale; in my extensive research — looking for this picture — I found scales that go up to 500 lbs.) on a standard, upright, balance scale:
The trick is to add a counterweight, hanging it from the little piece of metal sticking out at the top of the beam on the top. I find the large rubber bands used by the post office (available from them for free) work well; I use three of them looped end to end.
I use a screwdriver for my counterweight, but anything you can hang freely from the beam will work. Here’s how to do the calibration:
Step on the scale yourself and record the reading [assuming you weigh less than your scale’s limit, for cryin’ out loud!] Then add the counterweight and re-weigh yourself. The difference in the two readings is what your counterweight adds. For example, my approximately 3 oz. screwdriver adds 94 lbs. to my scale’s 350 lbs.
When you weigh a patient over 350, begin with the counterweight in place and then add your known differential to the measurement you get.
From the comments on the previous post, the double-scale method (two scales, one foot on each, add the readings) should work mathematically, but it strikes me as cumbersome. Technically, there’s no limit using this technique, except that after a certain point, larger people won’t physically fit on the scale.