Ugly bags of mostly water
One way to calibrate the hand dynamometer would be to loop a line over it and use it to slowly lift a known weight. F=MA and you know how tricky that gets, so we'll keep acceleration to near zero. I did that recently with some twine and a crow bar (estimated weight: 10 pounds), as well as 40 pounds of water. It seems to read accurately and consistently with the known weights. I think what I'll do is fill a plastic garbage bag with five gallons of water, loop the ends over the dynamometer, and slowly lift. Then I'll water the plants from the bag.