Colour tells you who dyed it, and nothing else

Both common glycols are colourless

Propylene glycol is recorded as a colourless viscous liquid; the ILO-WHO International Chemical Safety Card aggregated by PubChem describes it as a “COLOURLESS ODOURLESS HYGROSCOPIC VISCOUS LIQUID.” Ethylene glycol is recorded the same way — “Clear, colorless, syrupy, liquid,” with a note that it is a solid below 9 °F. Neither substance is inherently pink, blue, green or yellow.

So any colour in a coolant drum was put there. Dow states that DOWFROST HD is “Dyed fluorescent yellow-green for leak detection purposes.” That is a purpose, attached to one named product, chosen by one formulator. It is not an industry code, and there is no cross-manufacturer convention that lets you read a chemistry, a concentration, an inhibitor type or a service life off a colour. Two fluids that must never be mixed can be the same shade, and one product can ship dyed and undyed.

Identify the fluid from the label, not the sight glass

When a technician reports “the loop is green,” record it as an observation about appearance and leave the identity field unresolved. The identification that matters comes from the fill record, the delivery documents and, where those are missing, an investigation the equipment owner and the fluid supplier define together. Colour is useful for exactly the job Dow assigns it — spotting that something has leaked — and it is useful for noticing that this drum does not look like the last one, which is a reason to ask a question rather than an answer to one.

Where a site standardises on a colour internally, write the convention down, date it, and keep it with the fill records. An undocumented local convention becomes a hazard the moment a contractor who has never seen it makes a top-up decision from it.

Storage is governed by the delivered liquid, not the name

Published closed-cup flash points for the neat glycols are high: propylene glycol is reported at 210 °F (99 °C) closed cup, and ethylene glycol at 232 °F (111 °C) closed cup. OSHA defines a flammable liquid at 29 CFR 1910.106(a)(19) as one with a flash point at or below 199.4 °F (93 °C), sorted into four categories, and adds at (a)(19)(v) that a liquid with a flash point above 199.4 °F which is heated for use to within 30 °F (16.7 °C) of that flash point must be handled as a Category 4 flammable liquid. A hot glycol loop can therefore change the handling question without changing the chemical.

Two details decide whether any of that applies to what you actually received. First, 1910.106(a)(14)(iii) requires that a mixture of compounds with different volatilities be flash-tested “in the form it is shipped” — a diluted or contaminated glycol is not the neat substance. Second, the published figures themselves are method-dependent: the same aggregated record gives propylene glycol 210 °F closed cup, 104 °C by Pensky-Martens closed cup and 225 °F open cup. Classification of a specific delivered liquid is the employer’s determination, made on the exact product’s safety data sheet and the site procedure, not on a value copied from a compound record.

Shelf life is a condition statement, not a date

The aggregated storage guidance for propylene glycol is that it is hygroscopic and “should be stored in a well-closed container, protected from light, in a cool, dry place,” and that at cool temperatures in a well-closed container it is stable. For ethylene glycol the equivalent entry is to store in tightly closed containers in a cool, well-ventilated area away from oxidising agents, at ambient temperature. Hygroscopic matters commercially: an open or badly sealed container drifts in water content, and water content is what the concentration on your purchase order was specifying.

An inhibited fluid adds a second clock, because the inhibitor package is the part with a service life. Ask the manufacturer for storage conditions and any stated shelf life for the exact formulation, put the container condition and the lot identifier into the receiving record, and keep the certificate of analysis with them. Alliance sends a Certificate of Analysis when a customer asks, at no charge — put the request in the purchase order so it arrives with the paperwork rather than months later.

• What does the fill record say the loop contains, independent of what it looks like?
• What storage conditions and shelf life does the manufacturer state for this exact formulation?
• Has the delivered container been open, and is the water content still within the specification?
• Who classifies this liquid for storage, and on which document?

Requirement → evidence → decision boundary

An editorial checklist for your review—not a table of product specifications.

Revision record

1.0 · September 12, 2026 — Source-based editorial edition. No original testing or product qualification is claimed.

For engineering and purchasing teams defining closed-loop fluid requirements. This collection does not size a ground loop, prescribe a concentration, or approve a substitution.