Flow Proportioned Sample Collection Procedures
{illustration of lab beakers} 
The Regulations require flow proportioning for all daily composite samples. See RCSA Section 22a4303(a)(3) for more information. This requirement is often a major area of noncompliance for permittees. You can call 8604243018 for questions and assistance regarding sampling procedures.

Permittees that have a constant discharge rate as demonstrated by recorded flow data can collect and combine equal volume aliquot samples taken at equal intervals over a full operating day.
Permittees that have a fluctuating discharge rate, however, must collect a series of
aliquot samples over a full operating day and combine them proportionally to the flow rate at the time each aliquot is collected or by the total flow discharged between each hourly aliquot.
Sample Collection Methods
Listed below are four widely used methods for collecting flow proportioned samples. (Please remember that prior to using any of the following methods, preliminary calculations should be performed to ensure that an adequate final sample volume will be obtained.) 


Constant aliquot sample volume, variable time interval (maximum allowable time between each aliquot is one hour unless otherwise approved by the commissioner), samples collected based on flow rate. When using this method you need to know four things:
 Minimum Daily Flow. The minimum daily flow should be based upon recent historical data. Since the actual flow on the day of sampling will likely be different from the historical minimum, the total sample volume will usually vary from the required amount. To ensure that an adequate sample volume will be collected, you may wish to use in your calculations a conservative value for the minimum daily flow rate (i.e., a somewhat lower value than what the historical records indicate).
 Hours of Discharge
 Sample Volume Required by Contract Laboratory
 Required Sample Volume divided by Minimum Daily Flow (ml/gallon)
Example of Constant Aliquot Sample Volume 
Minimum Daily Flow 
22,000 gallons 
Hours of Discharge 
10 
Required Sample Volume 
10,000 ml 
10,000 ml divided by 22,000 gallons equals 0.45ml/gallon (round 0.5) 
If sample period is set at 1,000 gallon intervals 0.5ml/gal x 1,000 gal equals 500 ml aliquot sample volume 
After each 1000 gallons of discharge, collect a 500 ml aliquot sample. This will give you the required sample volume plus a safety of 1000 ml. (Always use a safety factor to guarantee that enough samples are collected.)
This method can be used for manual collection or collection by an automatic sampler that is interfaced with a flow meter.
Variable aliquot sample volume, time interval between samples remains constant (maximum allowable time between each aliquot is one hour unless otherwise approved by the Department), based on the total flow between aliquot sample collection. When using this method you need to know four things:
 Minimum Daily Flow
 Sample Volume Required by Contract Laboratory
 Required Sample Volume divided by Minimum Daily Flow
 Hourly Flow Rates (See example .)
Example of Variable Aliquot Sample Volume 
Minimum Daily Flow 
22,000 gallons 
Required Sample Volume 
10,000 ml 
10,000 ml divided by 22,000 gallons equals 0.45ml/gallon (use 0.5) 
Required aliquot volume of 0.5ml of sample per gallon of discharge 
Constant time interval, variable aliquot sample volume based on instantaneous flow at the time of aliquot sample collection or the total flow per hour. When using this type of sampling you need to know the following items:
 Range of instantaneous flow
 Average instantaneous flow based on historical data (or total hourly flow)
 Sample volume required by the laboratory performing the analysis
 Number of aliquot samples to be taken (at least one per hour)
 Sample of volume required divided by the number of aliquots
 Factor for flow proportioning (number 5 above divided by 2 above)
Example:
 Range of instantaneous flow: 10 gpm to 100 gpm
 Average instantaneous flow: 34 gpm
 Sample volume required: 4000 ml
 Number of aliquots: 5
 Volume required divided by the number of samples: 4000 ml/5 samples=800 ml
 Factor: 800 ml/34 gpm = 23.5 ml/gpm (round to 24)
Example of Constant Time Interval 
Time 
Flow Rate, gpm 
Avg. Instantaneous Flow Rate, gpm 
Volume Required = Sample Volume/Number of Aliquots 
Factor =
Volume Required/Avg. Instantaneous Flow Rate 
Aliquot
Volume, ml = Factor * Flow Rate 
07:00 
25 
34 
800 
24 
600 
08:00 
100 
34 
800 
24 
2400 
09:00 
10 
34 
800 
24 
240 
10:00 
15 
34 
800 
24 
360 
11:00 
20 
34 
800 
24 
480 
Total Volume=4080 ml 
This type of composite sample is collected by having a continuous drip through a small tap valve that flows only while a discharge is occurring. Flow rate is controlled by pressure in the discharge line from either pumping or gravity. The samples generated tend to be larger than needed. When properly installed, this method requires very little manual effort.
Content last updated July 2011