Calculation
ΔImax = class·Ifull/100; frel = 100·ΔImax/Ireading
The class refers to full-scale value, not the current reading.
Busch 2006, Abschnitt 10.2 Genauigkeitsklasse
Meter error from accuracy class
The class refers to full-scale value, not the current reading.
MINTSI
02
Result
Select a target and calculate.
Understand the inputs
- Maximum relative error in % — Largest percentage deviation relative to indicated current; this value rises sharply at low scale readings.
- Accuracy class in % — Number printed on the scale; 1.5 means ±1.5% of full-scale value, not of the reading.
- Full-scale current Ifull — Upper scale value of the current range actually selected.
- Indicated current Ireading — Current indication for which relative error is to be assessed.
Example
Class 1.5 on a 200 A range gives ±3 A; at a 50 A reading this is at most ±6%.
Assumptions and limits
Accuracy-class specification of an analogue pointer instrument with a full-scale-referenced limit; resolution, reading, temperature and connection errors are excluded.
Understand Meter error from accuracy class
This calculator derives the error limit of an analogue ammeter from its printed accuracy class and shows why an oversized range degrades the reading.
What does this quantity describe?
Accuracy class is a percentage of full-scale value. The absolute limit therefore stays constant across the scale, while its percentage of a small indication increases.
Formula and variables
ΔImax = class·Ifull/100; frel = 100·ΔImax/Ireading
ΔImax = (class/100)·Ifullfrel = ΔImax/Ireading
| Symbol / input | Meaning |
|---|---|
| Maximum relative error in % | Largest percentage deviation relative to indicated current; this value rises sharply at low scale readings. |
| Accuracy class in % | Number printed on the scale; 1.5 means ±1.5% of full-scale value, not of the reading. |
| Full-scale current Ifull | Upper scale value of the current range actually selected. |
| Indicated current Ireading | Current indication for which relative error is to be assessed. |
Choose the inputs correctly
Read class from the scale. Ifull is the end value of the range actually selected, not the instrument's greatest capability. Ireading is the uncorrected indication.
How to use the calculator
Identify the active range, then enter its full-scale value, class and reading. Use the absolute limit for reporting and relative error to judge range utilisation.
Worked example
Class 1.5 on 200 A gives ΔImax=3 A. At 50 A this is ±6%; at 150 A it would be only ±2%.
Understand the result and units
A large relative value may simply mean poor scale utilisation rather than a defective meter. Busch therefore recommends readings in the upper third.
Class and result are percentages; full scale and reading must use the same current unit.
Typical applications
Laboratory range selection, checking legacy panel meters and teaching measurement uncertainty.
Assumptions, limits and common mistakes
For full-scale-referenced analogue classes only. Digital ±(% reading + digits) specifications, resolution, parallax and environmental effects need another model.
Common mistake: Do not apply the class directly to the indication. Class 1.5 does not generally mean ±1.5% of the displayed value.
Frequently asked questions
What is “Meter error from accuracy class” used for?
Laboratory range selection, checking legacy panel meters and teaching measurement uncertainty.
Where do the input values come from?
Read class from the scale. Ifull is the end value of the range actually selected, not the instrument's greatest capability. Ireading is the uncorrected indication.
What does the result not cover?
For full-scale-referenced analogue classes only. Digital ±(% reading + digits) specifications, resolution, parallax and environmental effects need another model.