Calibration - Laser Turbidity Sensor

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Precision Measurement Engineering, Inc.

Discussing Calibration

LASER TURBIDITY SENSOR

The Laser Turbidity sensor may be calibrated in two ways. The most accruate way is to obtain a 5 gal black polyethelyene bucket, place calibration solutions in the bucket, and make measurements with SCAMP having its sensor guard in the normal position. This most closely matches the actual deployment situation. The disadvantage is that large amounts of calibration standard solutions must be mixed.

An alternate method is to use small amounts of calibration solution that are poured into SCAMP's clear sensor cover. The disadvantage here is that there is a slight error due to the reflectivity of the sensor cover. The instructions below are for calibration using this alternate method.

Mix 1000 ml of 100 NTU solution from the 4000 NTU stock solution. This is accomplished by placing 25 ml of 4000 NTU stock into a 1000 ml graduated cylinder, then adding distilled water to make 1000 ml. This will become the stock solution for further dilutions.

Set SCAMP Laser Turbidity channel calibration coefficients to C0=0.0, C1=0.0117188, C2 = 0.0, and C3 = 0.0. Set Units to "Volts". Select Noff = 0, Noffpolar = 1, NGain = 255.

Begin at the lowest turbidity of interest.
Carefully rinse SCAMP's sensors by pouring about 100 ml of distilled water into the sensor cover, rinsing the interior surfaces, then pouring the water out. There will be a small amount of water retained within the sensor cover. Shake as much of this out as possible.
Rinse the mixing container with distilled water.
Mix 100 ml of the desired turbidity standard solution by weight, assuming the density of both water and stock solution is 1 gm/ml.
Pour the standard into SCAMP's sensor cover. Verify there are no bubbles present on the turbidity sensor near the sensing volume. Hold SCAMP upright in a dark place and record 1000 or more scans. Note that the formazin particles will settle slightly over a few minutes time. Perform the measurement quickly.
Repeat the above steps, working up to the maximum turbidity.

The results of calibrating SCAMP SN0034 are shown in the table and figure below.

Weight of 100 NTU stock	Total Weight of Standard	NTU of Standard	File Name	Sensor Output Voltage
0	100	0	FILE1	0.0307
10.44	101.05	10.33	FILE2	0.0984
20.82	100.77	20.66	FILE3	0.1660
31.47	102.12	30.82	FILE4	0.2280
41.12	100.58	40.89	FILE5	0.2865
50.57	100.04	50.55	FILE6	0.3469
60.61	100.04	60.59	FILE7	0.4143
71.16	100.62	70.72	FILE8	0.4755
80.03	100.09	79.96	FILE9	0.5392
90.54	100.32	90.25	FILE10	0.6175
100.85	100.85	100.00	FILE11	0.6960

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A linear least square fit of the data gives

Turbidity EU (NTU) = -3.9075 + 153.3055 * Sensor Output (Volts)

The C1 calibration coefficient can not be entered directly into the C1 box on the SCAMP Control Dialog Channel tab. It must be adjusted by multiplying by 0.01172. The values to be entered into the Laser Turbidity calibration coefficients are C0 = -3.9075, C1 = 1.797, C2 = 0.0, C3 = 0.0. Change the Units to 'NTU'. Add a comment giving the date of calibration or other information.

This completes the turbidity calibration. SCAMP should be tested by pouring a standard into the sensor cover, recording a data file, and examining the file to be sure that the correct NTU units are shown.

Range of Calibrated Response

The Laser Turbidity sensor electronics can produce up to approximately 2.5 Volts. The above calibration shows that about 379 NTU will cause the sensor to produce this maximum. The Span Limits may be changed as required to give additional sensitivity and reduce the range available. If this is done the calibration may optionally be repeated.

Calibration Standard Solution

4000 NTU Formazin solution, 500 ML (Cost USD $45.80) Avaliable from Hach Company (www.hach.com) their P/N 246149.

Dilution of Formazin solution:

Tstd = Tstock * (Vstock)/(Vstock + Vwater)

Tstd = turbidity of standard to be mixed up

Tstock = turbidity of stock solution (4000 NTU)

Vstock = volume of stock solution to which water is added

Vwater = volume of distilled water added to stock solution