Cooling tower log sheet: what to record and read together?

This guide covers the routine readings a service engineer or operator logs at each cooling tower visit. It does not set control limits, which belong to each site's treatment program, and it does not replace legionella sampling, which follows its own method and laboratory rules.

What do you need before starting a cooling tower log sheet?

Before starting a cooling tower log sheet, you need the site's control chart, a clean sample bottle, calibrated test kits, the makeup and blowdown meter locations, and a sampling risk assessment. The control chart is the program's list of treatment chemicals, target levels and required tests; US military guidance notes that water treatment service companies commonly supply these control charts (UFM 3-230-13, 2026).

Prepare these items before the first visit:

1. The control chart and the previous visit's log, so today's values can be compared on the same lines.

2. A clean 1-litre glass or plastic sample bottle with a cap (UFM 3-230-13, 2026).

3. Test kits and reagents within date, with each reagent's Safety Data Sheet (SDS) at hand; handle and dispose of spent reagents as the SDS and local waste rules require.

4. A handheld conductivity meter calibrated against a standard, so you can check the controller reading.

5. A sampling risk assessment. UK guidance says a risk assessment should be conducted before any sampling or monitoring activity, and that respiratory protective equipment should be worn in listed cases, such as when the system has yielded high legionella results before (HSE HSG274 Part 1, 2024).

Agree with the site which meters are read and who reads them, because the log needs the same meters every visit.

Which cooling tower water readings should you record at every visit?

The cooling tower water readings to record at every visit are pH, conductivity and inhibitor or tracer level in the tower water, plus the makeup conductivity, the meter totals, the chemical used and the oxidant residual where an oxidizing biocide is fed. US military guidance recommends that recirculating water "should be tested for pH, conductivity, and inhibitor content", with calcium hardness and M alkalinity as tests that "may also be tested" (UFM 3-230-13, 2026).

Record each visit in this order:

1. Date, time and sample point for every sample, tower and makeup.

2. Tower water: pH, conductivity, inhibitor or tracer level, and oxidant residual if one is fed.

3. Tower water on the program's schedule: calcium hardness, total (M) alkalinity, chloride and temperature.

4. Makeup water: conductivity at each visit; calcium hardness and alkalinity on the program's schedule.

5. Makeup and blowdown meter totals, and whether each is metered or estimated.

6. Chemical added since the last visit, from drum or tank levels and pump settings.

7. Controller reading beside your handheld reading.

US military guidance recommends that cooling tower logs "contain results (including dates) of all chemical tests performed, COC, and the amount of chemicals added" (UFM 3-230-13, 2026). COC means cycles of concentration. Logging the amount of chemical added beside the water readings is what lets you read dose against cycles later.

Why do sample time and sample point matter in a cooling tower log?

Sample time and sample point matter in a cooling tower log because the same tower gives different values at different places and times, and only a consistent sample can be compared with the last visit. US military guidance says "It is best to always collect the sample for each system in the same manner and from the same location for consistent results" (UFM 3-230-13, 2026).

Three timing rules come from published guidance. First, do not sample right after a chemical addition: the same guidance says the sample "is collected after the chemicals have been allowed to mix thoroughly with the system water", and recommends testing within 1 hour (UFM 3-230-13, 2026). Second, flush the sample line first; the guidance gives 10 seconds of flushing at the recirculating pump discharge, or until no sediment remains (UFM 3-230-13, 2026). Third, for dip slide and other microbial samples where biocide is shot dosed, UK guidance says sampling should be done when the residual biocide is at its lowest, ideally at the same time each week, at a point remote from the biocide dosing point (HSE HSG274 Part 1, 2024).

The makeup sample needs the same care. US military guidance says one makeup sample per week is usually adequate, but if makeup water quality varies, the makeup should be sampled at the same time as the cooling water (UFM 3-230-13, 2026). Write the time and point on the log line, not in a separate note.

Which cooling tower readings should you read together?

Cooling tower readings should be read together in pairs, because a single value in range can hide a problem that shows only when two readings disagree. The table decides which disagreement points to a control problem at this visit.

