It is recommended that calibration is done on a daily basis, or at least prior to each use.
During calibration, observe the slope of the mV readings between the two points that are being used for calibration. Slope is defined as the change in voltage read by the sensor with a ten-fold change in concentration (mV/decade).
The slope of the WQ-FL sensor following a two-point calibration should be 58.2 ± 5 mV/decade at 25°C. Every ±0.5°C change in temperature moves the mean value of the slope ±0.1 mV.
Follow the procedure outlined below to complete a calibration:
- Dilute the 1000 ppm fluoride standard solution as necessary to prepare two calibration standards that differ in concentration by a factor of 10 and bracket the expected sample range. Consult Diluting Stock Standards for a step-by-step guide to preparing calibration standards from a stock standard solution.
- Make sure that the fluoride ionic strength adjuster (ISA) is added to each calibration standard in a 1:1 ratio. For example, 50 mL of ISA should be added for each 50 mL of diluted calibration standard. ISA is not considered part of the dilution.
- For 10 ppm: 49.5 mL of distilled water + 0.5 mL of 1000 ppm Standard + 50 mL of ISA
- For 100 ppm: 45 mL of distilled water + 5 mL of 1000 ppm Standard + 50 mL of ISA
3. Check that the sensor reference fill solution level is above the reference fill line.
4. Plug the sensor into an available USB port on a computer running WQSensors software. Click the Settings tab and then select “Show mV values?”
5. Click the Sensors tab and data should be displayed on screen as shown.
6. Slide the fastfill sleeve on the sensor down, rinse the sensor tip in distilled or deionized water and blot dry. Keep the sensor vertical so that the reference fill solution does not spill out.
7. Place the sensor in the first standard and wait 1-2 minutes for readings to stabilize.
8. Click Calibrate, highlight the concentration reading in ppm, enter the concentration of the standard and press Enter or click the Calibrate button again. A short animation with the letter C will display on the bottom left hand corner of the screen.
9. Rinse the sensor and blot dry before placing it into the second calibration standard. Again wait 1-2 minutes for the reading to stabilize, enter the concentration of the standard in ppm, and press Enter.
10. Rinse the sensor and blot dry. Close the fastfill sleeve and cover all calibrations standards tightly to prevent contamination. Calibration is complete.
Calibration should be done as often as every one to two hours for maximum accuracy. Calibration drift can be checked by immersing the probe in a standard with a known concentration and letting the reading stabilize.