# A Stamped Scale Is Not a Calibrated Process: Why Food Factories Must Check Their Instruments Every Shift, Not Once a Year

*By Isaac Muthui, CEO, Tamalaki Business Network Ltd*

Walk through any food factory and you will see numbers everywhere. The pasteuriser chart says the milk held its temperature. The intake scale says the farmer delivered so many kilograms. The checkweigher says every pack is above its declared weight. The metal detector light stays green.

Now ask a different question: "How do you know those numbers are true?"

Every record in a food safety file, every intake payment and every batch cost is only as good as the instrument that produced it. A thermometer that reads two degrees high makes an under-pasteurised batch look safe. A scale that drifts makes your yield, your stock and your supplier payments wrong at the same time. A metal detector that has never been challenged with a test piece is a green light, not a control.

This article is for production leads, QA officers, engineers, store managers and owners of small and mid-sized food and agro factories in Kenya and East Africa: dairies, millers, bakeries, juice and beverage makers, edible-oil processors, meat and poultry plants, and spice and snack packers. It gives you an eight-step **calibration checklist for food factories** that you can run with your own team. It is operations guidance, not legal advice; confirm the specifics with the Weights and Measures office in your county, KEBS and your customers' specifications.

## What the standards and the law actually say

**Codex, the international baseline.** The [General Principles of Food Hygiene (CXC 1-1969)](https://doi.org/10.4060/cc6125en) comes back to calibration several times. Section 9.3.2 says that, where appropriate, monitoring equipment should be calibrated to ensure that temperatures of food processes are accurate. The section on time and temperature control says temperature monitoring and recording devices should be checked for accuracy and calibrated at regular intervals or as needed. On physical hazards, it says detection or screening devices that are appropriately calibrated, such as metal detectors and X-ray detectors, should be used where necessary. And in the HACCP chapter, it lists calibration among verification activities, with "calibrating or checking the accuracy of instruments used for monitoring and/or verification" given as an example.

**The East African hygiene code.** [EAS 39:2001, the hygiene code of practice for the food and drink manufacturing industry](https://law.resource.org/pub/eac/ibr/eas.39.2001.pdf), says the manufacturer is responsible for identifying, monitoring and recording critical parameters, such as heat treatment and cleaning temperatures, and adds that measuring instruments used to monitor critical parameters should be calibrated at appropriate intervals so that their accuracy stays within defined limits (clause 4.3.5). It asks that metal detector reject mechanisms be checked whenever the detector is checked (4.6.5), keeps glass thermometers out of production areas unless encased in metal (4.4.4), bans mercury thermometers from processing areas (4.6.6), and lists "the calibration status and procedures used to calibrate all devices used to assure the safety of the finished product" among the records a plant should keep (14.3.3).

**Trade scales and Kenyan law.** Scales used in trade, including the ones you buy raw materials on and sell finished goods by, fall under legal metrology. The [State Department for Trade](https://trade.go.ke/weights-and-measures) explains that the Weights and Measures Department derives its mandate from the Weights and Measures Act (Cap. 513) and the Trade Descriptions Act (Cap. 505), that counties verify weighing and measuring equipment used in trade, and that traders must make sure every piece of equipment they use for trade bears a valid stamp of verification. It also says that if an inspector reports equipment as inaccurate, it must be repaired and re-stamped before it goes back into use, and that if you suspect a piece of equipment is inaccurate, you should take it out of use and call an inspector. Counties run this as an annual exercise. In February 2026, for example, Nairobi City County [announced its annual verification and stamping](https://thekenyatimes.com/latest-kenya-times-news/nairobi-county-announces-inspection-of-weights-and-measures-targeting-traders/) of weighing and measuring instruments under Section 27(1) of Cap. 513, with fixed instruments verified in position.

**Accredited calibration.** For instruments outside legal metrology, such as process thermometers, data loggers, pH meters and reference weights, the usual route is a calibration laboratory accredited to ISO/IEC 17025. The [Kenya Accreditation Service (KENAS)](https://www.kenas.go.ke/accreditation/schemes/calibration) runs a calibration accreditation scheme to that standard, covering fields that include mass, temperature, humidity, pressure, volume and density. KENAS notes that without regular calibration, equipment can drift from its specification and give imprecise measurements, with risks to quality and safety.

