Here is an interesting article that I found on a diesel forum on the web for all of us do-it-yourselfers. I copied it because the link below doesn't connect. It outlines a way to check the accuracy of your torque wrench... .
http://www.dieseldoctor.com/messageboard/data/229.html
Subject: How to calibrate a torque wrench in your own shop.
-------------------------------------------------------
Put the square drive of the wrench in a vise, making sure
that the body of the wrench isn't touching (only the square
drive touches the vise)
Ratchet the wrench to a horizontal position.
Okay, first, how to do the "micrometer" or "Click" type of
torque wrench (the beam needle type is below that, but bothe
start with the wrench held horizontal by clamping the square
drive in a vice in such a way that ONLY the square drive is
in contact with the vice).
Measure out from the center of the square drive (this coincides
with the center of the fastener, the wrench may or may not ratchet
about this same point).
You can determine the center of the square drive by the point
at which two diagonal lines from the corners of the square intersect.
Mark this distance on the handle. Choose somewhere convenient,
like at an even number of inches near where your hand would be.
Write down this distance in inches or feet, depending if your
wrench is marked in inch pounds (in. lb) or foot pounds (ft lb).
Get a weight, somewhere near the amount of force you might
typically extert on the handle (say, 20 lb, or 40 lb).
Multiply the distance from the center of the square drive by
the weight you will use.
If you wrench is a clicker type, set it to the number you
got when you multiplied the distance by the weight (inch lb
or ft lb). If it isn't the clicker type, read this anyway,
and then read the section on beam/pointer type wrenches.
Using bailing wire, hang the weight at the mark on the handle.
If the wrench clicks, lift the weight, move it closer to the handle,
let the weight hang at the new location. You might start by moving it
an inch or two, and see if it still clicks.
If it still clicks, keep moving it closer until it doesn't, then gradually
move farter out (away from the vice) until you find the transition
point between where it clicks, and where it doesn't.
Measure this distance. This is a new distance. You can now
use the ratio of the new distance, and the first (expected)
distance to determine a calibration factor.
If the wrench didn't click when you first tried hanging the weight,
find how much farther out you have to hang the weight to make it
click. try to find the transition point. Measure this distance.
You will similarly use this distance to determine a calibration ratio.
Let:
Ts = the torque setting on the wrench.
D1 = distance measured first (to the mark you made)
D2 = distance to point at which wrench actually clicked.
Ta = actual torque the wrench is applying.
So, if you set your wrench to a given torque value (Ts)
the torque it is actually applying is:
Ta = Ts x (D2/D1)
And, if you want a certain actual torque (Ta) applied to a fastener,
you would set your wrench to a value given by this equation:
Ts = Ta x (D1/D2)
Okay, put the wrench in the vise as described.
Mark it at 24" from the center of the square drive.
Get a 20 Lb weight.
Set the wrench to 480 inch lbs or 40 ft lbs.
Hang the weight on the line.
If it clicks, move it towards the vice, if it diesn't, move it
towards the free end (away from the square drive).
Find where the threshold between where it wil & won't
click is. Lets say that's at 26 inches.
Okay, the torque it applied when you first hung the weight was
480 inch lb.
The torque it took to make it click at that setting was 26 in. x 20 lb
= 520 in lb (divide by 12 to get ft lb)
But from now on, with this info, you can do the following...
If you want to apply a certain amount, say 50 ft. lb. ,
Just multiply 50 by 24/26 (or 12/13 if you're watching)
so 50 x 12/13 = 46. 15
So, to torque a bolt to 50 lb, set your wrench to 46 "and a hair" ft lb.
Easy enough, eh?
Actually, once you know the ratio (like 12/13 = . 923)
all you have to do is multiply the torque you want on the
bolt by that number (. 923) to tell you what setting to
put the wrench at.
I'll have to give instructions for the beam / needle (pointer)
type wrench later.
#ad
Okay, for the beam / needle pointer type torque wrench:
Mark the torque wrench handle at a known distance from
the center of the square drive (you don't really even have
to mark it, just choose a convenient number like 10", 12",
24"... ). Record this distance (D1).
Multiply the distence above by the weight.
Now move the hang point of weight along the handle until
the torque wrench needle is pointing to the value of torque
calculated above.
Measure the distance from the center of the square drive
to the point that the weight is now hanging, and that is the
the other distance (D2). These two distances will be used to
calculate a calibration ratio.
Now the equation is similar:
Let:
Ts = the torque that you will read on the pointer
D1 = the initial distance to apply the actual torque
D2 = the distance to make the needle point to the value calculated
Ta = the calculated torque from the cosen distance X known weight.
Okay here is an example.
I'll choose 24" (D1) and 20 lb again. We know that if we hang
a 20 lb weight 24" fron the center of the square drive, we are
applying 480 inch Lb (40 ft lb).
So, now we will move the weight until the needle points to
480 inch lb, or 40 ft lb. (if the needle was pointing to less
than the known torque, move the weight away from the square
drive (which is in the vice).
Now measure that distance (D2). Let's say it's 26"...
So, to torque a fastener to a desired value (say, 50 ft lb)
Using Ts = Ta x (D1/D2) = 50 x 24/26 = 50 x 12/13= 46. 15
So if you pull on the torque wrench until the needle points to
46. 15 ft lb you will be applying the desired 50 ft lb.
And there you have it.
Essentially, your torque wrench was reading lower than the
torque it was actually applying, so if you pulled on it until
it read the torque you wanted, you would be overtightening
the fastener.
And, once again, Actually, after you know the ratio
(like 12/13 = . 923) all you have to do is multiply the torque you
want on the bolt by that number (. 923) to tell you what setting to
read on the wrench to get that actual torque.
Attention: TDR Forum Junkies 
holding a tool 
of some sort - much like the old Snap-On and Ridgid calendars.
