HPC Samplers / Testers for liquid testing

Swimming pools can be hotbeds for bacterial infection despite water treatment programs. Therefore, they need to be tested regularly especially if there are periods of increased use. Bacteria can originate from people, animals, equipment, pipes, filters or other contaminants that are in the environment.

HPC (heterotrophic plate count) samplers are sensitive tests that will allow you to test for low microbial limits making them suitable for applications such as swimming pool testing where the target values are low.

The Pool water treatment advisory group, a UK based independent organisation recommends the following in their code of practice (https://www.pwtag.org/code-of-practice/):

"Swimming pool water should be microbiologically tested each month to monitor for the presence of potentially harmful microorganisms.

Hydrotherapy pools, even those not in a healthcare setting, should be tested weekly.

Whenever a microbiological sample is taken it is important that a pool waterchemical test of free and combined chlorine and pH is taken at the same time, from the same location as a reference.

The aerobic colony count should not be more than 10 colony forming units (cfu) per millilitre of pool water after incubation for 24 hours at 37°C.

• A colony count in excess of 100cfu/ml is unsatisfactory
• A consistently raised colony count of 10 to 100cfu/ml is unsatisfactory and should be investigated."

These tests can help you assess the general quality of the pool water alongside / in between lab tests.

Due to the incubation requirements of 37 degrees C, you will require a variable temperature incubator. Our variable temperature incubators can be found here:

• Small MX10: Click here to view
• Medium MX25: Click here to view
• Large MX45: Click here to view

Please be aware that capacity information on the individual incubator pages relates to dipslides and not HPC samplers which are a different shape / size.

Before testing using the HPC sampler, you will need to dechlorinate / neutralise the chlorine in the sample by using sodium thiosulphate which can be purchased at well known online stores / lab stores. Please take care when using sodium thiosulphate and always follow any instructions included with your purchase.

Sample Collection

For sampling, The Pool Water Treatment Advisory Group recommends the following method:

"Sampling should be taken with the pool in use, preferably when heavily loaded or immediately afterwards. The deep end is the best place, and away from inlets. Leisure pools with complex water flows to different areas may demand several samples.

Sample containers should be of a material that will not affect the sample either microbiologically or chemically. They may be glass, shatterproof plastic-coated glass or plastic. A 500ml sample bottle is the norm.*

The bottle should be sterile and contain an agent that neutralises the pool disinfectant: sodium thiosulphate (18mg/l) is the agent for chlorine and bromine-based disinfectants. That will deal with up to 5mg\l of free chlorine; above that the test may be invalid.

To take the sample, the stopper or cap is first removed, making sure that nothing touches the inside of the bottle or cap.

While the bottle is being plunged into the water the long axis should be kept approximately horizontal but with the neck pointing slightly upwards to avoid loss of the neutralising agent. The bottle is then quickly immersed 100-300mm below the pool surface, at which point the bottle is tilted upwards to allow it to fill. On removal from the water, the cap is immediately replaced, the sample shaken to disperse the neutraliser."

Reference: https://www.pwtag.org/microbiological-sampling-september-2014/

*Only 18ml sample is required for testing with the HPC samplers.

Step by step guidance for swimming pool sample collection

Based on the recommendations above (from the The Pool Water Treatment Advisory Group)

After taking the sample and neutralising the chlorine, you can then conduct the HPC sampler test.

1. Take a sterile 500ml sample bottle (preferably with a cap or stopper).
2. Remove the cap or stopper (making sure that the cap does not touch anything / become contaminated).
3. Make sure that the bottle contains the neutraliser sodium thiosulphate. Consult manufacturer's guidelines for the neutraliser to water ratio (usually 18mg/l).
4. Quickly immerse the bottle within the pool water (100-300mm below the surface) with the long axis of the bottle horizontal and the neck slightly upwards to avoid loss of the sodium thiosulphate.Tilt the bottle upwards to allow it to fill with water.
5. Remove the bottle from the water, replace the cap and shake the bottle to evenly disperse the sodium thiosulphate.

After taking the sample and neutralising the chlorine, you can then conduct the HPC sampler test. The instructions on how to test the neutralised pool water using the samplers can be found below.

How to use the HPC test

Collect Sample

Pour sample into the 18ml sample chamber (fill up to the marked line).

Drop Swimming Pool HPC Sampler into the chamber

Drop paddle into sample solution and close the paddle within the chamber.

Lay Swimming Pool HPC Sampler down

Lay the sampler down on its side with the filter side facing down for 30 seconds. The pad on the paddle will absorb 1ml of liquid sample.

Empty Chamber

Empty the tester, discarding the sample liquid. Shake the paddle and chamber vigorously to remove excess sample droplets.

Incubate Swimming Pool HPC Sampler

Place the paddle back within the chamber ensuring a tight seal. Place in the incubator for 24 hours at 37 degrees C (lay sampler device down on its side again with filter side facing down).

Count the colonies

Count the dots (colonies) and compare with the chart provided.

• Pour sample into the 18ml sample chamber. 
• Drop paddle into sample solution, close the paddle within the chamber. Lay sampler device down on its side with filter side facing down for 30 seconds. The pad on the paddle will absorb 1ml of liquid sample.
• Empty the tester, discarding the sample liquid. Shake the paddle and chamber vigorously to remove excess sample droplets.
• Place the paddle back within the chamber ensuring a tight seal. Place in the incubator for 24 hours at 37 degrees C (lay sampler device down on its side again with filter side facing down).
• Count the colonies on the pad. Counted colonies can be recorded as cfu/ml since the sample volume is 1ml. 

Reading the HPC sampler results

Colony Count	10	30	100
Example Image

What is the difference between the HPC sampler / tester and your other dipslides?

There are a few differences. One is that our other dipslides are semi quantitative tools which means that when evaluating the results, the user counts the colonies e.g 12 and then compares this with the chart. In this case the chart would say that this is 10/3 (1000 cfu/ml) so 12 colonies equals 1000 cfu/ml.

The HPC samplers are quantitative tests. This means that if you count 12 colonies, the result is 12 cfu/ml. This is a more accurate way of testing as it is more in line with / equal to laboratory tests. The semi quantitative dipslide tests still offer a good guide of the level of micro-organisms in the sample. However, the quantitative HPC samplers offer more accurate results when testing samples with lower CFUs. The samplers would be ideal for applications where recommendations for micro-organism levels are low e.g dental DUWLs (100 - 200 cfu/ml), laboratory reagent grade water (10 cfu/ml).

So to recap, dipslides are semi quantitative e.g 12 colonies = 10/3 (1000 cfu/ml)

HPC samplers are quantitative e.g 12 colonies – 12 cfu/ml

The other differences are that the HPC samplers do not require a separate incubator and the HPC samplers have a much longer shelf life (up to 18 months compared to 6-8 months).