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C. When does it make a lot of difference?
First, the accuracy of temperature compensation to 25°C determines the
accuracy of any TDS conversion. Assume we have industrial process
water to be pretreated by RO. Assume it is 45°C and reads 1500 µS
uncompensated.
1.
If NaCl compensation is used, an instrument would report 1035
µS compensated, which corresponds to 510 ppm NaCl.
2.
If 442
TM
compensation is used, an instrument would report
1024 µS compensated, which corresponds to 713 ppm 442
TM
.
The difference in values is 40%.
In spite of such large error, some users will continue to take data in
the NaCl mode because their previous data gathering and process
monitoring was done with an older NaCl referenced device.
Selecting the correct Solution Type on the Ultrameter
III
will allow the
user to attain true TDS readings that correspond to evaporated weight.
If none of the 3 standard solutions apply, the User mode must be used.
Temperature Compensation (Tempco) and TDS Derivation below, details
the User mode.
XXII. TEMPERATURE COMPENSATION (Tempco) and TDS
DERIVATION
The Ultrameter
III
contains internal algorithms for characteristics of the
3 most commonly referenced compounds. The solution type in use is
displayed on the left. Besides KCl, NaCl, and 442
TM
, there is the User
choice. The benefit of User mode is that one may enter the tempco and
TDS conversion values of a unique solution via the keypad.
A. Conductivity Characteristics
When taking conductivity measurements, the Solution Selection
determines the characteristic assumed as the instrument reports what
a measured conductivity would be if it were at 25°C. The characteristic
is represented by the tempco, expressed in %/°C. If a solution of 100
µS at 25°C increases to 122 µS at 35°C, then a 22% increase has
occurred over this change of 10°C. The solution is then said to have a
tempco of 2.2 %/°C.
Tempco always varies among solutions because it is dependent on their
individual ionization activity, temperature and concentration. This is why
the Ultrameter
III
features mathematically generated models for known
salt characteristics that also vary with concentration and temperature.
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Summary of Contents for POOLPRO PS9TKB
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