Bad Measurements Happen! Technicians are Fallible! The Wrong Equipment is Used!
Almost everyone has had problems related to bad measurement practices—some of which have resulted in serious devastation. There was the BP oil refinery explosion in Texas, the Hubble telescope with improper focus, the Space Shuttle explosion, a Stealth Bomber crash, Cox Health’s overdosing of 152 cancer patients, and another BP oil rig disaster. All of these tragedies were the result of poor decision-making, and all of them were preventable.
I'd like to ask you to pause for a moment, and take 20-30 seconds to think about measurement issues that may have affected you:
· Have you, or any of your technicians, ever overloaded a load cell?
· Have you, or someone you know, ever used the wrong equipment to try to accomplish a certain task?
· Have you signed a certificate you were unsure about?
· Do you know of any bad measurement practices in your organization that are not being corrected, or do complaints fall on deaf ears?
· How about your calibration provider: Have they ever admitted to making a mistake? If the problem was not corrected, did it just go away?
Let's face it: We are not perfect. And neither is any measurement practice. But if we aim for perfection, we can achieve excellence, which is what we do at Morehouse. We mitigate measurement risk by making better measurements, and by replicating the use of all instruments to lessen the possibility of devastating errors. We educate our customers on best measurement practices. And when you send an instrument to our lab for calibration, we guarantee your satisfaction. If you still need another reason to come to Morehouse, remember: the alternative could be disastrous.
Our goal is to solve problems before failures occur. Solving measurement problems requires adherence to the three rules.
Rule #1. Know the Right Requirements - This first rule involves knowing what is needed to accomplish the task at hand. It should include the establishment of reasonable measurement tolerances that are based on system performance, as well as an understanding of what equipment or calibration provider is needed to ensure the proper Test Uncertainty Ratio (T.U.R.) and reduce measurement risk. The more accurate the system, the higher the costs will be to procure the equipment and have it calibrated. Maintenance may also be an issue. If the wrong equipment is chosen, more frequent calibration cycles may be required*.
*This is in reference to stability criteria. If the requirement is 0.1 % of applied, and the stability of the device is 0.2 % over a one-year period, the device would need to have the calibration interval shortened.
SAE AS9100 7.3.1 states, ”Design and development planning shall consider the ability to produce, inspect, test and maintain the product."
For most tests, a T.U.R. of 4:1 will meet the guidelines set fourth in ANSI Z540.1 of ensuring that the total risk is less than 4 %. T.U.R. is covered in greater detail in Rule #2.
Below is a chart showing what Calibration and Measurement Capability is required to maintain a 4:1 T.U.R. If you are using a device with a tolerance of 0.1 % of applied force, the table indicates that you would need to choose a calibration provider with a CMC of about 0.02 % or better.A lab with a CMC of 0.05 % cannot meet a T.U.R. of 4:1 to calibrate a device with 0.1 % accuracy. A CMC of 0.05 % is what most secondary labs maintain.
Note: To maintain a CMC of better than 0.02 %, multiple transfer standards are often required.
This table shows that calibration with deadweight primary standards is often required to calibrate any instrument with tolerances of 0.1 % of applied, or better. If the tolerance required is 0.05 %, or better, some type of deadweight or lever system must be used.