Hach 900 MAX - Manual (Page 45)

Section 3
Page 43
8990startup.fm Basic Programming Setup
Flow Proportional Constant Time, Variable Volume Sampling (CTVV)
A Level-Velocity Sensor Input must be logged and electrically connected for
the CTVV feature to work correctly.
CTVV samples are taken at user-specified constant (fixed) intervals. However
the actual volume of each sampling is based on the known average flow rate
of the site, the actual metered flow rate for each specific interval, the total
sample volume desired, the user-specified collection period, and the specified
Sampling Interval. Depending on the flow volumes of a stream and various
intervals within the Collection Period, the volume of individual sampling can
vary. Also, the total sample collected during the entire period can carry slightly
above or below the Total Volume Desired. For these reasons, the
manufacturer advises to use a sampling container that has a larger volume
than the Total Volume Desired value.
• Sample Distribution menus are not available when CTVV is selected.
• A level-velocity sensor input must be logged and electrically connected for
the CTVV feature to correctly work.
• The Flow Pacing Mode menu within the Flow Proportional menus
sequence will not remember the previous setting whenever you re-enter
this menu, it will revert to the CVVT default. When re-entering this menu,
press
CHANGE CHOICE to select the sampling method, CVVT or CTVV.
Example:
This is an example of how the instrument determines sample volume, based
on user inputs and actual metered flow volume.
The user entered values are the following:
• Average Flow Rate (historical, site specific): 150 gph
• Sampling Interval: 2 minutes
• Total Sample Volume Desired: 1500 mL
• Collection Sampling Period: 30 minutes
Calculation 1: Total Number of Samples
Calculation 2: Average Sample Volume
Calculation 3: Sample’s Volume per unit of Flow Rate
Calculation 4: Actual Sample Volume to be Collected
Sample Period
Sampling Interval
----------------------------------------------
30 min.
2 min.
------------------- 15 samples total within specified period = =
Total Sample Volume Desired
Total Number of Samples
--------------------------------------------------------------------------------
1500 mL
15 samples Total
--------------------------------------------- 100 mL/sample = =
Avg. Sample Volume
Avg. Flow Rate
--------------------------------------------------------
100 mL/Sample
150 gph
------------------------------------------ 0.7mL/gph = =
Sample Volumes per unit of Flow Rate Actual metered Flow Rate
×
0.7 mL/gph 150 gph
×
105 mL for this sample interval =