Beyond the 25% Benchmark: A Practical Guide for Maintaining PG25 Performance in TCS Loops

11/04/2026

By Dakota Bunn, PhD

Propylene glycol (PG)-based coolants are widely used in direct-to-chip liquid cooling applications for their low toxicity, compatibility with sensitive equipment, and their ability to inhibit microbial growth [SOURCE1]. However, one of the key tradeoffs when using PG-based coolants is the inherent reduction in heat transfer when compared to treated water solutions. This is driven by the lower specific heat and thermal conductivity of propylene glycol/water mixtures compared to water, which can limit overall cooling efficiency [SOURCE2]. As a result, many data center operators are increasingly asking a practical question: “What is the minimum PG concentration required to effectively control microbial growth while minimizing the thermal penalty?”

From a practical design perspective, this seems like a simple question. In practice, it’s far more nuanced, as microbial behavior in PG-based cooling systems is not governed by a single concentration threshold.

At concentrations of roughly 25%, PG is deemed biostatic, meaning it can inhibit or slow the growth and development of biological organisms. This is because ~25% PG solutions (“PG25”) effectively pull water out of microbial cells, limiting their growth and metabolic activity [SOURCE3]. However, PG is biodegradable and can serve as a microbial food source under certain conditions [SOURCE4], which is why understanding the various microbial, chemical and environmental factors that can impact PG-based microbial control is important for data center operators.

It is worth noting that technology cooling systems (TCS) are not fully closed sterile environments. After typical commissioning procedures, residual fluids and environmental access points can remain, allowing introduction of microorganisms into the loop. Frequent blade changeouts can additionally allow introduction of microbes. In practical terms, this means every system starts from a non-sterile baseline where new diverse organisms are continually being introduced [SOURCE5].

The increased microorganism biodiversity is extremely important because the same PG concentration is not universally inhibitory, and different bacteria are able to metabolize PG as a food source at different concentrations [SOURCE6, 7, 8]. Therefore, once one organism begins metabolizing PG, it starts to decrease the PG concentration in the system, generates byproducts and nutrients, and alters the local chemistry in the loop, allowing additional microorganisms to start to grow and perform metabolic processes, creating a snowball effect. Subsequently, as the PG concentration in the TCS loop decreases, the risk of microbial growth and loss of control in the system increases exponentially (Figure 1).

Graph illustrating the relationship between propylene glycol concentration and biostatic performance in TCS coolant loops. Propylene glycol maintains biostatic properties at concentrations of 25% or higher. When concentration level falls below 25%, these properties are compromised, increasing the potential for microbial growth.

This situation is further complicated, because cooling loops are not homogeneous and are constantly in flux. Low-flow regions, and dead legs can allow biofilms to establish and persist in protected niches, even when bulk fluid looks controlled. These biofilms not only result in decreased heat transfer, but they allow microorganisms to produce organic intermediates and acids that can shift system chemistry over time and potentially lead to microbially induced corrosion [SOURCE9]. Ultimately, biofilm development results in decreased system performance and elevates the likelihood of unplanned downtime.

While a PG concentration of ~25% reduces risk, effective system management requires a broader, proactive approach. Here are three steps system engineers can take to support stable, reliable performance without added stress:

Design clean
Good system design up front can prevent diagnosis and remediation headaches throughout the TCS life cycle. Aim to minimize stagnant zones and dead legs. Designs that maintain proper flow throughout the system can help limit localized areas that are ideal for microbial growth.
Continuous monitoring of PG25 health parameters
PG25 coolant parameters often begin to shift before operational issues arise. Monitoring key chemical indicators in real time, such as glycol concentration, conductivity, pH, temperature and turbidity, and can help operators catch and correct out-of-spec conditions before they escalate.
Routine PG25 sampling and lab analysis
Incorporating routine microbiological checks allows for early detection of biological activity. Consider ATP or culture-based methods on a quarterly basis and lab-based chemistry analysis focused on critical contaminants, key corrosion indicators, and corrosion inhibitor levels.

Overall, the widely cited 25% PG threshold should be viewed as a practical guideline rather than a definitive boundary. Because TCS loops are dynamic, ongoing attention to flow dynamics, chemistry, and biological activity is a must. By combining smart design with disciplined monitoring, engineers can optimize both operational efficiency and cooling capacity.