# Before the Coolant Flows: How to Keep PG25 Ready for the Rack Canonical: https://www.reliabilityengine.com/insights/before-pg25-reaches-rack-coolant-storage-logistics Markdown: https://www.reliabilityengine.com/insights/before-pg25-reaches-rack-coolant-storage-logistics/markdown Site: Reliability Engine Author: Brenda Pak Author profile: https://www.reliabilityengine.com/authors/brenda-pak Published: 2026-07-29T09:00:00.000Z Updated: 2026-08-12T06:53:45Z Categories: Liquid Cooling, Cooling Systems, Data Center Design, Operations The level alert arrives on a Friday afternoon. The site has spare coolant, but the container was opened months ago and nobody can confirm what touched it. The loop needs fluid. The shelf has a question mark. Avoiding that moment begins long before a technician lifts a hose. PG25 can spend months waiting on a pallet, in a storeroom, or at a remote module. Its seal, lot, storage conditions, and route to the loop matter during every one of those quiet days. Treat premixed coolant like a controlled spare part, not a communal jug. The fluid and its history should reach the rack together. The first job of coolant logistics is not moving fluid. It is removing doubt. ## The 30-second version - Buy and store the exact approved product. PG25 is a concentration shorthand, not proof that two formulations are interchangeable. - Protect the seal and the history. Lot, dates, storage conditions, destination, and seal status should travel with the package. - Keep the transfer path dedicated and clean. A clean drum can still meet a contaminated hose, pump, or funnel. - Forecast the next need. Remote modules turn a small top-up into a route, timing, and inventory decision. ## PG25 on the label is not the whole specification PG25 usually points to a coolant with roughly one-quarter propylene glycol. It does not identify the water quality, inhibitor package, approved materials, storage limits, shelf life, or test methods. Those details belong to the named product. Castrol ON Direct Liquid Cooling Fluid PG 25, for example, is supplied pre-diluted and ready to use. Its operating guide says to keep the container sealed until use, protect it from sunlight, and store it below 30 C. The same guide gives that product a 36-month shelf life and calls for testing before use after that period. That guidance belongs to that product. Another PG25 may have different instructions. The transferable rule is to preserve the identity and follow the approved fluid plan. ![Sealed coolant drums and pails on secondary containment beside a dedicated transfer cart with a pump, capped hose, absorbent pads, and a sealed sample bottle.](https://cdn.sanity.io/images/7c899jfp/production/a132af33d0cbd5cfc33a61b96326c119725bc0e1-1200x630.png?w=1200&fit=max&auto=format) ## One fluid, two operating worlds On a large campus, sealed packages can wait in one controlled facilities area. Teams can record each lot, issue it against a work order, and move it to the CDU only when needed. A remote modular site changes the geometry. Space is tight, service visits are less frequent, and the next approved package may be hours away. A small planned top-up can become a scheduling problem before it becomes a cooling problem. ![A technician checking a compact coolant reserve compartment at a remote modular data center with a service truck in the distance.](https://cdn.sanity.io/images/7c899jfp/production/d2d866da105592e054bd373cea92033e36e81b06-1600x900.png?w=1200&fit=max&auto=format) The answer is not simply more inventory. Excess stock occupies scarce space and continues aging. The better answer is enough matching fluid, in the right package, on a route the operations team can actually support. ## The ideal storage story is boring Nothing leaks, nothing is borrowed, and nobody has to guess. Five habits make that possible: - Keep the original identity. Record the product, lot, received date, expiry or retest date, and approved destination. - Protect the package. Follow the supplier limits for sunlight, temperature, orientation, containment, and seal status. - Use dedicated transfer equipment. Keep hoses, pumps, caps, and other wetted tools clean and assigned to the approved fluid. - Separate sealed and opened stock. An opened remainder is no longer equivalent to untouched inventory. Castrol advises checking stored leftovers before use, including appearance, pH, and glycol concentration. - Retain the evidence. A representative sample and a complete maintenance record are far more useful than a memory that the fluid looked clear. ## Ten liters can change the whole loop Consider a 100 L loop at 25 percent propylene glycol. It contains 25 L of glycol. Add 10 L of deionized water and the total volume becomes 110 L while the glycol stays at 25 L. The new concentration is about 22.7 percent. That is mass balance, not a recommendation to top up with water. The approved action depends on the system and fluid supplier. The example shows why a small maintenance shortcut can quietly change the formulation. Open Compute Project guidance lists 24.5 to 29.5 volume percent propylene glycol for the PG25 fluid it describes. Castrol ON publishes 24.5 to 29 volume percent for its product. Use the applicable product and project limits, not the letters PG25 as a substitute for measurement. ## Turn inventory into a fluid forecast A stock count answers what is on the shelf today. A fluid forecast answers what each site is likely to need before the next service visit. Useful inputs include makeup history, planned maintenance, unresolved losses, service cadence, package size, shelf life, and a defined reserve. Across a fleet, that can turn a surprise truck roll into a planned stop. Change the fleet assumptions below. The result is an illustration, not a manufacturer target, but it makes the logistics tradeoff visible. ## Give every liter a timestamp The useful record is not only how much fluid was purchased. It is where the fluid went and what the loop did after it arrived. - Package record: product, lot, volume, seal state, location, received date, and retest or expiry date. - Transfer record: who moved it, which loop received it, what equipment was used, and how much was added. - Loop response: level, pressure, flow, concentration, pH, and the approved checks before and after the event. - Exception record: leak, spill, unknown fluid, open remainder, off-spec result, or any break in custody. When inventory events and telemetry share one timeline, a declining level can trigger the next delivery before the site reaches a forced choice. A concentration change after a top-up already has a cause attached to it. ## Ready means five questions have answers - Which approved fluid belongs in this loop? - Where is the nearest sealed package from the same approved supply plan? - Will it remain inside its specified storage or retest window when the site needs it? - How much will the site probably need before the next planned visit? - What evidence will confirm that the transfer left the loop inside its approved condition? Reliability Engine connects makeup events, fluid analysis, loop telemetry, and service history. That gives teams an early view of changing demand and a clean record of what entered the system. The best emergency container is sealed, approved, traceable, and already on the right route. ## References 1. Castrol ON Direct Liquid Cooling Fluid PG 25 Operating Guide 1. Castrol ON Direct Liquid Cooling Fluid PG 25 Product Information 1. Open Compute Project: Guidelines for Using Propylene Glycol-Based Heat Transfer Fluids 1. Dow: DOWFROST LC 25 Heat Transfer Fluid