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Onsite Calibration: Is it always the best choice?

Posted by The Metrologist on Oct 3, 2016 11:48:19 AM
The Metrologist

 

 

But I Want It Done at My Facility

Onsite calibration services have made the life of the calibration purchaser simpler. No more boxing items up and shipping them off to a lab and then waiting. Yes, waiting while production is down or product is stacking up waiting to be inspected before it can ship. If product isn’t leaving the door, then you’re not getting paid, and that doesn’t make anybody happy.

So, let’s have the calibration company come to our facility and perform all of our calibration work. Sure, it will cost a little extra, but we get the same thing…

Or do We?

Onsite calibrations are a great convince but they sacrifice one important part of a calibration company, the calibration lab. The calibration lab, large or small, is a place that has been designed and refined for one purpose, to perform calibrations. In the world of dimensional calibrations. It is controlled to 68°F because in the world of metrology that is the temperature where one inch is, well, one inch. At 70°F a one-inch gage block is actually 1.000013 inches. This trend continues from 68°F at a rate of 6.5μin per inch per degree Fahrenheit. If you are comparing two items made of the same material, this isn’t a big deal because they both grow or shrink at the same rate.

What if they are made of different materials?

Aluminum expands at nearly twice the expansion rate of tool steel. This means a six-inch aluminum caliper would be larger by 0.000420 inches (420 μin) than a six-inch gage block at 80°F, and if the six-inch gage block came directly from the “cold room” the difference would be 0.000890 inches (890 μin).

How Does the Calibration Company Handle This?

Simple, we expand the measurement uncertainty to include the worst case scenario. We monitor the conditions when we perform calibration activities and consider this in our reported measurement uncertainty.

The calibration company may also use different instruments to perform the calibrations in the field then they would in the lab. In the lab the “best tool for the job” is always the first choice, but what if that tool isn’t mobile or is extremely delicate, it won’t likely be seeing any field work.

“Double Down” on Expanding Uncertainties

So far the calibration provider has expanded uncertainties to account for the calibration environment and is possibly using second choice field equipment. We now have two sources of larger than advertised measurement uncertainty. If you’ve chosen your calibration provider based on their published “Scope of Accreditation,” then you might be disappointed when you receive your calibration certificates.

So Why Do You Offer Onsite Services?

When uncertainties associated with portable calibration equipment are very small and the tolerance that is required is very large, onsite calibration is a great option. There are a large number of situations where onsite calibrations are appropriate, but some are not, and unfortunately the calibration provider is not always the best one to make this decision.

The purchaser of calibration services is ultimately responsible for ensuring that they receive service that it “fit for use.”

You may want to read: How Do I Know if My Lab is Capable?

How Do I Get the Service I Really Need?

When purchasing onsite services, it is always best to discuss them with your calibration provider. Ask questions and make them aware of any special needs you have. Most calibration providers service multiple industries, so it’s always a good idea to communicate industry specifics they might not be aware of.

Always ask what the deviations from the scope of accreditation will be, and if something sounds too go to be true, it probably is.

Topics: Measurement Uncertainty

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