Measurement Uncertainty
Measurement Uncertainty for VGT Accredited Test Methods
When scientists make a measurement or calculate some quantity from their data, they generally assume that some exact or "true value" exists based on how they define what is being measured (or calculated). Scientists reporting their results also calculate a range of values that they expect this "true value" to fall within, based on expected errors associated with that measurement. This range is called the Measurement Uncertainty and is assessed at a particular result value.
When assessing whether a result is within compliance or not, it may be necessary to take account of the measurement uncertainty range that the result reported may fall into. As an example, if a Total Suspended Solids result is reported as 50 mg/L, the measurement uncertainty associated with that result has been calculated as ±2 mg/L, then it is possible that the “true value” is somewhere between 48 and 52 mg/L.
The measurement uncertainty for VGT’s accredited tests is given below. For further information, please contact the laboratory.
Test |
Method ID |
Level MU Assessed |
Measurement Uncertainty (MU) (+/-) |
Including Sampling (+/-) |
pH |
VGT WI01 APHA 4500 H-B |
7.00 |
0.01 |
Creeks: 0.01 Dams: 0.04 Groundwaters: 0.1 |
Conductivity |
VGT WI02 APHA 2510B |
1412 |
10 |
Creeks: 15 Dams: 20 Groundwaters: 27 |
Solids - Total suspended (TSS) |
VGT WI03 AS3550.4 |
50 |
2 |
|
Solids - Total dissolved (TDS) |
VGT WI49 AS3550.4 |
746 |
10 |
|
Turbidity |
VGT WI04 APHA 2130B |
20 |
0.2 |
|
Dissolved oxygen (DO) |
APHA 4500O-G |
8.8 |
0.2 |
|
Oil and grease |
APHA 5220B |
10 |
1 |
|
Biochemical oxygen demand (BOD) |
APHA 5210B |
198 |
4 |
|
Particulate matter - PM2.5; Particulate matter - PM10; Suspended particulate matter – Total |
VGT WI44 |
50
|
2 |
|
Ash; Combustible matter; Solids - Insoluble |
VGT WI14 AS3580.10.1 |
4 |
0.8 |
|
References:
Nordtest Guides:
- NT TR 537 Handbook for Calculation of Measurement Uncertainty in Environmental Laboratories: https://www.nordtest.info/images/documents/nt-technical-reports/NT_TR_537_edition4_English_Handbook_for_calculation_of_measurement_uncertainty_in_environmental_laboratories.pdf
- NT TR 604 Uncertainty from sampling - A Nordtest Handbook: https://www.nordtest.info/images/documents/nt-technical-reports/NT_TR_604ed2_Nordtest_Handbook_for_sampling.pdf
Eurachem Guides:
- Quantifying Uncertainty in Analytical Measurement: https://www.eurachem.org/index.php/publications/guides/quam#translations
- Measurement uncertainty arising from sampling: A guide to methods and approaches: https://eurachem.org/index.php/publications/guides/musamp