The key row is inhibitor or tracer level against cycles. Chem-Aqua's bulletin says cycles "must be considered in relation to inhibitor tests", because when cycles are high or low the inhibitor may be proportionally high or low (Chem-Aqua, 2024). Where feed follows makeup, a low inhibitor reading at low cycles points first to excess blowdown; if inhibitor is low by more than the cycles explain, check the pump and feed rate (Chem-Aqua, 2024).

Two more pairs need the same habit. Calcium hardness, alkalinity and pH only mean something for scale when read with temperature, because the Langelier Saturation Index uses all of them; Chem-Aqua lists calcium hardness, total alkalinity and pH as the tower tests used to calculate it (Chem-Aqua, 2024). An oxidant residual that looks normal can still be weak if pH has risen, since UK guidance says the efficacy of chlorine and bromine "is compromised at higher pH levels" (HSE HSG274 Part 1, 2024). Write each pair on the same log line, so the comparison is made at the visit.

How do you check cycles of concentration from cooling tower log readings?

You check cycles of concentration from cooling tower log readings by dividing a tower water value by the same value in the makeup, then comparing that ratio with the makeup-to-blowdown meter ratio for the same period. UK guidance says concentration factors can be calculated by comparing "conductivity, TDS (total dissolved solids), magnesium hardness, chloride and silica" in the cooling water with the makeup (HSE HSG274 Part 1, 2024).

An illustrative case: tower conductivity of 2,000 µS/cm over makeup conductivity of 500 µS/cm gives 4 cycles. If the meter ratio for the same weeks gives 6 cycles, more water is leaving than the blowdown meter records. US federal guidance says these ratios should match the target cycles, and "If both ratios are not about the same, check the tower for leaks or other unauthorized draw-off" (US DOE FEMP).

Pick the cycles basis before you compare visits. Chem-Aqua notes that chloride may be used to calculate cycles "unless elevated by chlorine-based biocides" (Chem-Aqua, 2024). UK guidance adds that chloride may indicate brine loss from a malfunctioning water softener where one is fitted (HSE HSG274 Part 1, 2024). If the site feeds bleach or has a softener, use conductivity or silica for cycles and log which basis you used.

What cooling tower log sheet mistakes hide a problem?

The cooling tower log sheet mistakes that hide a problem are values without time or point, dose logged apart from water readings, estimated meter values written as measured, and a single cycles figure with no basis. Each one makes a disagreement between readings invisible.

• No sample time or point. A sample taken minutes after a shot dose or from a dead leg reads differently, and the log cannot show why.

• Dose kept in a separate notebook. Without chemical added beside cycles, a low inhibitor reading cannot be traced to feed or to blowdown. Lubinpla keeps each reading as measurement history in the project's conditions and reads it over the same period as the product usage records, the dose and application date.

• Estimates logged as meter readings. The US DOE data collection form asks whether makeup and blowdown are "Metered / Estimated / Not Tracked" (US DOE, 2018); keep that distinction on each line.

• Controller value copied without a check. A fouled probe gives a steady but wrong number; log your handheld value beside it.

• Results not checked when they look wrong. US military guidance says that when a result is out of limits, the sampling and testing must be checked to confirm the result is valid (UFM 3-230-13, 2026).

How often should each cooling tower reading be taken?

How often each cooling tower reading should be taken depends on tower size and the site program, and the two published schedules below differ because they serve different purposes. The table decides a starting frequency per reading.

The key row is tower pH and conductivity: both schedules put it at least weekly. US military guidance gives once per week for towers under 25 tons, twice per week for 25 to 100 tons, and daily above 100 tons (UFM 3-230-13, 2026). UK guidance calls its frequencies typical and "not universally applicable" (HSE HSG274 Part 1, 2024). Calculated values follow the readings they come from: UK guidance lists concentration factor and calcium balance as monthly calculated checks on the cooling water (HSE HSG274 Part 1, 2024). Sample more often when something changes, such as a new makeup source, a controller repair or a reading outside the program's range, and return to the routine schedule once the readings agree again. Keep the readings in one history per site, so trends show across visits; the practice is the same one used to keep a concentration and pH history per site for other fluids.