Coolant Sump Checklist: Concentration, pH and Bacterial Growth, Shift by Shift

Good coolant management comes down to checking the same few things every shift, writing them down, and acting on a short list of warning signs. The guidance used here, OSHA's Metalworking Fluids best practices manual and NIOSH Publication 98-116, both US federal documents, does not prescribe a fixed weekly timetable or a universal pH value: concentration and pH must stay within the range your fluid supplier recommends, and the size of the system sets how often you test. What follows is that guidance organised by timing.

Before the first test: put the paperwork in place

A sump that is "checked when someone remembers" will not be checked consistently. OSHA's manual makes a written standard operating procedure for fluid testing a core element of any fluid management programme and says it should state:

1. where and when samples are collected;
2. how samples are treated after collection;
3. which tests are performed;
4. a specific protocol for each test; and
5. the name of the person responsible for performing and recording the results.

The same section asks for a named person with overall responsibility for the system, a designated person who controls and logs every addition (concentrate, biocide, additives, water), and a data tracking system laid out so that trends can be spotted early. Operators should be trained to look at the fluid and report anything unusual, and their observations compared with lab data and additions. In the self-assessment checklist (Appendix 7), routine monitoring of water-miscible systems for concentration, pH, microbial levels, tramp oil and suspended particulates is rated critical.

Every shift: the five-minute sump check

• Level. Check it at the start of the shift. A level 30% below the full mark signals fluid loss or water evaporation, which concentrates the chemicals. Check the concentration: if too strong, add water; if correct, the loss was dragout, so add fluid at the right dilution. Confirm the concentration afterwards.
• Colour. Many synthetics are clear, semisynthetics transparent to milky, soluble oils milky white with no free oil layer. Grey or black often means bacteria; a yellow or brown tint suggests tramp oil; fading dye may mean the fluid is ageing.
• Smell. A strong "locker room" odour likely means biological growth: treat with biocide and evaluate, and if necessary discard, clean the sump and refill. Masking the smell does not remove microbes that can be carried into the mist.
• Surface. Floating chips, swarf or mould are not normal, skim or pump them off. For water-diluted fluids, swish the oil layer aside: if the surface is fully covered and closes back over within 5 to 8 seconds, there is too much tramp oil.
• Foam and machine condition. Heavy foam, dirty machines or trenches point to concentration, contamination, filtration or housekeeping problems.
• People. Skin or chest complaints from operators are listed by OSHA as a fluid warning sign in their own right.

What a useful log entry contains

Neither document prescribes a form, but taken together they name the fields worth logging for each machine or central system: date and shift, sump level, concentration, pH, tramp oil (visual), appearance and odour, any foam or floating matter, every addition (what, how much, by whom), and operator observations. OSHA's manual says the data should be tabulated so relationships and trends can be spotted, and that the system should give feedback early enough for corrective action before problems develop. Its self-assessment checklist then asks two critical questions that a log alone does not answer: are the collected data reviewed on a regular basis, and is corrective action taken in a timely manner when the data call for it?

On a regular schedule: concentration and pH

The documents in the base give no generic target figure: the window comes from the product data for your fluid. What they do explain is why drifting out of it matters. According to OSHA's manual, running too lean can shorten tool life, invite bacteria and fungi, and cause corrosion; running too rich can cause dermatitis, foaming and heavy residues. High alkalinity is also named among the fluid properties behind skin irritation.

When things change: bacteria, fungi and water quality

NIOSH says there are currently insufficient health data to set a numeric limit for bacteria or fungi in metalworking fluids, but warns that their hazard must not be minimised: water-based fluids are excellent nutrient sources, sulfate-reducing bacteria can produce hydrogen sulfide, and microbial products are suspected in some respiratory illnesses. The controls described in the base:

• Use the purest makeup water you can. Hardness minerals (calcium, magnesium) form scale and soap scum; sulfates feed bacteria that give a "rotten egg" smell; sulfates and chlorides encourage rust. OSHA's manual cites a zeolite softener followed by reverse osmosis, or deionisation.
• Dose biocide sparingly and legally. Biocides should be registered by the US EPA for metalworking fluid use and used under their registration conditions; overdosing can irritate or sensitise skin and airways. NIOSH treats biocides as preventive, not as a cure for overgrowth.
• Starve the microbes of tramp oil. Leaking hydraulic and slideway oil feeds bacteria and seals the surface. Apply only the way lube needed, run lubricators only with the machine, and skim during quiet time when pumps are off.
• Avoid long stagnation. The manual warns against idle periods over 8 to 10 hours in contaminated or under-dosed fluid.
• Keep rubbish out. Cups, food, cigarette ends and floor wash water must never go into sumps or return trenches; chip beds should be cleared regularly.

End of life: drain, clean and recharge properly

OSHA's manual explains why simply pumping out a sour sump fails: residual fluid carries residual bacteria, which then feast on the fresh charge. When additions stop working or microbial growth is unmanageable, it calls for a full drain, clean and recharge (Appendix 6):

1. Circulate a cleaner containing a compatible biocide, then pump out and dispose of the old fluid; drain the delivery lines if possible.
2. Remove chips and swarf from flumes, trenches, lines and sumps, taking off covers and guards to reach hidden areas.
3. Fill with fresh water and sump cleaner, circulate, and spray contaminated surfaces at high pressure; scrape off what will not shift.
4. Rinse with fresh water as many times as needed to remove the cleaner.
5. Change all filters and wipe out the filter canister.
6. Refill immediately with fresh fluid, run all axes through their full travel, and catch the first fluid from the coolant lines until it runs clean.
7. Leave a note on the machine recording when the fluid was changed.

Not sure whether a symptom calls for treatment or a full clean-out? You can describe it to the metalworking fluids knowledge base, for example, "The level in our sump keeps dropping between shifts, is that something to worry about?", and get the relevant OSHA or NIOSH passage quoted back.