Turb 2000 Series Instrument offset
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5 Instrument offset
In certain instances, it may be desirable to use an offset factor to calibrate the instrument rather than
performing a physical calibration of the instrument (as described in section 4.2). This procedure is not
recommended in lieu of regular instrument calibration but it can be used in situations where the number
of instruments used makes regular calibration prohibitive.
This calibration technique will make the instrument accurate only at turbidity levels in the immediate
vicinity of the grab sample and not in the full range of the instrument. Note that the OFFSET icon will be
illuminated whenever an offset used. The maximum offset is ± 1.00 NTU. If instrument variation is
greater than 1 NTU a full calibration is recommended.
The procedures are as follows:
1. Collect a grab sample of the process water that is being monitored by the instrument and record the
turbidity reported by the instrument.
2. Take the grab sample and measure its turbidity using a laboratory turbidimeter (contact the WTW
customer services department for examples of laboratory turbidimeters).
3. Compare the turbidity reported by the instrument to that obtained in the laboratory. If the readings
are very close, then no offset adjustment or calibration is required and the procedure may be
stopped at this step. However, if the readings are substantially different (but less that 1 NTU),
continue on in this procedure to utilize the offset option to improve the turbidity reading of the
instrument so that it will agree with the laboratory reading between calibrations.
4. Select the offset function of the instrument by pressing the MODE/EXIT button until the arrow
beside CONFIG is illuminated on the display. Refer to the following screen.
5. Push the OK
button until OFST is displayed on the lower row.
6. At this point, the lower row of the display will indicate the operational status of the offset function
(On or OFF). Change this status by using the and
buttons. Once the desired operational
status of the offset function has been set, press the OK button to accept it. If the option was turned
off, return to AUTO mode by pressing MODE/EXIT.
7. If the option was turned On, the upper row will display the offset required. This will add or subtract
the value of the offset to the measured NTU value. As an example if the Turb 2000 series measures
the sample at 0.16 NTU but the laboratory instrument reads the sample at 0.12 NTU, adding an
offset of –0.04 would result in the instrument displaying 0.12 NTU.
Select the desired offset level using the and buttons. Once the desired level has been set,
press the OK button to accept it.
8. This completes the offset configuration.
9. At this point, the instrument will remain in configuration mode (CONFIG). Press the
MODE/EXIT key to return to operation mode, AUTO.
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