Sure you can seek out a lab to calibrate your torque wrench, but lets have
some fun with this. Torque simply is force x distance. That's any force x any
distance. If you take a known weight, and suspend it from the end of a
cantilever, you can compare its torque with your wrench. Here's what I would
try. Lay your wrench on its' side on your bench. Attach your vice-grips to the
drive square, horizontally, and lightly. Now slide a piece of pipe over the
handle of the vice-grip, so you have a cantilever, maybe 3 feet long. Now, if
you hang a 5 gallon bucket of water from the end of your cantilever you have a
torque to compare to. 5 gallons water weighs 40 pounds, and if you measure the
distance from the drive square to the bucket handle to be 35.5 inches, you
just multiply 35.5 inches x 40 pounds which gives you 1420 inch pounds or
118.33 foot pounds. Now, depending on the wrench, you may have to find a more
suitable weight, or cantilever distance. Add in a foot pound or so for the
pipe itself. Well, some of the purists out there may be scoffing still, but
the same physics applied for Uncle Isaac a few centuries ago, and that's about
all he had to work with! PS, I am an engineer.