A scale calibration log records each time you check a scale with a known weight: the date, the scale, the test weight, the reading, the difference, and what you did. A useful check has three parts: test weights across the range you use, not only one, the same weight in the center and each corner of the pan, since a scale can read true in the middle and off at the edge, and the same weight several times, to see that it repeats. The tolerance comes from the scale maker's specification for that scale's capacity and division. Why it matters: the net weight on a label has to be an accurate statement of what's in the package, with only reasonable variations allowed, so a packing scale that reads 4 grams heavy packs every bag 4 grams light. Scales used to sell by weight at a counter fall under your state's weights and measures law and the NIST specifications. And a facility with preventive controls calibrates the instruments that monitor them, with a written method and frequency. In the example below, a cookie maker finds a packing scale off after it was knocked off the bench. Below are the checks, the rules, the example, and a free template.
The short version
- A scale calibration log records each check: date, scale, test weight, reading, difference, action.
- Check with test weights across the range you use.
- Check the center and the corners, and the same weight several times.
- Tolerances come from the scale maker's specification.
- A scale reading heavy packs light: net weight on the label has to be accurate.
- Counter scales that sell by weight follow your state's weights and measures law.
- Out of tolerance: recalibrate, then decide about what it weighed since the last good check.
- TaroStack keeps lots and holds, so a week of packs can be found and held. More below.
The checks
| Check | How | Catches |
|---|---|---|
| Zero | Empty pan, reading zero, and back to zero after a weight comes off | Drift, a dirty pan, a tare left on |
| Span | Test weights near the low end, the middle and the top of what you weigh | A scale true at one weight, off at another |
| Corners | One test weight in the center, then each corner | A damaged load cell or a pan off its seat |
| Repeat | The same weight five times | A scale that wanders |
| Daily check | One weight near what you weigh most, before the first run | A knock since yesterday |
Test weights come in classes by accuracy; keep a set matched to your scales, stored clean, and have them checked on the schedule their maker or your auditor sets. How a net weight is declared is in net weight labeling requirements.
The rules
| Rule | Where |
|---|---|
| Net weight | An accurate statement of the package's contents; reasonable variations from moisture or unavoidable manufacturing deviations, not unreasonably large ones (21 CFR 101.7(q)) |
| Selling by weight | Weight means net weight; commercial scales meet the NIST specifications and tolerances (Wis. Stat. 98.01, 98.03) |
| Preventive controls | Calibrate, or check the accuracy of, monitoring instruments, with a written method and frequency, and keep the records (21 CFR 117.165, 117.190) |
A small business that qualifies for the modified requirements doesn't write a food safety plan, but its labels still have to be accurate. The thermometer side is in thermometer calibration log. This is general information, not legal advice.
An example: an Appleton cookie maker
Fox River Cookie Co., the made-up cookie maker in Appleton, Wisconsin, I use in these examples, packs six-cookie packs labeled Net Wt 8.4 oz (240 g) for a regional grocery chain. On Monday morning the packing lead checks the three scales:
| Scale | Check | Should read | Read | Tolerance | Result |
|---|---|---|---|---|---|
| Packing scale, 6 kg × 1 g | Zero | 0 g | 0 g | ±1 | Pass |
| Packing scale | 200 g, center | 200 g | 204 g | ±1 | Recalibrated, reads 200 |
| Packing scale | 200 g, corners, after recalibrating | 200 g | 200 to 201 g | ±1 | Pass |
| Packing scale | 2 kg | 2,000 g | 2,001 g | ±2 | Pass |
| Floor scale, 150 kg × 50 g | 20 kg | 20,000 g | 20,050 g | ±100 | Pass |
| Bench scale, 600 g × 0.1 g | 100 g, five times | 100.0 g | 100.0 to 100.2 g | ±0.2 | Pass |
The packing scale read 4 grams heavy: packs weighed to 240 g on it held about 236 g. It was fine at the last weekly check on September 28, and someone remembers it falling on Thursday. Fox River holds the packs made since the 28th, weighs a sample of each day's lot on the corrected scale, and finds Thursday's and Friday's lots light; those are repacked, the rest released. The daily check, one 200 g weight before the first run, goes into the procedure. The plant's licensing is in food processing license.
Get the template
Download the scale calibration log. One sheet: each check's date, scale, method, the test weight and the reading, the tolerance and how often it's checked; the difference, pass or adjust, and the next check due fill in, with out-of-tolerance checks in red. It's an ordinary Excel file with no macros, and it works in Google Sheets, LibreOffice and Numbers.
Where this stops working
The log says the scale was off on Monday and fine on September 28. Which packs were made in between, which lots they were, and which of them already shipped to the chain's warehouse is a different question.
How TaroStack does it
TaroStack keeps each day's packs as a lot, so a scale problem leads to the packs it touched.
Each run a lot. Each day's packing is a batch with its own lot and date, so "since September 28" is a list of lots.
Holds kept out of stock. Lots in doubt can be put on hold, apart from what's available to sell, and released or reworked once they're checked.
Where it went. Each order records the lots it got, so if Thursday's lot already shipped, one recall report shows which warehouse has it. Your own fields on the batch record can hold which scale weighed the run.
| The job | Binder | In TaroStack |
|---|---|---|
| Packs made since the last good check | Search the run sheets | Lots by date |
| Keep doubtful packs from shipping | A sign on the pallet | A hold, kept out of stock |
| Which lots already shipped | Bills of lading | One recall report |
| Which scale weighed a run | Often not written | A field on the batch record |
Batches, lots, holds and recalls are on every plan, from $49 a month. The first 30 days are free, with no credit card. If you'd like a hand setting up, ask, and we'll do it with you.