Calibrating Turbidity - Xylem EXO Series User Manual

Calibration
5.7 4.4
Turbidity
Before calibrating, be certain that the probe is clean and free of debris. Solid particles, particularly those
carried over from past deployments, will contaminate the standards during your calibration protocol and
cause either calibration errors and/or inaccurate field data. Use a clean, spare sonde guard. Also, review the
basic calibration description on pg 65.
For proper calibration, you must use standards that have been prepared according to details in Standard
Methods for the Treatment of Water and Wastewater (Section 2130 B). Acceptable standards include (a)
formazin prepared according to Standard Methods; (b) dilutions of 4000 NTU formazin concentrate
purchased from Hach; (c) Hach StablCal
AEPA standards prepared specifically for the EXO turbidity sensor by the manufacturer.
2-point
Pour the correct amount of 0 NTU standard (clear deionized or distilled water) into the calibration cup.
Immerse the probe end of the sonde into the water.
In the Calibrate menu, select Turbidity, then select Turbidity FNU.
Click 2 Point for the Calibration Points. Enter 0 FNU for first standard value and 124 FNU for second
standard value. (0 must be calibrated first.)
- If the water to be evaluated is known to be low in turbidity, an appropriate choice of standards
might be 0 and 12.4. However, for general purpose measurements an appropriate choice of standards
is usually 0 and 124.
Click Start Calibration. Observe the readings under Current and Pending data points. While stabilizing,
click the Wipe Sensors button to activate the wiper to remove any bubbles. When data are Stable (or data
shows no significant change for approximately 40 seconds), click Apply to accept this calibration point.
- If the temperature of your field site is substantially different from the lab temperature, allow the
sensor to sample for 3-5 minutes at each calibration point before accepting it. This step ensures the
best possible temperature compensation when deployed.
Next place the sensors in the second calibration standard. Click Proceed on the pop-up window. Observe
the readings under Current and Pending data points. While stabilizing, click the Wipe Sensors button to
activate the wiper to remove any bubbles. When data are Stable (or data shows no significant change for
approximately 40 seconds), click Apply to accept this calibration point.
Click Complete. View the Calibration Summary screen and QC score. Click Exit to return to the sensor
calibration menu, and then the back arrows to return to main Calibrate menu.
Rinse the sonde in tap or purified water and dry the sonde.
3-point
Select the 3-point calibration option for maximum accuracy over a wider range. As for the 2-point procedure,
the first standard must be 0 FNU. Because of the linearity characteristics of the sensors, we recommend that
the other two standards have turbidity values of 124 and 1010 FNU. However, the user can select any values
that are deemed appropriate. The procedure for this calibration is the same as for a 2-point calibration, but
the software will prompt you to proceed to an additional solution to complete the 3-point procedure.
standards in various NTU denominations; and (d) AMCO-
TM
|
73
pg