Quick answer
Maintaining a CDU secondary loop means regularly checking glycol concentration, pH, conductivity, inhibitor level, particulates and microbiological growth, keeping filters clean, and topping up only with the same approved fluid. Numeric limits vary by fluid and hardware vendor and are not published here. Use your supplier's datasheet and your CDU manual.
Key takeaways
- Use the supplier's limits. We don't publish generic numbers.
- Concentration drifts with water top-ups.
- Filtration protects microchannel cold plates.
- Never mix fluid brands without supplier approval.
- Many suppliers offer sampling programs.
Monitoring checklist
| Parameter | Why | Tool |
|---|---|---|
| Glycol concentration | Freeze, microbial and corrosion protection | Refractometer |
| pH | Inhibitor health | Meter or strips |
| Conductivity | Contamination, inhibitor depletion | Meter |
| Inhibitor level | Corrosion protection | Lab analysis |
| Particulates | Cold-plate clogging | Filters, lab |
| Microbes | Biofilm | Lab / dip slides |
Routine tasks
- Log the readings on the supplier's schedule.
- Inspect and change the filters.
- Top up with the same pre-diluted fluid.
- Send samples for lab analysis as advised.
- Act on trends, not just on limits.
Common mistakes
- Topping up with water. It dilutes the inhibitors.
- Mixing brands.
- Ignoring slow conductivity rise.
Sampling best practice
- Sample from a point with active flow, after flushing the sample valve.
- Use clean, supplier-provided bottles.
- Label each sample with the loop, date and time.
- Ship promptly for lab analysis.
- Compare results with the baseline and trends.
Responding to out-of-range results
| Finding | Typical response (confirm with supplier) |
|---|---|
| Low concentration | Top up with fluid, not water; find the dilution source |
| Rising conductivity | Check for contamination or inhibitor depletion |
| Particulates | Inspect and replace filters; look for corrosion sources |
| Microbial growth | Supplier-guided treatment or fluid replacement |
What ASHRAE TC9.9 says to monitor
ASHRAE TC9.9 technical bulletins on TCS fluids say that, at a minimum, operators should measure and monitor pH, conductivity, glycol concentration and inhibitor reserve, and they also list turbidity, particulates and dissolved oxygen among the parameters to verify. For water-based TCS loops, ASHRAE's guidance calls for a pH of 8.0–9.5, required corrosion inhibitors and biocide, bacteria below 100 CFU/mL, and conductivity between 0.2 and 20 µmho/cm. For a glycol product, the supplier's datasheet limits govern.
| Parameter | FWS (facility side) | TCS (IT side) |
|---|---|---|
| pH | 7 to 9 | 8.0 to 9.5 |
| Corrosion inhibitor(s) | Required | Required |
| Biocide | Not specified | Required |
| Sulfide | <10 ppm | <1 ppm |
| Sulfate | <100 ppm | <10 ppm |
| Chloride | <50 ppm | <5 ppm |
| Bacteria | <1000 CFU/mL | <100 CFU/mL |
| Total hardness (as CaCO3) | <200 ppm | <20 ppm |
| Conductivity | Not specified | 0.2 to 20 µmho/cm |
| Total suspended solids | Not specified | <3 ppm |
| Residue after evaporation | <500 ppm | <50 ppm |
| Turbidity | <20 NTU | <20 NTU |
Values as reproduced in the ASHRAE TC9.9 water-cooled servers white paper (Table 1, citing ASHRAE 2014 Tables 5.3 and 6.2). These are water-quality guidelines for water-based loops. A glycol fluid such as PG25 should be managed to its supplier's own limits, which may differ.
Cleaning, flushing and commissioning
Many TCS problems start before the first fill. ASHRAE bulletin guidance is to build with clean, non-contaminating materials, then perform controlled flushing and filtration before the final fill, using flush water that meets the TCS water-quality guidelines or ASTM D1193 Type II, and to verify cleanliness through fluid testing. The OCP PG25 guideline likewise devotes sections to cleaning, flushing, commissioning and filtration. Debris left from fabrication, such as flux, oils or particles, can foul microchannel cold plates and consume inhibitors early.
