AI Server Liquid Cooling System Reliability Testing & Validation How Do the Flow and Pressure Characteristics of Liquid Cooling Components Affect System Performance?
Author: Liquid Cooling Series
May 29, 2026Artificial Intelligence / Testing and certification of productsIn previous liquid cooling articles, we explored Burst Testing for evaluating liquid cooling components under extreme pressure conditions, as well as the impact of Water Hammer and Pressure Impulse on system reliability.
However, beyond pressure resistance and fatigue durability, the fluid flow characteristics within a liquid cooling system are equally critical factors that influence cooling performance and overall system stability.
This article, we will further introduce two commonly used parameters in AI server liquid cooling systems: the PQ Curve (Pressure–Flow Curve) and Cv Value (Flow Coefficient). From our insights, how these parameters are used to evaluate the flow rate, pressure drop, and flow resistance characteristics of liquid cooling components? As well as their importance in the design and optimization of liquid cooling systems.
What Are PQ Curves and Cv Values in AI Server Liquid Cooling Systems?
In high-power AI server liquid cooling systems, cooling capacity is not the only factor that determines overall thermal performance. The efficiency of coolant flow throughout the system also plays a critical role in heat dissipation. Among the various fluid performance indicators, the PQ Curve (Pressure–Flow Curve) and Cv Value (Flow Coefficient) are two commonly used metrics for evaluating the flow characteristics of liquid cooling components.
Why Perform PQ Curve Testing?
PQ curve testing is primarily used to evaluate the pressure drop characteristics of liquid cooling components under different flow conditions, helping engineers understand the flow resistance performance of a product. The test results enable engineers to optimize pump selection, flow allocation, and system flow path design, reducing the risk of localized hotspots, uneven heat dissipation, and excessive pump loading.
To Verify Optimal Flow Rate of System
PQ curve testing helps identify the optimal operating point between the pump and the cooling system. Insufficient flow may lead to inadequate heat dissipation and equipment overheating, while excessive flow can increase pump power consumption, system pressure, and even the risk of coolant leakage.
To Optimize Cooling Efficiency
PQ curve testing determines the optimal flow condition that balances cooling performance and energy efficiency, helping reduce unnecessary power consumption while maintaining effective thermal management.
To Ensure Balanced Coolant Flow
In large-scale liquid cooling systems, coolant must be evenly distributed across multiple devices. PQ curve testing verifies that each branch receives sufficient and balanced flow, helping prevent localized hot spots caused by inadequate cooling.
To Validate Reliable Performance in Low-Temperature Environments
Liquid cooling systems commonly use ethylene glycol-based coolants. As temperatures decrease, the coolant flows less easily, resulting in higher flow resistance and increased system pressure. PQ curve testing verifies that the liquid cooling system maintains stable flow performance and structural reliability during low-temperature operation, transportation, or deployment in cold environments.
Why Are PQ Curves and Cv Values So Important?
As AI computing power continues to grow, server power consumption is reaching unprecedented levels. As a result, liquid cooling is no longer an optional technology—it has become a key solution for next-generation AI servers.
One of the biggest challenges in designing a liquid cooling system is ensuring that coolant flows accurately and consistently to every heat-generating component.
This is where Cv values and PQ curves play a critical role. Together, they provide the foundation for understanding and predicting how coolant will flow throughout the system.
Without these flow characteristics, it becomes difficult to accurately predict real-world system performance. Insufficient flow can reduce cooling efficiency and increase the risk of overheating, while excessive flow may lead to unnecessary pump power consumption, higher system pressure, and even an increased risk of coolant leakage.
One of the biggest challenges in designing a liquid cooling system is ensuring that coolant flows accurately and consistently to every heat-generating component.
This is where Cv values and PQ curves play a critical role. Together, they provide the foundation for understanding and predicting how coolant will flow throughout the system.
Without these flow characteristics, it becomes difficult to accurately predict real-world system performance. Insufficient flow can reduce cooling efficiency and increase the risk of overheating, while excessive flow may lead to unnecessary pump power consumption, higher system pressure, and even an increased risk of coolant leakage.
Understanding the Difference between Cv Value and PQ Curve
Take a valve as an example, the Cv value and PQ curve essentially describe the same flow behavior, but from different perspectives. The Cv value reflects the flow capacity of a valve, while the PQ curve illustrates the relationship between flow rate and pressure drop under different operating conditions.
A simple analogy is to compare them to a car. The Cv value is like the engine displacement, representing the vehicle’s fundamental performance capability. The PQ curve, on the other hand, is similar to an acceleration or power output curve, showing how the vehicle performs at different speeds or load conditions.
