Instruction manual

52
MODEL XMT-C SECTION 11.0
THEORY OF OPERATION
DEFINITIONS
1. LINEAR TEMPERATURE COEFFICENT OR TEMPERATURE SLOPE. The change in the conductivity of most
electrolyte solutions having conductivity greater than about 5 mS/cm at 25°C can be expressed by the following
equation:
C
25
=
In the equation, C
25
is the conductivity at 25°C, C
t
is the conductivity at t°C, and a is the linear temperature coef-
ficent. The linear temperature coefficent, sometimes called the temperature slope, has units of %/°C. In the equa-
tion, the temperature coefficent is expressed as a decimal fraction. The linear temperature coefficent depends to
some extent on both the temperature and the concentration of the salt solution. The temperature coefficient also
varies from salt to salt.
For maximum accuracy, the temperature coefficent must be appropriate for the salt or salts in solution, their con-
centration, and the temperature. Frequently the relationship must be determined by experiment. Fortunately, for
most dilute neutral electrolyte solutions, a linear temperature coefficent of 2.00%/°C (0.0200) works reasonably
well. The table below gives typical ranges for different electrolytes.
Slope (%/°C)
Neutral salts 1.8 - 3.0
Acids 1.0 - 1.6
Bases 1.8 - 2.2
High purity water Use standard correction
Temperature compensated conductivity measurements are important in the power industry. The table lists tem-
perature slopes for different types of treatment chemicals. The slopes apply across the range of concentrations
typically encountered.
Slope (%/°C)
Condensate treated with ammonia 2.00
Boiler water treated with phosphate/caustic 2.00
2. CATION TEMPERATURE CORRECTION. Cation conductivity, sometimes called acid conductance, is used in
steam power plants to measure salt contamination in the boiler feedwater and steam. The Model Xmt-C auto-
matically corrects for the variation in the conductivity of extremely dilute hydrochloric acid with temperature and
displays cation conductivity measurements. Cation conductivity temperature also applies to semiconductor
etch rinse baths, which contain trace amounts of acids.
3. RAW. Raw conductivity is the conductivity of the sample at the measurement temperature.
C
t
1 + a(t- 25)