8
Theory of Operation
Salt Dilution is a method of flow measurement that has been in use more than
55 years (Østrem, 1964), and is experiencing a renaissance in popularity in the
last 15 years as people discover its accuracy, relative ease, and convenience
(Hudson and Fraser 2002, Moore 2005, Richardson et al 2017). We have de-
veloped the QiQuac system to facilitate fast and accurate SDIQ measurements,
with uncertainty calculations, so that a user can leave the site confident that
their measurements are reliable.
In Salt Dilution, a measured amount of table salt, NaCl, is injected into the
watercourse and the conductivity is measured downstream of a turbulent sec-
tion of flow. [NaCl] and Electrical Conductivity are nearly linearly related (R2
of 1.000) over small ranges (ie >200 μS/cm)), but changes slightly over larger
ranges (Richardson et al 2017). Using the Calibration Factor (aka Concentration
Factor or CF), we can convert the measured conductivity to delta [NaCl] over
time. If we divide the measured mass of NaCl by the area under the [NaCl]-time
curve, we can calculate the Q. Using a dimensional analysis, it is easy to see
how this works:
(1)
The uncertainty of the resulting SDIQ can be <5% if all sources of uncertain-
ty are controlled for. However if things are not controlled, the uncertainty
can easily be >100% (for example if the mixing is not complete). One of the
benefits of the QiQuac is to provide QA/QC in the field on the quality of the
measurement.
Theory of SDIQ
Measurement
The dry salt (a.k.a. slug) method is based on the following expression:
(2)
where Q is stream discharge (m3s
-1
), M is the mass of salt injected (kg), CF
T
is a
calibration factor for converting temperature-compensated electrical conduc-
tivity to salt concentration, and A
BC
is the area under the “Breakthrough Curve”
commonly calculated as
(3)
C H A P T E R 2 : T H E ORY
