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-NO3 sensor following a two-point calibration should be 57.1 ± 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:

  1. Dilute the 1000 ppm nitrate standard solution as necessary to prepare two calibration standards that differ in concentration by a factor of 10 and bracket the expected sample range. Do not calibrate at 1 and 10. Calibrations may be done at concentration factors of 2 and 20, 3 and 30, 4 and 40, etc. The ideal range is 1 and 100. Consult Diluting Stock Standards for a step-by-step guide to preparing calibration standards from a stock standard solution.
  2. Make sure that the nitrate ionic strength adjuster (ISA) is added to each calibration standard in a 1:50 ratio. For example, 2 mL of ISA should be added for each 100 mL of diluted calibration standard. ISA is not considered part of the dilution.
  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?”

 

Click Settings tab to enable showing mV values

Figure 1: Click Settings tab to enable showing mV values

 

5. Click the Sensors tab and data should be displayed on screen as shown.

 

Sensors tab for WQ NO3

Figure 2: Click the Sensors tab to display ppm and mV data

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.

Enter the value of the first calibration standard and hit Enter to accept

Figure 3: Enter the value of the first calibration standard and hit Enter to accept

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.

 

REV: 13H19

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