Put together, the floor rule is simple: **know every instrument that matters, decide how accurate it must be, have it checked by someone qualified, check it yourself in between, and act on what you find.**

## Why "the scale was stamped in March" is not a calibration programme

The annual stamp matters, and you need it. But it is a snapshot of one day. Instruments drift, get knocked, get washed down, run out of battery or get "adjusted" by someone who thought the number looked wrong.

The risk is not theoretical. In August 2026, the Daily Nation [reported](https://nation.africa/kenya/counties/bomet/tea-factory-staff-arrested-over-manipulated-weighing-scales-5557240) a crackdown by the Tea Board of Kenya, the Weights and Measures Department and police on weighing at tea buying centres and factories in Konoin, Bomet County, after growers complained that their deliveries were being under-recorded. According to the report, the Tea Board said some digital machines displayed weights 1.7 kg below the actual weight delivered, one clerk pleaded guilty to using a weighing machine without a valid stamp, and officials said some machines lacked valid Weights and Measures seals or had not been calibrated for between two and seven years. That case was about alleged tampering, but the lesson for every processor is the same: when nobody checks the instrument, nobody can defend the number.

**Illustration (not real customer data):** a small dairy's pasteuriser thermometer has been reading about 1.5 °C high for weeks. The chart shows every batch at the target holding temperature, so every batch is released. Nobody has compared that thermometer with a reference since installation. When a customer's laboratory reports a problem, the plant cannot say which batches were really held at temperature, because the only evidence is the instrument that was wrong. A two-minute check against a reference thermometer at the start of each shift would have caught the drift on day one.

## The 8-step calibration checklist

### 1\. Build an instrument register

- List every instrument whose reading is used to make a decision: intake and dispatch weighbridges and platform scales, batching and formulation scales, checkweighers, filler volume settings, process thermometers and probes, chiller and cold-room loggers, pasteuriser and retort recorders, pH meters, refractometers (°Brix), moisture meters, milk analysers, pressure gauges on steam and air lines, flow meters and metal detectors.
- Give each one a unique ID tag, a location, an owner, and what it is used for.
- Mark which instruments monitor a critical control point or a food safety limit, which are used in trade (buying or selling by weight or volume), and which are used for costing and stock. One instrument can be all three; the intake scale often is.
- Add new instruments to the register before they go into use, not after the first audit.

### 2\. Decide how accurate each instrument must be, and how often to check it

- For each instrument, write down the acceptable tolerance in terms of the process. If your pasteurisation limit is a temperature, your thermometer's tolerance has to be tight enough that a "pass" reading really means the limit was met.
- Set two frequencies: an external calibration interval (for example, once a year, or more often for critical or heavily used instruments), and an in-house check frequency (per shift, daily or weekly). Base both on risk, the manufacturer's guidance and the instrument's own history. EAS 39 asks for calibration "at appropriate intervals"; the register is where you show what "appropriate" means for you.
- Tighten intervals for instruments that keep failing checks. Relax them only when the history shows they are stable.

### 3\. Get every trade instrument verified and stamped

- Present every scale, weighbridge and measure used in buying or selling for verification during your county's annual exercise, and keep the verification certificates on file. Fixed or bulky equipment is usually verified in place, so notify the county office in good time.
- Check for a valid stamp before you buy or install a new trade scale, and use only repairers licensed by Weights and Measures.
- After any repair or adjustment of a trade instrument, have it re-verified and re-stamped before it is used again.
- Our [net quantity and pack weight checks](https://factoraerp.com/en/blog/net-quantity-pack-weight-checks-food-factories-kenya) explain why the scale behind your pack-weight samples has to be one you can defend.

