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Technical Note:
A Practical Guide to Flow Measurement
potential error to a negligible 0.3%. Remember that
blanks should also be diluted 10-fold.
Turbidity causes a second effect which is generally
unrecognized and is a potential error, although only in
rare situations. All glass and gelatin optical filters are
somewhat fluorescent. Scattered excitation light
strikes the emission filter and causes it to fluoresce. If
the blank and standard are prepared from the sample
water, this additional fluorescence simply yields a
higher blank than the same water without turbidity
and there is no error. If the dye concentration is low
enough so that the added contribution is significant,
then a change in turbidity will cause an error in the
opposite direction from that previously discussed.
The effect is usually small and seldom a problem with
dye concentrations above one ppb (actual
ingredient). It should be borne in mind, however, that
some of the worst scattering can occur in the
absence of visible particles.
In the volumetric flume study (32), the injection rate
was set to yield a rhodamine B concentration of one
ppb at the highest flow (about 200 cfs). There was
violent turbulence and many visible air bubbles in the
early sections of the flume. Samples and blanks were
collected far enough down the flume that the visible
bubbles were absent. For convenience, however,
water for the standards was collected from the river
just prior to entry. At 200 cfs, the dye technique
consistently calculated a flow 8% lower than the
volumetric determination. The error reduced
approximately linearly with reduction in flow. The
problem was massive levels of micro air bubbles that
were causing a blank increase of 0.08 ppb. In other
words, the apparent dye concentration was 0.08 ppb
higher than the true dye concentration.
concentration of 10 ppb had been used in the
beginning, the bias at 200 cfs would have been 0.8%,
about 0.4% at 100 cfs, and would have probably been
ignored. Unfortunately, sufficient dye was not
available to make this increase after the problem was
diagnosed.
4.
The final problem is sample temperature. It is
simplest to have your calibration standard and your
samples at the same temperature. Float your
carefully sealed standard in the stream. In the
laboratory, place all the samples together, away from
any source of heat. For precise studies, it is best to
incubate both standard and samples in a water bath.
The sample compartment of the fluorometer will be a
few degrees above ambient temperature, but there
will be no effect on the sample in the few seconds
required for measurement. However, don’t attempt to
remeasure any sample until it has re-equilibrated,
because the cuvette will have begun to warm up. If
you wish to periodically recheck samples, or for some
reason wish to have them in the fluorometer for an
extended period, use the 10-030 Sample Holder,
which is temperature-regulated.
If you need to correct various samples to a standard
temperature, Wilson contains a simple graph from
which an accurate correction is easily obtained (17).
On the other hand, if you have a pocket calculator
handy, the equation is:
F r = Fs e
[n(Ts - Tr)]
Where:
Fr
Upon discovery, there was sufficient time to invent a
practical means of removing the bubbles. Thus, water
taken at the sample point was used to prepare
standards and all standards, samples and blanks
were measured 30 minutes after the water collection.
This ploy was probably not totally successful due to
the different treatment given to the standards
(pipetting, mixing, etc.). (The researchers didn’t think
to chill the samples, an ultrasonic bath was not
available, and the director of the hydraulic laboratory
indicated that it would take from 24-48 hours for the
bubbles to disappear). Note that if a dye
Sample Temperature
is the calculated fluorescent reading at the
reference temperature, Tr.
Fs is the observed fluorescence reading of the
sample at the time of reading the sample
temperature, Ts.
e
is the base of natural log.
n is a temperature coefficient for whatever dye is
used. For Rhodamine WT, rhodamine B, pontacyl
brilliant pink B, and fluorescein, the values have been
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Revision A 05/31/2001
Page 17 of 29