Instruction manual
51
MODEL XMT-C SECTION 11.0
THEORY OF OPERATION
SECTION 11.0
THEORY OF OPERATION
11.1 CONDUCTIVITY / RESISTIVITY / % CONCENTRATION
Liquids can only conduct electrical currents when they contain particles that carry charges. These particles are
called ions, and they are produced when acids, bases, and salts are mixed with water. The conductivity of a sub-
stance determines how well it can carry electrical currents and is used to indicate the concentration of acids, bases,
and salts in water.
Conductance is the reciprocal of resistance. The traditional unit of conductance is mho, a term representing the
reciprocal of ohm. Recently, the unit siemen has replaced the mho, but the amount of conductance is exactly the
same. Liquid water has relatively low conductivity, so measurements are expressed in millisiemens (.001 siemen)
or microsiemens (.000001 siemen), and abbreviated as mS or μS, respectively.
The Model Xmt-C conductivity transmitter is a device used to measure conductivity in most chemical processes.
Conductivity is a function of ion concentration, ionic charge, and ion mobility. Ions in water conduct current when
an electrical potential is applied across electrodes immersed in the solution.
Model Xmt-C conductivity transmitter uses conductivity probes with electrodes (contacting). Model Xmt-T is
designed for use with inductive (toroidal) probes for measurement of high conductivity. Generally, contacting (elec-
trode) probes are used for conductivity below 200 microsiemens, such as water rinses in metal finishing or ultra-
pure boiler water applications. The electrode design is more sensitive for low level measurement and these water
solutions tend to be non-fouling.
For % concentration measurement, the Model Xmt-C transmitter uses the measured temperature and absolute
conductivity and applies specific algorithms that have been developed for each of the substances available in the
instrument.
11.2 TEMPERATURE CORRECTION
The conductivity of an electrolyte solution depends strongly on temperature. To allow comparison among meas-
urements made at different temperatures, conductivity values are usually converted to the value at 25°C. The
Model Xmt-C performs the correction automatically following one of three temperature correction algorithms.
1. User-programmable linear temperature coefficient (slope)
2. Ultra-pure water
3. Cation conductivity (dilute hydrochloric acid)
Temperature correction can also be turned off. If temperature correction is off, the Model Xmt-C displays the raw
or non-temperature corrected conductivity. Temperature corrections apply whether the measurement is in con-
ductivity or resistivity units.