Adjustment and Calibration
18
Background of temperature coefficients
This is a topic that is frequently misunderstood and often neglected in hemodialysis.
A solution’s conductivity will change according to temperature. With increasing temperatures, the measured
solution’s conductivity will increase, too. To achieve meaningful measurement results, the conductivity
value displayed is compensated to 25°C. In other words, the display is always converted to a solution
temperature of 25°C. The temperature coefficient which the displayed value is compensated with is
expressed as %/°C. Unfortunately however, different solutions also have different temperature coefficients.
To achieve an exact display, the instrument will have to be adjusted to the temperature coefficient of the
current solution. The commonly used temperature coefficient for dialysates is 2.07 %/K. For naturally
occurring solutions, a value of 1.97 %/°C is frequently used. Many measuring devices not specially tailored
to dialysis will use this value.
Dynamic temperature compensation
Dynamic temperature compensation is achieved by calculating a temporary temperature coefficient for the
measured temperature using a set of two coefficients for 25°C and 37°C respective. This temporary
temperature coefficient is then applied to the actual measured conductivity to calculate the conductivity at
the common reference temperature. The following table shows the usage of dynamic temperature
coefficients applied to a NaCl solution of 14mS/cm.
Temperature Conductivity NaCl 14 mS/cm
without compensation
Temperature
coefficient
Conductivity
Readout
25 °C
14,00 mS/cm
2,000 %/K
14,00 mS/cm
31 °C
15,71 mS/cm
2,035 %/K
14,00 mS/cm
37 °C
17,48 mS/cm
2,070 %/K
14,00 mS/cm
The values 2.00 %/K at 25°C and 2.07 %/K at 37°C are defaults for the respective dialysis
machine or values entered by the user. For demonstration purposes the temperature
coefficient for 31°C is mentioned, which is calculated by the HDM97Pocket from the values
for 25°C and 37°C by linear interpolation.
The calculation below shows the drastic effects of an incorrect temperature coefficient.
Example calculation for an incorrect temperature coefficient, using a dialysate with a temperature coefficient
of 2.07 %/°C:
Conductivity
of solution
mS/cm
Temperature of
solution
°C
Temperature
coefficient
of meter
%/K
Meter
display
mS/cm
Difference
in values
%
14,00
37,0
2,07
14,00
0,00
14,00
37,0
1,97
14,17
1,21