### 4\. Calibrate reference standards and critical instruments externally, with traceability

- Keep a small set of reference tools: certified check weights that cover the range of your scales, a reference thermometer, and pH buffer solutions within their expiry dates.
- Send the reference tools, and the critical instruments themselves where practical, to a calibration laboratory accredited by KENAS for the right field, such as mass or temperature. Ask for the certificate to show the readings found, any adjustment made and the measurement uncertainty.
- Read the certificate when it comes back. A certificate that shows your thermometer was 1 °C out before adjustment is a reason to look back at the batches it measured, not just a document to file.
- Store reference tools carefully. A dropped check weight or a reference thermometer used daily on the line stops being a reference.

### 5\. Run quick in-house checks between calibrations

- **Scales:** at the start of each shift, and after moving or cleaning, check zero and weigh one or more check weights near the range you use. Record the reading.
- **Thermometers and probes:** compare against the reference thermometer at a temperature near the one you control. A mix of crushed ice and water should read close to 0 °C, which makes a simple check for chiller and cold-room probes.
- **pH meters:** calibrate with fresh buffers before use, as the manufacturer instructs.
- **Metal detectors and checkweighers:** at the start of the run, at set intervals and after changeovers, pass the test pieces for each metal type (or a known under-weight pack) through the line and confirm that the reject mechanism actually removes them. EAS 39 asks for the reject mechanism to be checked whenever the detector is checked.
- Record the instrument ID, the expected value, the reading, pass or fail, the time and who checked. Our [dairy milk intake checklist](https://factoraerp.com/en/blog/dairy-milk-intake-checklist-kenya) shows how much depends on intake readings being right.

### 6\. Label status and remove what is overdue or unsafe

- Put a status label on each instrument with its ID, last calibration date and next due date.
- Make "overdue" mean "out of use". If an instrument is past its due date, it should not be used to release product or to pay a supplier.
- Keep glass thermometers out of production areas unless they are encased in metal, and keep mercury thermometers out of processing areas altogether, as EAS 39 sets out.
- Fold calibration dates into your [preventive maintenance](https://factoraerp.com/en/blog/preventive-maintenance-basics-food-agro-factories-kenya) plan, so the engineer and QA work from the same calendar.

### 7\. When an instrument fails, act on the product, not just the instrument

- Take the instrument out of use straight away and label it. For trade instruments, that is also what the State Department for Trade tells traders to do when accuracy is in doubt.
- Work out the window at risk: everything measured since the last good check. That is why frequent in-house checks matter; they keep the window small.
- Assess the product in that window. If a critical limit may not have been met, put the affected batches on a [QA hold](https://factoraerp.com/en/blog/qa-hold-checklist-before-batch-leaves-warehouse-kenya) until QA decides. If product has already left, you need to find it fast, which is what a [mock recall](https://factoraerp.com/en/blog/how-to-run-a-mock-recall-food-traceability-kenya) tests.
- For scales, check the knock-on effects too: supplier quantities and payments, yields, and stock. A scale error usually shows up later as an unexplained [stock count](https://factoraerp.com/en/blog/stock-count-reconciliation-checklist-food-factories-kenya) difference or a strange [batch yield](https://factoraerp.com/en/blog/batch-costing-yield-tracking-agro-processors-kenya).
- Log a non-conformance with the cause, the corrective action, the owner and the date it was closed. Repair, recalibrate or replace, and re-verify before returning the instrument to use.

### 8\. Keep the calibration file and review it

- Keep together: the instrument register, calibration certificates, verification certificates for trade instruments, in-house check records, status label history, failures and the corrective actions taken. EAS 39 lists calibration status and procedures among the records a food plant should keep.
- Have a qualified person review critical monitoring records regularly; EAS 39 (clause 4.3.6) asks for records of processes that assure food safety to be inspected at least daily and signed.
- Review the register every few months: which instruments failed, which intervals should change, and which new instruments were added.
- Keep records for longer than the shelf life of your products, so that you can answer a question about any batch still on the market.

## Habits that make calibration stick

- **No ID, no use.** Every instrument that matters is in the register and tagged.
- **Every shift starts with a check.** Thirty seconds with a check weight or a reference thermometer is the cheapest control in the plant.
- **Overdue means out of use.** A past-due label is a stop sign, not a suggestion.
- **A failed check opens a hold.** Instrument errors are batch problems.
- **The certificate gets read.** "As found" readings tell you about the past, not just the future.

