This page will familiarize you with commonly used methods to estimate the accuracy of experimental data and quantities derived from this data.
Errors in experimental data consist of two components:
- Fixed or Systematic errors - cause repeated readings to be in error by a similar amount e.g. improperly calibrated or installed equipment.
- Random errors - caused by electronic fluctuations, friction etc.
The following technique for estimating the propagation of errors/uncertainties into the result is that proposed by Kline and Mclintock. We use specified uncertainties for primary experimental measurements e.g. p = 500Pa ± 10Pa, where ±10Pa is the uncertainty.
Take measurements with estimated uncertainty for each measurement. Then estimate the uncertainty in the result R as a function of independent variables xi. Thus
Let wR be the uncertainty in the result and wi the uncertainty in the independent variables. Then the uncertainty in the result is
The velocity indicated by a Pitot static tube is given by:
where Δp is the dynamic pressure indicated by the Pitot tube and r is the gas density at the measurement point. Thus the uncertainty in the velocity may be estimated as:
As an example, assume the measured independent values and their estimated uncertainties are:
Δp=600Pa ± 0.5%
ρ=1.18kg/m3 ± 0.03 kg/m3
Estimate the uncertainty in the velocity wv
∂v/∂Δp = 0.5(2/ρΔp)0.5
∂v/∂ρ = -0.5(2Δp)0.5/ρ1.5
thus the uncertainty in the velocity wv is:
wv = 0.413m/s
wv / v = 0.413 / 31.89 = 1.29%
How do we estimate the uncertainty?
1) By repeating experimental readings. Then estimate uncertainty within a certain confidence interval. Pertains to random errors as for a sufficient number of readings random errors tend to approach a Gaussian distribution allowing statistical analysis.
Expanding on point 1), suppose we have repeated a set readings. Let n be the number of readings and xi a particular reading.
then the arithmetic mean is given by
the deviation from the mean for each reading is given by
di = xi-xm
the average of the absolute value of the deviations is given by
and the root mean square deviation or standard deviation is given by
for a large number of readings or
for a small data set (n < 20)
From statistical analysis define
Then the uncertainties are
- σmean for a 68.3% probability
- 2σmean for a 95.5% probability
- 3σmean for a 99.7% probability
Thus the repeated data may be expressed as
- xm ± σmean for a 68.3% probability (i.e. there is a 68.3% probability that the measurement will lie within ± σmean etc)
- xm ± 2σmean for a 95.5% probability
- xm ± 3σmean for a 99.7% probability