Building a maintenance log
| Record | Why |
|---|---|
| Date, loop ID, sample point | Traceability |
| pH, conductivity, glycol % | Trend detection |
| Inhibitor reserve (lab) | Corrosion protection remaining |
| Filter differential pressure and changes | Contamination load |
| Top-up volume and fluid lot | Dilution and leak tracking |
| Actions taken | Audit trail for warranty discussions |
See who makes data-center and car coolants
Browse suppliers →Who does what
- Facilities team: FWS water treatment, the CDU primary side.
- CDU vendor or service partner: CDU operation, filters, alarms.
- Fluid supplier: sampling program, lab analysis, treatment advice (several suppliers list such services; see Ecolab).
- IT owner: hardware approvals and warranty conditions.
Related guidance
- What is PG25?
- Direct-to-chip vs immersion
- NVIDIA DSX Ready coolants
- DC coolant comparator
- Coolant suppliers directory
- Data-center liquid cooling hub
Warning signs between scheduled samples
- Filter differential pressure rising faster than usual
- Unexplained top-up volume (possible leak or weeping fitting)
- Discoloration or cloudiness in sight glasses or samples
- Rising conductivity trend in CDU telemetry
- Flow or pressure alarms on specific racks (possible cold-plate fouling)
Investigate trends early. Inhibitor depletion and fouling are much easier to correct before cold plates are affected.
Spare fluid on site
Keep sealed containers of the same pre-diluted fluid, from a known lot, for top-ups. Store them per the SDS, and note the shelf life on the label.
Bottom line
Monitor the ASHRAE minimum parameters, follow supplier limits, and act on trends.
ASHRAE water-quality guidance: facility side vs IT side
ASHRAE TC9.9 stresses applying the facility water (FWS) table to building loops and the much stricter technology cooling system (TCS) table to IT-side loops. Mixing them up is, according to its water-cooled servers white paper, a common and costly error. The figures below are for water-based loops. Glycol products follow their supplier's limits.
| Parameter | FWS (facility side) | TCS (IT side) |
|---|---|---|
| pH | 7 to 9 | 8.0 to 9.5 |
| Corrosion inhibitor(s) | Required | Required |
| Biocide | Not specified | Required |
| Sulfide | <10 ppm | <1 ppm |
| Sulfate | <100 ppm | <10 ppm |
| Chloride | <50 ppm | <5 ppm |
| Bacteria | <1000 CFU/mL | <100 CFU/mL |
| Total hardness (as CaCO3) | <200 ppm | <20 ppm |
| Conductivity | Not specified | 0.2 to 20 µmho/cm |
| Total suspended solids | Not specified | <3 ppm |
| Residue after evaporation | <500 ppm | <50 ppm |
| Turbidity | <20 NTU | <20 NTU |
Values as reproduced in the ASHRAE TC9.9 water-cooled servers white paper (Table 1, citing ASHRAE 2014 Tables 5.3 and 6.2). These are water-quality guidelines for water-based loops. A glycol fluid such as PG25 should be managed to its supplier's own limits, which may differ.
Short checklist
- Readings logged
- Filters checked
- Same fluid for top-ups
More questions answered
Can sampling be automated?
Some CDUs monitor conductivity and other parameters. Lab analysis still adds detail.
Frequently asked questions
How often should data center coolant be tested?
Per your fluid supplier's program.
Can I top up PG25 with water?
Not unless the supplier says so. Use the same pre-diluted fluid.
Related guides
Sources
- Dow: DOWFROST LC 25 product page
- Ecolab: coolants for direct liquid cooling
- ASHRAE TC9.9 white paper: Water-Cooled Servers: Common Designs, Components, and Processes
- ASHRAE TC9.9 Technical Bulletin (TCS cleaning and commissioning)
- ASHRAE TC9.9 Technical Bulletin (TCS fluid monitoring)
- OCP: Guidelines for Using Propylene Glycol-Based Heat Transfer Fluids in Liquid Cooled Racks (Rev 2.0, Sep 2026)