Calibration ties into the rest of your quality system: the scales behind [supplier intake](https://factoraerp.com/en/blog/supplier-intake-checklist-food-manufacturers-kenya), the probes behind [water quality](https://factoraerp.com/en/blog/water-quality-checklist-food-factories-kenya) and cleaning temperatures, and the evidence you need when handling a [customer complaint](https://factoraerp.com/en/blog/customer-complaint-handling-checklist-food-factories-kenya).

## Where an ERP helps

A register on a clipboard, a set of check weights and a reliable calibration lab can run this checklist in one small plant. It gets harder with many lines, many instruments, several shifts and a lot of batches to trace back when a check fails.

FactoraERP tracks production batches and lots, stock status including QA holds, non-conformance records, stock adjustments with reason codes, and batch costs and yields. That means a failed calibration check can be logged as a non-conformance with an owner, linked to the batches produced since the last good check, those lots can be put on hold, and any correction to stock gets a reason code instead of disappearing. See [factoraerp.com](https://factoraerp.com).

## Free download: eTIMS readiness checklist for factories

Quality records, stock records and invoices increasingly have to tell the same story for Kenyan manufacturers, and the scale that weighs a delivery feeds all three. If you want a free **eTIMS readiness checklist for factories**, sign up for the FactoraERP Insights newsletter at <https://isaacmuthui.substack.com>. You will get practical operations notes for food and agro manufacturers. No spam; unsubscribe anytime. For background, see our guide to [eTIMS for food and agro manufacturers](https://factoraerp.com/en/blog/etims-for-food-agro-manufacturers-kenya).

## FAQ

**What should a calibration checklist for a food factory include?**\
An instrument register, a tolerance and check frequency for each instrument, Weights and Measures verification for trade instruments, external calibration of reference standards and critical instruments by an accredited laboratory, in-house checks between calibrations, status labels, a procedure for failures that includes assessing and holding affected product, and a calibration file that is reviewed regularly.

**Is the annual Weights and Measures stamp the same as calibration?**\
No. Verification and stamping under the Weights and Measures Act (Cap. 513) confirms that an instrument used in trade is accurate and compliant when the inspector tests it. It is a legal requirement for trade instruments. It does not replace your own routine checks between verifications, and it does not cover process instruments such as thermometers or pH meters.

**How often should food factory thermometers and scales be calibrated?**\
Codex says temperature monitoring and recording devices should be checked for accuracy and calibrated at regular intervals or as needed, and EAS 39 says instruments monitoring critical parameters should be calibrated at appropriate intervals. Set the interval by risk, use, the manufacturer's guidance and the instrument's history, for example an external calibration once a year and in-house checks every shift or day for critical instruments. Shorten the interval for any instrument that keeps failing.

**Who can calibrate instruments in Kenya?**\
Trade instruments are verified and stamped by Weights and Measures inspectors, and repairs should be done by licensed repairers. For process instruments and reference standards, use a calibration laboratory accredited to ISO/IEC 17025 for the relevant field; KENAS runs Kenya's calibration accreditation scheme.

**What should we do if a thermometer or scale fails a check?**\
Take it out of use, work out what was measured since the last good check, put any product that may not meet its limits on QA hold, log a non-conformance with the cause and corrective action, and repair, recalibrate or replace the instrument before using it again. For a trade instrument, have it re-verified and re-stamped.

**Do metal detectors need calibration?**\
They need regular checks. Codex refers to appropriately calibrated detection devices, and EAS 39 asks for the reject mechanism to be checked whenever the detector is checked. Pass certified test pieces for each metal type through the detector at start-up, at set intervals and after changeovers, confirm they are rejected, and record the result.

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*Isaac Muthui, CEO, Tamalaki Business Network Ltd*

*Originally published at* [*factoraerp.com*](https://factoraerp.com/en/blog/calibration-checklist-food-factories-kenya)*.*