what are dipslides and how to take them
If you are responsible for operating and maintaining a water system such as an evaporative cooling system or chilled water system, you may need to take dip slides.
DIP SLIDES – WHAT ARE THEY?
The HSE guidance recommends the use of dip slides for monitoring bacterial levels and the effectiveness of biocides.
Read more about HSE guidance documents here
Dip slides are commercially available plastic slides, which are coated with sterile nutrient agar – a medium on which many microorganisms will grow, but not legionella. Bacteria in the cooling water will grow on the agar and form visible colonies. Comparison with a chart will indicate the number of bacteria in the water, expressed as colony forming units per millilitre (cfu/ml).
HOW TO TEST
Dip slides should be used to sample the system water downstream of the heat source. The water sample is usually taken from the return line to the tower. If a sample point is used, it is important to flush it to ensure a representative sample before the slide is dipped.
The dip slide should be placed into its sterile container and into an incubator for a minimum of 48 hours, usually set at 30°C
WHEN TO TEST
Cooling system water should be tested weekly. The dip slide is usually taken as part of the weekly inspection of the system.
Read more about inspecting the system here
WHAT DO THE RESULTS MEAN?
If the control strategy is effective, the dip slide counts should reflect a system under control. If an unusually high result is obtained, the test should be repeated immediately and, if confirmed, appropriate action taken.
