45
A
dvanced
S
ensor
T
echnologies,
I
nc. U.S.A.
Website:
www.astisensor.com
IOTRON
TM
Trademark of ASTI
IOTRON
TM
pH / ORP / ISE / DO / Conductivity Measurement Products Lines
APPENDIX “B”
Temperature Considerations for Calibrating pH Sensors with pH Buffers – Part 2 of 2
The HiQDT touchscreen controller automatic calibration mode computes the exact values of the pH 1.68, 4.00, 6.86, 7.00,
9.18, 10.01 and 12.45 buffers in the automatic calibration mode for anywhere between 0 to 60
°
C. If calibrating with pH
buffers in the temperature condition below 0
°
C or above 60
°
C automatic calibration mode cannot be used (manual mode
must be used instead). The HiQDT touchscreen controller software can also perform manual calibration to any pH value
for Offset, Slope Low (Acidic) or Slope High (Alkaline). In this way this controller is not limited to pH 1.68, 4.00, 6.86,
7.00, 9.18, 10.00 and 12.45 buffers for calibration but rather can perform offset and slope calibrations to any value desired.
Temperature compensation only accounts for the change in the mV response of the pH sensor itself with temperature.
The type of temperature induced shifts such as those demonstrated in the table above for the pH buffers are NOT
corrected in default Nernstian temperature compensation scheme. For process solutions the change in the pH value with
temperature can be significantly more pronounced than for pH buffers which are inherently designed to shift in only the
most minimal way due to changes in temperature, dilution, evaporation and other typical conditions in field use.
Thankfully the HiQDT-pH sensors allow for a user defined temperature compensation coefficient to account for the NET
temperature effects. The temperature impact on the pH sensor and the temperature impact on the measured solution
cannot be cleanly separated (deconvoluted). It is, however, possible to determine the effective net mV per °C change and
enter this as a custom temperature compensation coefficient. Contact the ASTI factory for assistance with such situations
requiring special temperature compensation schemes. The default temperature compensation setting is the classical
Nernstian 198
μ
V (0.198mV) per °C with the allowable range of 000-999
μ
V to any custom value for your given process.
The temperature compensation coefficient can be changed by the Windows software or handheld communicator.