Sommer GmbH
9
5. Tracer dilution method
5.1. Measurement principle
The tracer dilution method requires a slug injection of a defined amount of tracer substance (in this case
salt) into the water body to be measured. Prior to the injection at least one conductivity probe has to be
placed in the water body downstream of the injection site. The conductivity sensor is used to measure
and record the concentration of the tracer substance in the water.
The conductivity sensor outputs the currently measured conductivity of the water. To define a relation of
the measured values with the actual concentration the probes need to be calibrated to determine the
calibration factor (k). This calibration factor is not only depending on the used tracer substance and the
probes but also on the river’s or canal’s water. Therefore Sommer Messtechnik strongly recommends
performing a calibration of the probes prior to every measurement.
The calibration of the probes is of equal importance as the absolute accuracy of the used probes with
regards to the accuracy of the discharge determination. Accordingly Sommer Messtechnik suggests using
tracer solutions with a concentration of 10 g/l, which can be carefully and thoroughly prepared by the end
user (for more information see chapter 9.4).
The maximum deflection must be at least 200 µS/cm and/or 100% higher than the base value.
The discharge calculation is based on the injection amount (m), the previously determined calibration
factor (k) and the measured values during the passage of the tracer substance in the form of the area
between the curve and the base value (A).
Q … Discharge
m … Amount of tracer substance
k ... Calibration factor
A … Area covered by the curve
(measured values)
µS / cm
Base value
Ma
ximu
m
def
lec
tion
>
2
0
0 µS
/cm and
/o
r 10
0%
