Key points
- Measure the coolant's conductivity before specifying a magnetic flow meter.
- Size for the real flow range, not the pipe size.
- Pressure drop matters on CDU pumps: full-bore meters add none.
- Heat-removal accuracy depends on flow and ΔT together.
Step 1: Identify the loop
Liquid cooling has two hydraulic loops separated by the CDU heat exchanger. The facility water system (FWS) carries chilled or tower water from the plant. The technology cooling system (TCS) circulates coolant through cold plates or immersion tanks. The Open Compute Project and ASHRAE TC 9.9 use these terms in their liquid cooling guidance.
Step 2: Check the coolant
A magnetic flow meter only works above a minimum conductivity (≥50 µS/cm for the LDG-SUP-A100; about 2 µS/cm for low-conductivity models). Facility chilled water is normally well above this. TCS coolant may be deionised water or propylene glycol with inhibitors, which can sit close to or below the limit. Measure a sample rather than assuming. More detail: minimum conductivity for magmeters.
Step 3: Match the technology
| Technology | Best loop | Conductivity needed | Pressure drop | Notes |
|---|---|---|---|---|
| Electromagnetic | FWS, large TCS | Yes | None | Stable, no moving parts |
| Turbine (SUP-LWGY, FLC420) | TCS, rack manifolds | No | Low–moderate | Compact; needs clean coolant |
| Ultrasonic (inline/clamp-on) | TCS, retrofit | No | None | Clamp-on avoids cutting pipe |
| Vortex | FWS | No | Moderate | Needs minimum velocity |
| Coriolis (FCC-300) | Test benches, reference | No | Moderate | Direct mass, highest accuracy |
Step 4: Size, output and materials
- Flow range: the meter's normal flow should sit mid-range, not near its minimum. Turbine and vortex meters lose accuracy at low velocity.
- Output: 4–20 mA for robust loops, Modbus RTU for many sensors on one CDU controller, pulse for totalising.
- Materials: stainless steel bodies and EPDM or FKM seals compatible with glycol and inhibitors.
- Companions: pair each flow point with matched Pt100 temperature transmitters for ΔT and a SUP-PE100 for pump ΔP.
Worked example
A CDU rated 1 MW at ΔT = 10 K on water (cp ≈ 4.18 kJ/kg·K) needs ṁ = 1000 / (4.18 × 10) ≈ 23.9 kg/s, about 86 m³/h. On DN100 pipe that is roughly 3 m/s, inside a magmeter's ideal 0.5–5 m/s band. If the TCS coolant is DI water, select a turbine or ultrasonic meter at the same size instead.
Related questions
Can I use a clamp-on ultrasonic meter on a CDU?
Yes, especially for retrofit or verification, provided the pipe material, wall thickness and straight run are suitable.
What accuracy is needed for heat-removal reporting?
Many operators target overall heat measurement within a few percent; that means flow around ±0.5–1% and matched temperature sensors.
Does Supmea make data-center-specific meters?
Yes, for example the FLC420 turbine flow meter and SUP-PE100 pressure transmitter, designed for liquid cooling.
Sources and further reading
- ASHRAE TC 9.9 data center resources
- Open Compute Project
- Supmea FLC420 turbine flowmeter for data centers
Product specifications quoted here are summarised from Supmea datasheets. Confirm the figures for your exact model before final design.