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Accuracy, calibration and the accuracy logHow much drift is acceptable?
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Guidance Note 3 · p.134. Your instrument drifts over the years, and an instrument reading wrong is worse than no instrument at all. Two ways to stay on top of it:
| What it is | Rough cost | |
|---|---|---|
| Calibration | Sent to the manufacturer, returned with a certificate of calibration saying it reads within guidelines. Yearly, or every couple of years. | £50–100 |
| Accuracy log | Your own record — a spreadsheet will do — of checking the instrument yourself against known values between calibrations. | Your time |
The check itself is straightforward. Put your leads on a source you know the value of and see whether the meter agrees:
Checking against known values
Known 1 MΩ source -> insulation test should read 1 MΩ Known 10 MΩ source -> insulation test should read 10 MΩ Known 0.25 Ω -> continuity should read 0.25 Ω Acceptable drift = up to 5%, and you would hope for 1–2% A 10 MΩ source reading 7 MΩ -> way past 5%. The instrument is out.
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- 18th Edition
- Building Regs
- PAT
- Fundamental Inspection & Testing
- Initial Verification
- Periodic Inspection & Testing