The weekly dip slide is probably the most-done and least-understood check on a cooling tower. Someone dunks a little plastic paddle in the water, pops it in an incubator, reads it a couple of days later and writes a number in the log book. But what is that number actually telling you, and what should you do with it?
August is a good month to ask. Warm weather and heavy loads mean counts can move quickly, and the dip slide is usually the first place you'll see it. Here's a plain-English guide.
What a dip slide measures
A dip slide is a plastic paddle coated with a growth medium. You dip it in the cooling water, incubate it, and bacteria in the water grow into visible spots (colonies) on the surface. By comparing the slide with a chart supplied by the manufacturer, you get an estimate of the total viable count, or TVC: roughly how many bacteria per millilitre were in the water.
Two important points:
- It's a general count, not a Legionella test. A dip slide tells you how active the bacterial population is overall, which shows whether the biocide regime is keeping up. Legionella needs a separate lab test, which HSG274 Part 1 expects at least quarterly.
- It's an estimate. Dip slide results are read in broad bands, usually powers of ten. That's fine, because what you're looking for is which band you're in and whether it's changing.
How often?
HSG274 Part 1 expects microbiological activity in cooling water to be checked at least weekly, and dip slides are the usual way of doing it. Your written scheme should set the exact frequency and say who does it.
Taking a dip slide properly
Small details make a big difference to whether the result means anything:
- Check the slide. Make sure it's in date and has been stored as the manufacturer says. Don't touch the agar surface.
- Sample the right water. Take it from a point that represents the circulating water, not right next to where biocide is injected, and not from a dead leg.
- Watch the timing. Avoid sampling straight after a shot dose of biocide, or you'll be measuring the dose, not the system. Your scheme should say when in the dosing cycle to sample.
- Dip, drain and label. Immerse the slide for the time the instructions say, let excess water drain, put it back in its tube and label it with the system, date and time.
- Incubate. Follow the manufacturer's instructions and your scheme for temperature and time. That's commonly around two days in an incubator.
- Read and record. Compare both sides of the slide with the chart, record the result and the date it was read, and note anything unusual, such as fungi or a slide that's completely covered.
What the results mean
HSG274 Part 1 sets action levels for TVC in cooling water. As a simplified summary, which matches our positive result response page:
- Up to 104 cfu/ml: under control. Carry on with the programme and routine monitoring.
- Above 104, up to 105 cfu/ml: review the programme and dosing, and re-test to confirm the count.
- Above 105 cfu/ml: take corrective action straight away, review the controls and re-test.
Your written scheme should have the exact responses for your site. Write the action levels at the top of the record sheet so whoever reads the slide knows straight away whether it's a normal week.
Trends matter more than single results
A single reading in the lower band is reassuring. But a run of readings tells you far more. Watch for:
- A slow creep. Counts edging up band by band over several weeks often means dosing is slipping or the system's getting dirty.
- A sudden jump. Usually something specific: an empty drum, a failed pump, a process leak, a change in operation.
- Counts that don't come down. If a high result doesn't improve after corrective action, the cause hasn't been found yet.
Our post on dosing failures covers the common causes of a rising count.
Imagine a tower where dip slides have read in the lowest band all year. Then, over three hot weeks, they step up one band and stay there. Nothing's over an action level yet, but the trend is clear. A check finds the bleed valve sticking and the water more concentrated than it should be, which has been eating into the biocide. Fixing the valve brings the counts back down before anything needed escalating.
Common mistakes
- Reading the slide too early, or forgetting it in the incubator for a week
- Using out-of-date slides, or ones stored somewhere hot
- Sampling right after a biocide dose, which gives a falsely low result
- Recording "OK" instead of the actual band
- Writing down a high result and taking no action
Dip slides or lab TVCs?
Some sites send TVC samples to a lab instead of, or as well as, using dip slides. Lab counts are more precise but take longer to come back and cost more. Dip slides are quick and cheap, and done properly they're a perfectly good weekly early warning. Many sites use dip slides weekly and lab samples at set intervals or after a high reading.
In short
Take the slide from the right place at the right time, incubate it properly, record the actual result and watch the trend. Our monitoring and sampling service can do it for you, or train your team and pick up the chemistry and lab work. All results go straight into your log book, and anything out of range gets a clear next step.
Not sure what your dip slides are telling you?
Send us a few weeks of results and we'll talk you through them, or take the weekly checks off your hands altogether.