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Torque wrench accuracy - 20% of full scale?

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Patrick Boyle

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Jun 28, 2010
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Western Montana
I'm shopping for a in/lb wrench to cover the lower scale, so I'm trying to make sure I'm covered. My confusion is related to what is actually meant when manufacturers say "x% accurate to 20% to 100% of range." With my understanding, if you were looking at a 5-75 ft/lb wrench, it's only accurate above 19ft/lb. The scale actually covers a total of 70ft/lb, therefore 20% of 70 is 14. So the bottom 14ft/lb of the range that the wrench covers is not accurate to spec.

I've seen a lot of people calculate the percentage starting at zero no matter what the wrench actually starts at. For instance, I've heard that a 20-100 ft/lb wrench is accurate across the entire range because 20% of 100 is 20. I've also heard that a 10-80ft/lb wrench is accurate above 16ft/lb because 20% of 80 is 16.

I hope I'm explaining this well enough. Which is correct?
 
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reptilezs

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Mar 23, 2010
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Patrick Boyle said:
I've seen a lot of people calculate the percentage starting at zero no matter what the wrench actually starts at. For instance, I've heard that a 20-100 ft/lb wrench is accurate across the entire range because 20% of 100 is 20. I've also heard that a 10-80ft/lb wrench is accurate above 16ft/lb because 20% of 80 is 16.

I hope I'm explaining this well enough. Which is correct?
Click to expand...

that
 
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SMKS

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Patrick Boyle said:
I'm shopping for a in/lb wrench to cover the lower scale, so I'm trying to make sure I'm covered. My confusion is related to what is actually meant when manufacturers say "x% accurate to 20% to 100% of range." With my understanding, if you were looking at a 5-75 ft/lb wrench, it's only accurate above 19ft/lb. The scale actually covers a total of 70ft/lb, therefore 20% of 70 is 14. So the bottom 14ft/lb of the range that the wrench covers is not accurate to spec.

I've seen a lot of people calculate the percentage starting at zero no matter what the wrench actually starts at. For instance, I've heard that a 20-100 ft/lb wrench is accurate across the entire range because 20% of 100 is 20. I've also heard that a 10-80ft/lb wrench is accurate above 16ft/lb because 20% of 80 is 16.

I hope I'm explaining this well enough. Which is correct?
Click to expand...

My understanding has always been that it's 20% of the max number. So, a 10-100 ft lbs torque wrench would be accurate from 20 - 100 ft lbs.

Using that calculation, a 5 - 75 ft lb wrench would be accurate above 15 lb ft.


EDIT - I just checked the CDI webpage. They agree with me, saying their wrenches are accurate from 20% - 100% of full scale.

I've attached a screen shot.
 
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