GUM Uncertainty Budget Guide for Calibration Labs
What Is GUM?
The GUM (Guide to the Expression of Uncertainty in Measurement) is the internationally agreed method for evaluating and reporting measurement uncertainty, published by BIPM, IEC, ISO, and other international bodies. ISO 17025 clause 7.6 requires laboratories to "identify the contribution to measurement uncertainty" and report expanded uncertainty on all calibration certificates.
Uncertainty Budget Structure
1. Type A Uncertainty (Statistical Evaluation)
Evaluated from repeated measurements using the standard deviation of the data set:
u_A = s / √n
where s is the standard deviation of n repeated measurements.
2. Type B Uncertainty (Other Evaluation)
Evaluated from sources other than repeated measurements, including:
- Reference standard calibration certificate (stated U and k)
- Instrument resolution (±0.5 × resolution)
- Reference material certificate of conformity
- Drift of the reference standard between calibrations
- Temperature coefficient effects
3. Combined Standard Uncertainty
All components are combined using the law of propagation of uncertainty:
uc = √(u_A² + u_B1² + u_B2² + ...)
4. Expanded Uncertainty
The combined uncertainty is multiplied by a coverage factor k to achieve the desired confidence level:
U = k × uc
Typically k = 2 for approximately 95% confidence (assuming normal distribution).
Example: Thermometer Uncertainty Budget
| Source | Value | Distribution | Divisor | u (°C) |
|---|---|---|---|---|
| Reference thermometer (U=0.10°C, k=2) | 0.100 | Normal | 2 | 0.050 |
| Instrument resolution (±0.05°C) | 0.050 | Rectangular | √3 | 0.029 |
| Repeatability (10 readings) | 0.015 | Normal | 1 | 0.015 |
| Drift of reference (±0.02°C/year) | 0.020 | Rectangular | √3 | 0.012 |
uc = √(0.050² + 0.029² + 0.015² + 0.012²) = 0.058 °C
U = 2 × 0.058 = 0.12 °C (k=2, ~95% confidence)
Common Mistakes
- Forgetting drift: A reference standard 12 months past its last calibration has accumulated drift uncertainty that must be included.
- Using U instead of u: Calibration certificates state expanded uncertainty U. Divide by k before using in your budget.
- Skipping Type A: A repeatability study must be performed — at least once per measurement range.
- Ignoring temperature: For dimensional measurements, thermal expansion is often the dominant contributor.
LabSync GUM Calculator
LabSync includes a GUM uncertainty calculator built directly into the Job Item card. Technicians enter each uncertainty component, and the system calculates combined and expanded uncertainty automatically. The result flows directly into the certificate generated by the Designer — no separate spreadsheet needed.