As server compute density surges from the 5-8 kW per cabinet typical of traditional general-purpose computing to 30-130 kW per cabinet in High-Performance Computing (HPC), AI training, and edge inference scenarios, the drastic increase in Thermal Design Power (TDP) has pushed air cooling to its physical limits regarding chip junction temperature control and system energy efficiency. This has accelerated the application of liquid cooling technologies for thermal management across multiple domains. According to data from the China Academy of Information and Communications Technology (CAICT), China's liquid cooling market for intelligent computing centers reached 18.4 billion yuan in 2024, a year-on-year increase of 66.1%, and is projected to climb further to 130 billion yuan by 2029.
 
Against this background, eco-friendly cooling media represented by PG25 (25% propylene glycol aqueous solution) are gradually replacing traditional pure water as the preferred working fluid for cold plate and immersion liquid cooling systems, owing to their superior thermal conductivity, low viscosity and anti-freeze capabilities.   

In diverse liquid cooling equipment—ranging from CDU side loops and immersion tanks to energy storage cold plates and vehicular cooling modules—thermodynamic stability relies highly on the phase continuity and volumetric stability of the coolant.   When the loop experiences coolant evaporation (ethylene glycol solution evaporation rate >3% per year), micro-leaks (<0.1mL/min), or air bubble ingress, even minute liquid level fluctuations in the reservoir or cold plate channels (<5% volume change) can directly trigger centrifugal pump cavitation, leading to a sharp drop in flow rate and the formation of local hot spots; furthermore, a continuous decline in liquid level is often a precursor to failures such as quick-connect seal degradation, cold plate micro-cracks or pipeline corrosion.  Therefore, deploying high-precision liquid level sensors (pressure rating >6bar, high media compatibility) in CDU reservoirs, Manifold distribution units, immersion tanks and energy storage loops has become a critical technical requirement for preventing cavitation, maintaining system air tightness (pressure differential <0.05bar), and achieving early leak warnings. Application scenarios are extending from data centers to global liquid cooling systems including new energy storage, communication base stations, industrial variable frequency drives and intelligent transportation.

Modern data center liquid cooling systems typically employ a "primary side – secondary side" dual-loop architecture: the primary side connects outdoor cold sources to the Coolant Distribution Unit (CDU), while the secondary side is responsible for delivering the coolant to server cold plates or immersion chambers.At critical nodes within these two loops—particularly at piping branches implemented via tee fittings—the deployment of highly reliable liquid level monitoring devices is essential to achieve the following functions:

1）Leakage Warning: Real-time monitoring of the presence status of the PG25 solution within the piping; immediate alarm triggering upon detection of a leak.

2）Flow Verification: Confirmation of normal coolant circulation to prevent dry-run risks caused by pump failures or air locks.

3）System Protection: Interlocking with the CDU to automatically cut off circulation pumps when liquid levels are abnormal, protecting expensive GPU/CPU equipment.

Traditional mechanical float switches are prone to mechanical fatigue and contact adhesion in data center environments characterized by high pressure and high-frequency vibration. Meanwhile, capacitive sensors are limited by variations in the coolant's dielectric constant and interference from liquid hang-up. Consequently, with solid-state design with no moving parts, millisecond-level response times and superior media compatibility, optical liquid level switches have emerged as the preferred technological route for this application scenario.

To cope with the stringent requirements for liquid cooling systems in data centers, SST Sensing has launched LLHP series high performance liquid level sensors to provide the leading monitoring solution in the industry.

Advantages of LLHP liquid level sensors:

1） Pressure & Temperature Performance

The LLHP series offers a pressure rating of 25 bar (363 psi) and covers operating temperatures from -25°C to +80°C (Standard) or -40°C to +125°C (Extended). The combination of a 316 stainless steel housing and a polysulfone optical probe ensures long-term compatibility with PG25 coolant.

2） Electrical Interfaces & Installation

Supporting NPN, PNP and Push-Pull output logic, the device features a supply voltage range of 4.5–45 VDC. With an output current of up to 800 mA, it can directly drive solenoid valves or PLCs. Available thread specifications include 3/8" BSP, 1/2" BSP, 1/2" NPT and 3/4"-16 UNJF, suitable for side-mount installation on DN100/DN60 pipelines.

3）High-Reliable Solid-State Design

Utilizing infrared photoelectric detection principles, the switch features a solid-state design with no moving parts. It has an IP67 rating and millisecond-level response time.

To facilitate rapid decision-making by engineering teams, the following section presents a horizontal comparison of core metrics tailored for data center liquid cooling scenarios, highlighting the application boundaries and performance differences of various technological approaches.

As the comparison illustrates, SST’s LLHP series demonstrates distinct advantages in both pressure resistance and output drive capability. The 800 mA direct load drive capacity allows the sensor to directly control shut-off valves or alarm devices, simplifying system wiring; meanwhile, the 25 bar pressure rating ensures its safe deployment in high-pressure primary side pipelines.

For more technical details and project implementation, please contact Apollosense Ltd.