Acclimation time prior to calibration

[CaliperJim]

Hi. We calibrate micrometers, calipers, and protractors in house. The lowest device measurement tolerance we have is 0.0005. The shop floor where the devices are being used is uncontrolled. How long do I need to allow the devices to sit in the calibration room before calibrating? Thank you very much.

 

[Dazzur]

If you're talking about ISO 9001:2015 here, you don't. Just calibrate them, it doesn't call for specifics, just that you have a suitable environment, what that suitable environment is up to you.
Not every organisation is going to have the resources for Calibration Labs.

 

[Michael_M]

I am not familiar with the specifics of ISO 17025 (Calibration) however, I believe it is 24 hours. That being said, with a tolerance of .0005 (inches I assume) it would take some drastic heat difference to show growth or shrinkage.

When you say "tolerance we have is .0005" do you mean the device can only read to .0005 or that is the out of calibration failure? If it's the out of calibration failure, I would assume your calipers fail frequently and your micrometers rarely fail.

Unless you have specific requirements from customers that state how you have to do something, I would not worry about it.

 

[Sidney Vianna]

How long do I need to allow the devices to sit in the calibration room before calibrating?

In my experience, most organizations allowed for a 24-hour soaking time in the lab for the majority of typical handheld instruments. In the case of massive instruments, eg, 50-60” micrometers, I have seen 48 hours as soaking time.

 

[CaliperJim]

When you say "tolerance we have is .0005" do you mean the device can only read to .0005 or that is the out of calibration failure? If it's the out of calibration failure, I would assume your calipers fail frequently and your micrometers rarely fail.

We have micrometers that read to .0001 but our tolerance is .0005. Our tolerance for calipers is .001.

 

[CaliperJim]

If you're talking about ISO 9001:2015 here, you don't. Just calibrate them, it doesn't call for specifics, just that you have a suitable environment, what that suitable environment is up to you.
Not every organisation is going to have the resources for Calibration Labs.

Our customers require us to adhere to MIL-STD-45662A and we are ISO 9001:2015 certified.

 

[dwperron]

One way you can figure out thermal expansion of the tools is using the formula
▲L = L * ▲T * CE
Where ▲L is the thermal expansion, L is the length of the tool / part being measured, ▲T is the temperature difference in the measurement, and CE is the coefficient of expansion for tool steel. The CE varies for different steel alloys, but a value of 6.5 ppm/°F is typically used
So suppose you have a 6 in caliper, and the factory floor temperature is 80 °F and your calibration space is 70 °F (a 10 °F difference that needs to be accounted for). Plugging in those numbers you get:
▲L = (6 in)(10°F)(6.5 ppm/°F) = 390 µin.
As you can see the number is small, but it can get significant. Suppose that you have a 24 in caliper and you bring it from the 80 °F factory to your 70 °F lab and you don't let the temperature of the caliper acclimate. then the thermal expansion that needs to be accounted for is:
▲L = (24 in)(10°F)(6.5 ppm/°F) = 1560 µin, or 0.0015 in. As you can see, that is now a considerable error.

 

[CaliperJim]

One way you can figure out thermal expansion of the tools is using the formula
▲L = L * ▲T * CE
Where ▲L is the thermal expansion, L is the length of the tool / part being measured, ▲T is the temperature difference in the measurement, and CE is the coefficient of expansion for tool steel. The CE varies for different steel alloys, but a value of 6.5 ppm/°F is typically used
So suppose you have a 6 in caliper, and the factory floor temperature is 80 °F and your calibration space is 70 °F (a 10 °F difference that needs to be accounted for). Plugging in those numbers you get:
▲L = (6 in)(10°F)(6.5 ppm/°F) = 390 µin.
As you can see the number is small, but it can get significant. Suppose that you have a 24 in caliper and you bring it from the 80 °F factory to your 70 °F lab and you don't let the temperature of the caliper acclimate. then the thermal expansion that needs to be accounted for is:
▲L = (24 in)(10°F)(6.5 ppm/°F) = 1560 µin, or 0.0015 in. As you can see, that is now a considerable error.

How can I determine the adequate soak time for the tools? Our calipers range from 0-6 to 0-24, our micrometers range from 0-1 to 8-12, our height gauges are 0-24.

 

[Miner]

If I were to design a study to determine the adequate soak time, I would use a repeated measures design as follows.

1. Note the ambient temperature for the measurement device on the shop floor.
2. Bring the device into the lab and immediately measure the calibration standard(s) (3-5 different sizes).
3. Repeat at defined time intervals (e.g., hourly) for 24 hours (the typical recommended soak time).
4. Repeat steps 1-3 over the course of several days.
5. Analyze as a repeated measures design and determine at what time interval the measurement results have stabilized (this will probably vary depending on the shop floor temperature (i.e., larger delta T = longer soak time)).