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ATI A15/76 Turbidity Monitor Part 6 – Operation

O&M Manual

Rev-H (8/11)

Manual Low Level Offset Adjustment

While not normally required, the A15/76 contains an adjustment routine that allows the operator to

adjust the lower end of the measuring range to match values obtained by an alternate measuring device.

This routine should only be used when adjusting turbidity values below about 0.200 NTU.

To offset a low level turbidity value, follow the procedure below.

1. With sample flowing through the flowcell normally and a turbidity value below 0.200 NTU displayed on

the LCD display, pess the CAL key and the display will indicate “ZERO C.: X.X%”, with the X.X value

indicating the last entered zero offset, indicated as a percentage of the operating range.

2. Press CAL again and the unit will indicate “CAL ZERO1: %” until it has read a stable zero value and

then will indicate CAL ZERO1: X.X%”. Press ENTER and the procedure will be repeated for ZERO2.

Press ENTER again and it will again be repeated for ZERO3. The reason for this is that you are

zeroing the unit for all 3 operating ranges. After the ZERO3 value has been recorded, the display will

change to “SENS: 100%” (or some other value depending on the last calibration).

Setting Sensitivity (Span)

Adjusting the span of a turbidity system requires the use of either a mixed turbidity standard or

comparison with a reliable portable meter. Formazin turbidity standards can be prepared using reagent

solutions available from Fisher Scientific or other lab supply companies. The chemicals required are

Turbidity Standard 1 (Formazin Sulfate, 1%, Fisher part #LC26300-7) and Turbidity Standard 2

(Hexamethylenetetramine, 10%, Fisher part # LC26320-7). Stabilized turbidity standards available

commercially may also be used for this purpose.

Mixing a turbidity standard takes 24 hours. First, place 5 cc. of Turbidity Standard 1 and 5 cc. of

Turbidity Standard 2 in a 100 cc volumetric flask. Allow to stand for 24 hours at about 25° C. Then dilute

to 100 cc. with distilled water (preferably turbidity free water). This will provide a stock solution with a

turbidity value of 400 NTU. This solution should be discarded after about 30 days.

To set the span of the turbidity monitor, dilute the 400 NTU stock solution either 10:1 or 100:1,

depending on the desired final value. A 10:1 dilution will result in a 40 NTU value, which is the

recommended value for drinking water measurements under EPA guidelines. A 100:1 dilution will give

you a 4 NTU value, but dilution errors will increase proportionately. Once your standard is prepared,

follow the procedure below.

Note: When making adjustments using turbidity standards, the 4-20 mA output signal can be put

into a “hold” mode so that the output does not increase during calibration. This is very

important if high turbidity output signals can cause alarms or store erroneously high

values in process data logs. If you want to hold the output, press the CAL key from

display D2. The display will indicate either “AN. OUT: NO HOLD or HOLD”. If the display

indicates HOLD, press the ENTER key twice. If the display indicates NO HOLD, press the

UP key to change to HOLD and then press ENTER twice. The display will then read

“SENS: XX.X%”. Press the CAL key once and then proceed to add the standard as

outlined below.

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Analytical Technology A15 Especificaciones Pagina 25