A calibration shows how a scale weighs on the day the technician visited. What happens afterwards, nobody sees, unless you keep track of it yourself. A knock against the platform, a relocation or dirt under the weighing pan can suddenly shift the indication. If that goes unnoticed until the next calibration, a whole year of weighings may be called into question.
A simple interim check with a calibrated weight reduces that risk. Here you can read which weight you need, how often to check, how to carry out and record the check and what to do in the event of a deviation. Such a check does not replace the periodic calibration of your scale, but complements it.
Why an interim check is worthwhile
Standards such as ISO 9001 do not only ask for calibrated measuring equipment. They also require you to take action as soon as it becomes apparent that a measuring instrument is no longer fit for purpose. An interim check is the practical tool for that. During audits we see that a well-kept check log strongly complements the certificate, because it shows that the scale was also monitored between two calibrations.
A check is not a calibration. You do not determine a measurement uncertainty and you usually measure at one or two points. The aim is simpler: to see in time that something has changed.
Which check weight do you choose?
The check weight must be clearly more accurate than the scale you are checking. A useful rule of thumb comes from OIML R76: the error of the weight is at most one third of the permissible error of the scale at that load. For a scale that is not verified, you use the tolerance from your own quality system instead of the legal error.
Example: verified class III scale
A scale with Max 30 kg and a verification scale interval e = 10 g has, at 20 kg, which is 2,000 e, a permissible error of ±10 g according to the table for initial verification. One third of that is about 3.3 g. An M1 weight of 20 kg may deviate ±1 g and is amply suitable. An M2 weight (±3 g) just fits, an M3 weight (±10 g) does not.
Example: analytical balance
Suppose your procedure for an analytical balance applies a tolerance of ±1 mg at 100 g. One third of that is about 0.33 mg. An F1 weight of 100 g (±0.5 mg) is then too coarse; an E2 weight (±0.16 mg) does comply. For precision balances with a wider tolerance, F1 or F2 is usually sufficient.
Preferably choose a weight in the part of the range where you weigh most often, and possibly a second one close to Max. Check weights per class can be found in the webshop, for example F1 weights and M1 weights. More help with the choice can be found in which calibration weight do I need. Always use a weight with a valid certificate and handle it with care, see storing calibration weights.
How often do you check?
The frequency follows from the same risk as the calibration interval. Reference points that we often encounter in practice:
- Daily or at the start of every shift: balances for critical weighing-in, scales used to release products and checkweighers in production.
- Weekly: intensively used bench and floor scales and laboratory balances for routine work.
- Monthly: scales with limited use in a stable environment.
- Always in addition: after relocation, a fall or knock, overloading, cleaning during which the platform was removed, and when in doubt about a result.
It is better to start too often than too rarely. If the results prove stable after a few months, you can reduce the frequency with justification. See also how often to have a scale calibrated.
The check step by step
- Prepare. Make sure the scale has warmed up according to the manual, is clean and is level. Look at the spirit level.
- Let the weight acclimatise. Allow the check weight to take on the temperature of the room. With fine balances this can take a while.
- Check the zero. Zero the scale with an empty platform and check that the indication is stable.
- Place the weight. Put the weight in the centre of the platform. Use gloves or tweezers for small weights and suitable aids for heavy weights.
- Read. Wait until the indication is stable and record the value.
- Remove and check zero. Take the weight off and check that the scale returns to zero.
- Assess and record. Compare the difference with your limits and record the result.
In addition, carry out a more extensive check at a lower frequency, for example monthly. Also place the weight on the corners to monitor eccentricity, or weigh it several times in succession to assess repeatability.
Records and action limits
A check that has not been recorded does not exist for an auditor. For each check, record at least: the identification of the scale, date and time, the weight used with its number, the indication, the difference from the value of the weight, the verdict and the initials of the person who carried out the check. A logbook next to the scale or a simple digital form is sufficient.
Preferably work with two limits:
| Limit | Example | Action |
|---|---|---|
| Warning limit | Two thirds of your tolerance | Repeat the check, inspect the scale and surroundings, temporarily check more often |
| Action limit | Your tolerance | Take the scale out of service, look for the cause, have it calibrated or repaired |
Plot the results in a simple graph. Then you will see a gradual shift before a limit is reached.
What do you do in the event of a deviation?
First check the simple causes. Is the scale level? Is the platform touching something? Is there dirt underneath? Is there a draught or vibration, and is the weight clean and at temperature? Then repeat the check. If the deviation remains outside the action limit, take the scale out of service and mark it clearly.
Next, assess which weighings since the last good check may have been affected. That is exactly why regular checking pays off: the shorter the period, the smaller the consequences.
Do not adjust a verified scale yourself. Only use functions that the manufacturer makes available to the user without breaking a seal, such as the internal adjustment function on some balances. If adjustment requires access to sealed settings, the verification lapses and re-verification is required. Even with a non-verified scale it is wise to record an adjustment and then check again, so that the history is correct.
How MASSLAB helps
MASSLAB calibrates scales according to EURAMET cg-18 and calibrates check weights in classes F1, F2, M1, M2 and M3 in our own laboratory. If you include your check weights in the calibration visit, scale and weight are brought up to date in one go. If your logbook shows a deviation, we can calibrate the scale and, for a verified scale after adjustment, re-verify it as a Recognised Verifier (Erkend Keurder) under the Dutch Metrology Act. More information can be found at scale calibration.