Yokogawa 2-Wire Dual Channel Transmitter/Analyzer FLXA21 - Manual (Page 226)

App.2-10
IM 12A01A02-01E 5th Edition : Oct. 31, 2013-00
0 1000 2000 3000 4000 5000
1000
2000
3000
4000
5000
6000
7000
Concentration (mg/L)
Conductivity
(µS/cm, 25°C)
MgCI
2
Na
2
SO
4
MgSO
4
CaCI
2
NaCI
KCI
Figure 2 Relations of concentration and conductivity
n Calibration solutions for conductivity
The calibration (cell constant) of a sensor does not change unless the sensor is damaged.
It can also appear to change because of coating of the electrodes, or partial blockage.
It does not make sense to regularly recalibrate the FLXA21.
A calibration check, however, is another matter. When the objective is clearly defined as a
diagnostic exercise a regular check can bring an extra level of security and confidence to the
measurement.
Sensor damage, and/or coatings can be difficult to see and the calibration check can confirm
their presence, by a deviation from the known solution conductivity. The remedial action should
be to clean the sensor, and carefully check for blockage or damage (not simply to recalibrate).
Higher conductivity solutions should be used where possible. The lower the conductivity of the
test solution, the easier it is to contaminate. Carbon dioxide from the air can be quickly absorbed
to cause an error. All containers must be suitably clean, and all materials suitably pure. Outside of
a well-equipped laboratory these conditions are hard to meet.
Also note that the check must be performed with due regard to the cell constant of the sensor,
as it limits the effective working range. The documentation provided with the sensor should be
consulted to determine its maximum working value. By using a checking solution close to the
upper range limit of the sensor, the detection of contamination is better. A slightly contaminated
sensor may read perfectly at lower conductivity, but show significant errors at higher values. This
is caused by the early onset of polarization, the factor that limits the upper range of the sensor
in any case. Polarization is seen in contaminated sensors, and it is for this reason that there is a
sophisticated polarization check built into the FLXA21.