Campbell Scientific OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors - Manual (Page 26)

OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors
The NTU values of the standards will remain the same as long as the ratio of
particle mass (number of particles) to water mass (volume) does not change.
Evaporation causes this ratio to increase and dust, bacteria growth, and dirty
glassware can also cause it to increase. Therefore:
1. Always use clean glassware and calibration containers.
2. Don’t leave standards on the bench in open containers or leave the
standard bottles uncapped. Perform the calibration as quickly as
possible and return the AMCO solutions to their bottles.
3. Clean dirty sensors with a clean, alcohol-soaked cloth to sterilize it
before dipping it into the standards.
4. Transfer entire bottles between containers, avoiding aeration and
shaking excess fluid off the glassware.
Because of the intrinsic errors in the NTU value of formazin used by the SDVB
manufacture (GFS Chemicals) and the dilution procedures, the uncertainty in
the NTU value of an SDVB standard is ± 1% of the value indicated on the
standard bottle. Consequently, the NTU value of one liter of standard in an
uncovered 100 mm calibration cup will increase ~ 1% in 10 hours on a typical
summer day (R.H. = 90% and air temp. = 18 o C). For example, the NTU value
of a 2000 NTU standard in a 100-mm cup will increase by about 2 NTU (0.1%)
per hour. TABLE 8-2 gives the increases for some other, commonly used
standards.
TABLE 8-2. Change in NTU value resulting from one hour of
evaporation 1 of SDVB standard; in other words, loss of water but not
particles.
Nominal NTU Value
Calibration-cup Size
∅ mm (∅ in.)
250 500 2000 4000
100 (4) +0.26 +0.52 +2.10 +4.20
150 (6) +0.60 +1.20 +4.80 +9.70
8.1.1 Materials and Equipment
• OBS sensor with test cable
• Datalogger or averaging DMM with test leads, 12 V gel cell
• Large black polyethylene plastic tub for measuring the clear-water points
• 100-mm and 200-mm black PE calibration cups
The 100-mm cups are used with standards with NTU values greater than 250
NTU and the 200-mm cups are used with the 125 and 250-NTU standards.
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