|Package Fill Weight Control: A Basic Guide (Part 1)|
|Written by John Alleman|
|Wednesday, 16 June 2010 14:40|
Package Fill Weight Control: A Basic Guide (Part 1)
The issue of package weight/volume control can be confusing and complex. Yet, this issue potentially represents a significant productivity improvement opportunity for most NW Food Processors. This article is the first of two parts intended to demystify the regulatory requirements surrounding package weights, and provide a quick cost-benefit snapshot of improved fill control.
In any packing application where the product is being filled into individual packages that have a nominal labeled weight applied, there are 3 hazards to be aware of:
First, the weight (or volume) of each individual pack in an inspection lot cannot be below an absolute lower limit known as the Maximum Allowable Variation (MAV). NIST HB133 defines MAV as the “…a deficiency in the weight, or measure, of an individual package beyond which the deficiency is considered to be an unreasonable error."
The other legal requirement addresses the overall distribution of weights/volumes in the entire sample lot. The Average Error requirement stipulates that the average of ALL sampled units cannot be lower than a predetermined amount below the nominal labeled weight.
Difference is 1.5 g greater than Average Error limit (1.43 g)
In Figure 1 above, the example is for a lot of frozen peas filled into sealed poly bags with a labeled weight of 411 grams (14.5 ounces). The inspection sample consisted of 48 randomly chosen bags. The number of bags (on the vertical axis) have been plotted at their corresponding weights (on the horizontal axis). For example: There were 3 bags that weighed in at 403 g, 404g, 406 g, etc.
The MAV limit can be looked up in NIST HB133 and computes to 392.9 g. Thus, none of the sampled bags exceeded the MAV limit.
However the average error limit has been violated. The target (labeled) fill weight was 411 grams, while the entire sample of randomly chosen units from the production lot yielded an average of 409.5 grams.
Digging into the technical details, the Average Error limit allowed from NIST HB133 is 1.43 grams. As shown in Figure 1, the Average Error limit is the product of both the sample standard deviation and a correction factor from NIST HB133 that gets smaller as sample size increases. In this case, the actual average differed from nominal labeled weight by –1.5 grams, which is larger than the NIST HB133 allowable difference limit of 1.43 grams.
In general, the Average Error limit will not be violated as long as the target fill weight/volume is maintained at or slightly above nominal (labeled) fill weight/volume.