A flow test is the most useful diagnostic test for system performance and should be done before replacing or cleaning your membrane. Changes in production or water quality are normally caused by something other than the membrane unless the system has been left unused for a long time. These tests may seem daunting at first but if you approach them in a logical manner it is rather straightforward.


Voltage, total flow rate, feed pressure, recovery percentage, product flow rate, and product quality all need to be considered when diagnosing system performance.

The three main components of your system are the Feed Pump, Clark Pump, & Membrane(s).



1. Voltage

Please use a multimeter and take a DC reading off the system terminal block rather than a readout on a charging panel.

2. Feed Pressure

Please also note any asymmetry with the pressure as the Clark Pump is cycling.

3. Flow Rates (GPH or GPM)

Measure the time it takes to fill a bucket (or 1 Liter bottle) with a stopwatch. Record the two measurements below. Please record the following and note the examples:

  • Product Flow Rate “3:42 to fill up a 5 gallon bucket with product water”
  • ‘Total Flow’ Rate. “44.6 seconds to fill a 750 mL wine bottle from brine discharge + product”

4. Product Quality (PPM or taste)

If unit is running in run manual, please taste the water. You can also use a handheld salinity meter.



  1. Voltage

Shows us how much power is actually being fed directly to the motor

  1. Feed Pressure

Shows us what pressure is being delivered to the Clark Pump by the feed pump

  1. Flow Rates
  • Product Flow Rate shows us the amount of product water, in a given period of time
  • Total Flow Rate shows us the TOTAL amount of water that moves through the system, in a given period of time

Once we have values, we can compare the total flow rate vs. system spec nominal total flow rate, which checks on the Feed Pump Head. We can then divide the PRODUCT by the TOTAL FLOW and we get a percentage of total flow water recovered as product water, otherwise known as Recovery Rate %.

       4. Quality (PPM)

  • This shows us whether or not product water is within potable range



Before a flow test, change all filters and clean the sea strainer. Carefully check for water or air leaks, as air in the system will cause low production and erratic salinity. Look for air bubbles in the product flow meter, feed water hoses, and brine overboard hose. When the system has been running and is stabilized, use a multimeter to confirm at least 12.5 Volts are at the feed pump on 12-Volt DC systems; 25 volts on 24-Volt DC systems. Note that Spectra systems produce at spec levels when receiving a minimum of 13.2VDC and 26.4VDC respectively. If available, turn on engine or battery chargers.

Low voltage can be due to the ship’s battery, wiring between the battery & system, or a speed controller issue.

  • A 0.5 VDC decrease at the pump motor = Loss of 1/2 GPH of product water and 100ppm increase in product salinity
  • For every 1/10th of a GPM feed water flow loss, we will lose about 1/2 GPH of product flow and the salinity will go up 100 PPM


Run the system and watch the feed pressure very closely. If the feed pressure to the Clark Pump is asymmetrical from one stroke to another, this impedes performance. A difference of a few PSI is acceptable, but anything over that is an issue. The feed pressures should be within 5% of each other on each stroke, and the pressure spikes about the same. If the pressures are far different (asymmetrical) on each stroke look for one of these problems: (Clark Pump Asymmetrical Pump Strokes) If no asymmetry is noted, continue with this test.


You will need a stopwatch and container to measure two flow rates: the product water flow rate and the total/feed flow rate. Perform a flow test to confirm the Clark pump is functioning by confirming the ratio of product water to brine discharge is correct. First, measure the total system flow. Total flow is the product flow combined with the brine discharge flow (with Pressure Relief Valve closed). Then, capture the product flow by itself. Nominal flow rates for product flow are given in gallons per hour and nominal flow rates for total flow are given in gallons per minute. Calculations are simpler when a 1, 2, 3, 5, or 10-minute test are taken in a graduated bucket, graduated pitcher, or large measuring cup. However, if these are not available, you can count the number of seconds or minutes it takes to fill a container with a known quantity, i.e. 5-gallon bucket, milk jug, wine bottle, etc. Accuracy is important as we are interested in any deviation over 1% from the standard recovery rates (see below). The recovery rate is the percentage of seawater pumped through the system converted to potable water.


The ratio of product flow to total flow gives us our recovery rate, as a percentage. The Clark Pump is designed to have a fixed recovery ratio percentage of 7, 10, or 20% depending on your system.

Recovery Rate = [Product in GPH / Total Flow in GPH] x 100


Sample Calculations can be found here:

Flow Test Example

Compare your Total Flow Rate & Recovery Rate to your system’s nominal specs.

Quick Production Breakdowns of Systems



7% Clark Pump (Ventura 150)

6.5% Actual Recovery ---> 5.5% Minimum Acceptable

10% Clark Pump (Ventura 200, Cape Horn, Catalina, Newport 400)

9.0% Actual Recovery ---> 8.0% Minimum Acceptable

20% Clark Pump (Newport 700/1000)

18.5% Actual Recovery ---> 17.5% Minimum Acceptable


***Recovery Rate below minimum is indicative of an internal leak within the Clark Pump***

If the measured recovery rate is lower than the "Minimum Acceptable Recovery Rate", then it's time to send your pump in for an upgrade as it is worn out or pick up an upgrade kit to attempt on your own. Note that certain seals/o-rings can be challenging in the field without some of the specialized installation tools used at the factory.

See General Clark Pump Check-Up for some tips on checking out your Clark Pump.


Low feed flow and/or low feed pressure is commonly associated with a lack of voltage or a worn feed pump head. If you are making above the minimum value for Product Flow and the quality is below 500 ppm, the system is functioning normally.