In other words, the Cv value describes the intrinsic flow capacity of a component, whereas the PQ curve reflects how its flow and pressure characteristics change during actual system operation. Understanding both parameters helps engineers evaluate component performance and optimize liquid cooling system design.
A simple analogy is to compare them to a car. The Cv value is like the engine displacement, representing the vehicle’s fundamental performance capability. The PQ curve, on the other hand, is similar to an acceleration or power output curve, showing how the vehicle performs at different speeds or load conditions.
In other words, the Cv value describes the intrinsic flow capacity of a component, whereas the PQ curve reflects how its flow and pressure characteristics change during actual system operation. Understanding both parameters helps engineers evaluate component performance and optimize liquid cooling system design.
- Cv Value (Flow Coefficient): The Cv value represents a valve’s flow capacity and is a fixed characteristic of the valve. Under a specified pressure differential and fully open condition, it indicates the volume of fluid that can pass through the valve per minute. In simple terms, a higher Cv value means the fluid can flow more easily through the valve, resulting in lower flow resistance.
- PQ Curve (Pressure–Flow Curve): The PQ curve is a dynamic representation of how pressure changes with varying flow rates within a system. It helps illustrate the relationship between flow and pressure during actual operation, providing insight into overall system performance and operating characteristics.
Applications: Critical Components in Liquid Cooling of Secondary Loops
PQ Curve and Cv Value testing are primarily applied to the secondary loop of AI server liquid cooling systems. Since each component is designed differently, its flow resistance and pressure drop performance will also vary. PQ curve testing helps quantify these differences and evaluate real-world performance.
PQ Curve and Cv Value testing are commonly performed on the following liquid cooling components:
PQ Curve and Cv Value testing are commonly performed on the following liquid cooling components:
- Cold Plates
- In-Rack Manifolds
- Universal Quick Disconnects (UQD / UQDB)
- Various Cooling Valves (e.g., Reducing Valves, Check Valves, and Flow Control Valves)
Compliance with International Standards for Reliable and Consistent Testing
PQ Curve and Cv Value testing are conducted in accordance with recognized international standards and industry specifications, ensuring consistency, repeatability, and credibility of test results.
- OCP (Open Compute Project) : OCP defines flow rate and pressure drop requirements for liquid cooling components used in data center environments, including manifolds, quick disconnects (UQDs), and cold plates.
- These requirements help ensure stable operation and consistent cooling performance throughout the liquid cooling system.
- ISO 18869: Manufacturers commonly use ISO 18869 to evaluate the flow performance, pressure characteristics, and durability of liquid cooling quick disconnects (UQDs), helping ensure long-term reliability in liquid cooling applications.
- ISO 7241:This standard specifies dimensional and performance requirements for quick-action couplings, helping ensure compatibility, interchangeability, and operational safety among products from different manufacturers.
- ASHRAE TC 9.9:ASHRAE TC 9.9 provides guidance for liquid-cooled data center environments, including recommendations for coolant cleanliness and filtration. These practices help minimize blockage risks and maintain stable flow performance and cooling efficiency over time.
What Factors Influence PQ Curve Test Results?
To obtain PQ data that accurately reflects actual data center operating environments, test conditions must be carefully designed and controlled. Factors such as fluid properties, temperature stability, and test system configuration play a critical role in ensuring reliable and repeatable measurement results.
- Fluid Configuration
A mixture of 25% propylene glycol and 75% water (PG25) is commonly used during testing to balance freeze protection, flow characteristics, and heat transfer performance. In some applications, deionized water or pure water may also be used as the test medium. - Temperature Control
Fluid temperature is maintained through a dedicated temperature control system to ensure consistent test conditions. Standard testing is typically conducted at 25°C, while 45°C may be used for warm-water cooling applications to better simulate actual operating environments. - Test System Configuration
The test setup should be configured according to applicable standards or customer-specific requirements. Key considerations include piping length, sensor placement, and measurement point design to ensure that flow behavior and test conditions closely represent real-world system operation.
As AI servers and high-power liquid cooling applications continue to evolve, DEKRA iST provides comprehensive testing and validation services for liquid cooling performance and reliability. Our capabilities cover critical evaluations such as PQ Curve testing, Cv Value measurement, and other key validation programs.
Our Services:
- Burst Pressure Testing
- Water Hammer and Pressure Impulse Testing
- Flow Performance Characterization of Liquid Cooling Components
- (including UQDs and Cold Plates)
- 100 kW Thermal Performance Testing
- Hydrostatic Pressure Testing