With a Semi Conductor Breathalyzer, while the
sensor is generally quite accurate around the two calibration points of 0.03%
BAC and 0.10% BAC (UK limit 0.08%) the results can be less satisfactory outside
of these ranges, and particularly so at lower levels below 0.03%. Needless
to say, manufacturers all quote their accuracy as a figure +/- (x)% at the 0.10%
level, and while this is generally repeated at this level accuracy does diminish
at other measuring points.
A Fuel Cell, on the other hand, measures
evenly throughout the range, hence why they are the only types of sensor used
for Police grade detectors. Fortunately, while a Police standard detector
(such as the Dräeger 6510) costs around £700 there are now several Fuel-Cell
based Breathalyzers available at considerably less - the PT500 for
instance is just £199 Plus VAT.
The other reason for the price difference is
generally down to the sampling method. To get an accurate reading you need
an accurate and consistent sample, but whereas the Fuel-Cell type of
Breathalyzers also measure exactly 1.7ltrs of air each time (and expel the
remainder through an air valve) the semi conductor models use either time alone
(say a 3 second blow) or combination of time and pressure.
