Do you calibrate torque wrenches?
I was recently asked this question and the answer is yes. As is the case with all ISO 17025 accredited laboratories we do have limitations based on our scope of accredited services. In this specific case, the range was within our capabilities.
Companies change calibration services for a variety of reasons.
I asked specifically if they were having any issues with their current calibration service.
" Every time we get our torque wrenches back from calibration, I get complaints that they are not reading properly."
Interesting response so I asked if I could review their calibration certificates. Upon review several serious issues with their torque wrench calibration were discovered.
Torque Wrench Resolution
Resolution is the smallest change that can be detected.
As an example this company had a 600 ft lb torque wrench with a stated resolutions of 0.1ft-lbs.
There is often confusion between Resolution and Accuracy.
Imagine if you zoomed in on the image above and there were markings/subdivisions instead of just one big red dot bullseye. That is one way to think of resolution.
"The diagram in Figure 5.1 shows the same position (the point in the center of the bullseye) measured by two instruments. The two grid patterns represent the smallest objects that can be detected by the instruments. The pattern on the left represents a higher-resolution instrument."
Torque Wrench Accuracy
Accuracy on torque wrenches is commonly expressed as a percentage and directional.
|Mfg Part #||6004MFRMH|
|Type||MICROMETER 3/4" DR|
|Inch Resolution||5 IN-LB|
|Accuracy||+/- 3% CW, +/- 4% CCW|
The details above mean this specific torque wrench at 100 Ft-Lb would provide a result between 97-103 Ft-Lb ( +/- 3% Clockwise)
576 - 624 Ft-Lb at 600 Ft-Lb(+/- 4% CounterClockwise).
It has a resolution of 5 IN-Lb.
Resolution can be markings or a dial or a readout.
Often times resolution gives a false sense of confidence in regards to accuracy. They are not the same and just because a gage has a resolution lower than the accuracy it does make it more accurate.
What is the serious issue with torque wrench calibration?
As stated above, most torque wrenches have an accuracy between 3-5%.
Let's talk about 100 FT LB torque wrench at 4% accuracy. That would be an accuracy of 4 FT Lb.
In this specific case they stated " Every time we get our torque wrenches back from calibration, I get complaints that they are not reading properly."
In reviewing the calibration certificates from the laboratory they were using the stated measure measurement uncertainty was 5.3 FT LB.
Do you see the serous issue with the calibration of this torque wrench?
The measurement uncertainty of the standard used was greater than the accuracy of the torque wrench calibrated. That is a serious issue.
The calibration company had done nothing wrong. They clearly stated their measurement uncertainty.
As a consumer of calibration services you need to review the Scope of Accreditation to determine if the capabilities of the calibration laboratory meets your needs.
You may want to read: How Do I Know If My Lab is Capable?
Alliance Calibration offers the following measurement uncertainties for torque wrench calibration.
0.5.ozf.in to 1,000 lbf.in=0.33% of reading
10lbf.ft to 125 lbf.ft= 0.6% of reading
60 lbf.ft to 600 lbf.ft=0.71% of reading
1,000 to 2,000 lbf.ft= 2.4% of reading
As a consumer of calibration services you need to review the scope of accreditation and review the measurement uncertainties to determine if they meet your needs.