Frequency of scale checks

Our company has 3 large floor scales. 

- 2 floor scales with batch tanks sitting atop, 500-1500 lbs. batched at a time. 

- 1 floor scale for packaging, we package in bulk - so weights of 500 - 2300 lbs. 

We receive quarterly scale calibration from our scale contractor. 

We recently had a GMP inspection and were instructed scale weight checks were necessary (not indicating that I do not agree with this, although my Production Management sees differently). The batch tanks cannot easily be moved off of their floor scales. This would require a few hours of work prior to production activities, and we operate only one shift from 7-3pm. Additionally, we do not produce every day. Our production schedule is based off of orders received. 

- What is an acceptable frequency for larger range scales? My understanding is the tolerance range is greater, reducing the likelihood that the scales are out of range 

- What is an acceptable weight range for the check weights? 

- For these large scales, would a monthly external check suffice? Our past records indicate that the scales are rarely out of range during their quarterly checks. We have a small warehousing facility - sanitarily storing large 1000lbs check weights would prove to be an issue, coupled with the cost of recertifying my thought is to simply contract the task out. 

Thanks for any input anyone may have! 

For reference - we will be going for NSF Dietary Supplement GMPs certification in 2021. We manufacture bulk, liquid ingredients (concentrates) for supplements and cosmetics. 

Why would you change from quarterly to monthly? Even quarterly is quite frequent: from my experience of working for several industries and GFSI schemes (FSSC and BRC), annual calibration by the accredited service provider is completely sufficient especially if it's large range scales. When it comes to internal calibration, you could choose any frequency you want depending on the complexity of your calibration procedure.

I've done larger scales as a daily check with a reference weight the same as we'd do for a small set of bench scales, but for floor scales this is fairly easy once you've bought a weight of the relevant size - only takes a few minutes to pop it on with a forklift each morning as part of the startup process.

For the tanks on scales, this is more of a challenge and I've never had an issue with doing an external calibration similar to your approach, but then I've also never had to rely on those scales for anything beyond an indicative figure.

What do you put in those tanks, i.e. are they for holding a liquid? And if so, do you have any way to accurately (calibrated...) measure a volume of liquid being pumped into them? e.g. a known volume of a liquid with known density as an impromptu reference weight? I appreciate it's not directly linked to a traceable standard, but could be a practical alternative as an extra "check" in addition to the quarterly formal external one?

Hi All,

interim internal verification is acceptable between "traceable to national standards" calibrations. You can buy a weight or create a pallet of something in the right weight range. Use this pallet of material as a standard for interval comparison of weight. i.e. it should not alter form each weighing. You need to select the material which is not going to evaporate or absorb moisture .(change weight) as sealed pallet of flour perhaps?

Similar to what pHruit mentions, we would fill our tanks to the same level/measure line (using consistent temperature water) to confirm that the scale read the water weight the same each day when the tank was filled to the exact same level. This water was then used as an ingredient in the next batch of product that we blended so it did not get wasted. We would target a weight which would be the average "midpoint" of the weight range that the scales would typically be used for. We performed this each day at start up (and had scales calibrated annually by a 3rd party).