PG25 serves as a prevalent heat transfer fluid (coolant) within closed-loop liquid cooling architectures. It is formulated as an aqueous solution comprising 25% propylene glycol (PG) and water, supplemented with specific additive packages including corrosion inhibitors, pH buffering agents, and dispersants. This chemical formulation is meticulously engineered to ensure the long-term thermodynamic and chemical stability of PG25 when operating within thermal dissipation systems comprised of multi-metal wetted materials, such as copper and aluminum components.
A typical PG25 coolant should exhibit the following properties:
| Appearance | Clear and particulate free |
| Propylene Glycol | 24.5 ~ 29.5 vol% |
| pH | 8.0 ~ 10.5 |
| Freeze Point | -13 ~ -9℃ |
| Copper | <2 ppm |
| Iron | <2 ppm |
| Total Hardness | <20 ppm |
| Chloride | <5 ppm |
| Sulfate | <10 ppm |
| Sulfate | <10 ppm |
Cooling Technologies and Coolants
1. Direct Liquid Cooling (DLC): Propylene glycol (PG) has lower toxicity and improved oxidative stability. Therefore, propylene glycol/water mixtures (typically PG25) are commonly used as the working coolant in DLC systems.
2. Immersion Cooling: In immersion cooling, servers are directly submerged in a dielectric coolant, enabling efficient heat transfer through direct contact. Fluorinated fluids offer excellent thermal conductivity and stability, but carry environmental concerns associated with PFAS. Conversely, synthetic and silicone oils have a lower environmental impact, but exhibit higher viscosity and lower thermal conductivity.
| Cooling Technology | Coolant | Advantages | Disadvantages |
| DLC (Direct Liquid Cooling) | Ethylene Glycol (EG) | ower cost; lower freezing point
| Higher toxicity; relatively less environmentally friendly |
| Propylene Glycol (PG) | Lower toxicity; lower environmental impact | Higher cost than EG |
| Immersion Cooling | Fluorinated fluids | Excellent thermal conductivity; high stability | PFAS-related substances; expensive |
| Synthetic oil / silicone oil | Lower environmental impact | Higher viscosity |
Challenges and Solutions
Liquid cooling systems offer high heat dissipation efficiency, but they also face challenges such as high costs, leakage risks, and maintenance complexity. The following are common challenges and their corresponding solutions:
| Challenge | Impact | Solutions |
| Corrosion | Pipeline failure | 1. Add corrosion inhibitors. |
| 2. Keep coolant pH within a neutral-to-slightly-alkaline range to avoid corrosion of the base metal. |
| 3. Control metal ion contamination, including iron, chloride, and sulfate ions. |
| Microbial growth | Reduced cooling performance/failure Particulate contamination generation Pipeline clogging Piping wear and corrosion | 1. Maintain system cleanliness and replace coolant regularly. 2. Control nutrient levels. 3. Maintain sufficient coolant flow within the system. 4. Design and install appropriate filtration systems. |
| Sediment formation | 1. Control water hardness. 2. Keep coolant pH within a neutral-to-slightly-alkaline range to prevent precipitate formation (e.g., calcium sulfate). 3. Replace coolant regularly. 4. Install filtration systems. |
| Coolant degradation | Reduced cooling performance/system failure | 1. Monitor conductivity, pH, ion concentration, temperature, and microbial count. 2. Replace coolant regularly. |
| Coolant degradation | Short circuits/component damage | 1. If coolant contamination occurs, immediately clean and dry the affected area. 2. Regularly inspect system sealing integrity and pipeline condition. |
Particles and microbial contaminants in the coolant can negatively affect the operation of liquid-cooled servers in multiple ways. If not properly controlled, they may cause pipeline blockage and even create a risk of rupture. By implementing preventive and control measures such as regular filtration and coolant replacement, maintaining system cleanliness, and strengthening training and management, the impact of particulate contamination can be effectively reduced, ensuring efficient operation and long-term stability of liquid-cooled servers.
| Parameter | Specification |
| Total Suspended Solids (TSS) | Should be absent; Initial coolant: < 1 ppm; In-service loop: < 5 ppm |
| Total Dissolved Solids (TDS) | <1000 ppm |
| Electrical Conductivity | <1500 μS/cm @25℃ |
| Corrosion Byproducts / Ions | Cu： <0.2 ppm；Fe：<0.1 ppm |
| pH | 8.0 ~ 10.5 |
| Total Hardness | <30 ppm, Total Ca/Mg as CaCO3 |
| Turbidity | <5 NTU |
| Microbial Control | Initial coolant: < 1 CFU/mL; In-service loop: < 100 CFU/mL |
| Chloride | < 50 ppm for 304 Stainless Steel; < 500 ppm for 316 Stainless Steel |
Source：Guidelines for Using Water-Based Transfer Fluids in Single-Phase Cold Plate-Based Liquid-Cooled Racks, 2022, Open Compute Project
SGS provides coolant quality testing services
Through professional analytical testing technologies, contaminants, corrosive substances, and other potential issues in the coolant can be identified, helping prevent equipment failure and extend service life. In addition, regular quality testing helps improve operating efficiency, reduce maintenance costs, and ensure stable operation of server systems.
• Cleanliness analysis: in accordance with ISO 16232 / VDA 19 to identify and evaluate particulate contamination.
• Trace metal analysis: to quantify low-level contamination sources such as copper and iron.
• Trace ionic analysis: to detect corrosive species such as chloride, sulfide, sulfate, and nitrate.
• pH test: to evaluate coolant degradation and acidification over time.
• Total aerobic microbial Count: to monitor microbial proliferation in coolant systems.
• Total hardness test: to control coolant quality and reduce the risk of sediment formation and flow blockage.
• Other physical property tests: turbidity, water quality (conductivity, TDS, and TSS), etc.
SGS professional services help you monitor coolant quality and condition, ensuring stable operation of every server. Contact SGS today and let us help you enhance performance.
📧 semi.cs.tw@sgs.com ☎ +886 2 2299 3279 #7132-7134
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