Hach SD900 - Constant Time Variable Volume (CTVV) Sampling

1. Push MENU.
2. Go to PROGRAM SETUP>RESTORE.
3. Select YES.
Constant Time Variable Volume (CTVV) sampling
CTVV samples are taken at user-specified constant (fixed) time intervals. However, the actual
volume of each sample is based on the:
• Known average flow rate of the site
• Actual metered flow rate at the time of sampling
• Total sample volume desired
• User-specified collection period
• Specified sampling interval
The volume of individual sampling can vary and depends on the flow volumes of the stream at
various intervals within the collection period. The total sample collected during the entire period can
be slightly above or below the necessary total volume. For these reasons, the manufacturer
recommends that a sampling container is used that has a larger volume than the necessary total
volume. When CTVV is used for sampling:
• Sample distribution menus are not available when CTVV is selected, only composite sampling is
supported.
• The liquid sensor is always enabled.
• The sample distribution is composite.
• The run mode is non-continuous.
• The first sample is taken after the interval.
• Flow source is always 4–20 mA input.
The instrument finds the sample volume, based on user inputs and actual metered flow volume. For
example, the entered values are:
• Average flow rate (historical, site specific): 150 gph
• Sampling interval: 2 minutes
• Total sample volume desired: 1500 mL
• Collection period: 30 minutes
Calculation 1: Total number of samples
Collection period ÷ Sampling interval = 30 minutes ÷ 2 minutes = 15 samples
Calculation 2: Average sample volume
Total sample volume desired ÷ Total number of samples = 1500 mL ÷ 15 Samples = 100 mL/sample
Calculation 3: Sample volume per unit of flow rate
Average sample volume ÷ Average flow rate = (100 mL/sample) ÷ 150 gph = 0.66 mL/gph
Calculation 4: Collect actual sample volume
• If the 4–20 mA flow source is used and the flow rate measured is 140 gph, use sample volumes
per unit of flow rate x actual metered flow rate.
0.66 mL/gph x 140 gph = 92.4 mL. The sampler will pull a 90 mL sample (rounded off from
92.4 mL).
• If the flow pulses are used and every pulse count is 2 gallons and 2 counts are received in
2 minutes, the flow rate is 120 gph.
Sample volumes per unit of flow rate x actual metered flow rate
0.66 mL/gph x 120 gph = 79.2 mL. The sampler will pull a 80 mL sample (rounded off from
79.2 mL).
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