Intervals / Tolerances / Ranges

Every week, PME Services has new potential customers calling. “Hey PME, I need a quote on a gage. What is the cost?” As much as we would like to give a quote on this, we are missing some key information. Things like manufacture, model number, and description. A caliper gage is not the same cost as a pressure gage. Once PME takes on such work, we need information from the customer about intervals, calibration ranges, and tolerances. Since PME is most likely not going to be present at the customers location for validation of their tooling, we need to know these 3 pieces of information.

According to ISO 17025, calibration laboratories are not to assign tolerances or calibration intervals for the customer, but they can report these numbers based on customer provided information. In many cases the customers do not know how to come up with answers to these three questions. For beginners, there are a few general guidelines that help: using manufacturer manuals or sales brochures can provide information about tools needing calibration. If the values listed here are good enough for your applications, you are welcome to tell PME to “use manufacturers tolerance.” If you do not have a specific range, then PME can calibrate the entire range of the machine. Of course, with things like ovens and incubators, checking the whole range is very expensive and very time consuming. If you are only using at a single check point, we would be more than happy to check that specific value.

Lastly, calibration intervals. This is the tricky part. Commonly, most companies use 12 months. Many manufacturers would recommend 6 months or 12 months, with the majority leaning towards 12 months. However, this may not always work for you. So, things to take into account would be usage, robustness of the gage, if the device is being moved around a lot, environmental conditions, and how other similar items have acted in the past. Depending on your quality system, it is not uncommon for customers to start everything on 6 months. If there are no out of tolerances after 3 calibration cycles, that they can start increasing the interval. Three or more calibrations allows you to plot the data and estimate when a tool should drift out of tolerance. This is commonly referred to as “Interval Analysis.”