Calculator Inputs
Example data table
Typical inputs for a 20 V range, 20000-count meter. Values are illustrative only.
| Reading | Range | Counts | Spec | Computed basic error | Basic interval |
|---|---|---|---|---|---|
| 12.345 (base) | 20 (base) | 20000 | ±(0.5% + 2 digits) | ±0.061725 | 12.283275 to 12.406725 |
| 1.999 (base) | 2 (base) | 2000 | ±(1.0% + 1 digit) | ±0.02999 | 1.96901 to 2.02899 |
| 100.0 (base) | 200 (base) | 20000 | ±(0.2% + 5 digits) | ±0.025 | 99.975 to 100.025 |
Formula used
- LSD = Range / Counts (or direct LSD if provided).
- Basic error = |Reading|·(Percent/100) + Digits·LSD + |Fixed|.
- Temp error = |Reading|·(ppm·10⁻⁶)·|ΔT| (optional).
- Cal error = |Reading|·(Cal%/100) + |CalAbs| (optional).
- Worst-case: Combined = Basic + Temp + Cal.
- RSS (expanded): u = √(u₁²+u₂²+u₃²), Expanded = k·u, using rectangular-to-standard conversion uᵢ = term/√3.
These computations help translate common meter specifications into usable bounds for engineering decisions.
How to use this calculator
- Enter the displayed reading and choose the same prefix seen on the meter.
- Enter the selected range full-scale and the meter counts, or provide a direct LSD.
- Type the accuracy spec percent and the digits term from the datasheet.
- Optionally add temperature and calibration/reference terms for deeper uncertainty.
- Click Calculate and review the basic and combined intervals above the form.
FAQs
1) What does “digits” mean in meter accuracy?
Digits represent additional error in least significant digits. The calculator multiplies digits by the LSD step size to convert that term into an absolute error.
2) How do I find the LSD for my range?
Use LSD = Range ÷ Counts. For a 20 V range on a 20000-count meter, LSD is 0.001 V. You can also enter a direct LSD.
3) Should I use worst-case or RSS?
Worst-case is conservative and sums all bounds. RSS is typical in uncertainty budgets when sources are independent, and it returns an expanded uncertainty using a k factor.
4) Why does temperature affect accuracy?
Many specifications assume a reference ambient range. A ppm/°C coefficient approximates additional drift when your ambient differs from that reference, increasing uncertainty for larger ΔT.
5) What is the “calibration/reference” term?
If you compare the meter against a source, that source has uncertainty too. Adding those percent and absolute terms helps reflect the total measurement uncertainty.
6) Can I check if a measurement meets a tolerance?
Yes. Choose percent or absolute tolerance, then compare PASS or FAIL. PASS means the uncertainty bound is within your allowed error for that reading.
7) What if my reading is near zero?
Percent-of-reading becomes small near zero, while digits and fixed terms dominate. If the reading is exactly zero, relative percentages are displayed as zero for practicality.
8) Does this replace a full metrology analysis?
No. It is a practical engineering estimate based on common datasheet formats. For accredited work, follow your lab’s uncertainty procedure and instrument specifications.