Liquid-cooled AI racks get attention for their power and cooling numbers, but the part that makes a liquid-cooled tray serviceable is a small coupling. The Open Compute Project has now published version 2.0.0 of its Universal Quick Disconnect (UQD) specification, which brings its earlier hand-mate and blind-mate documents into one. NVIDIA's MGX 1.2 and Meta's IcePack both lean on blind-mate liquid cooling, so these couplings sit in the middle of the 2026 OCP announcements.
This article summarizes what the OCP specification defines, explains how NVIDIA's separate MQDB family fits in, and lists what it leaves open for connector engineers.
In short:
- UQD is a manually coupled quick disconnect, UQDB is the blind-mate version, and a hybrid mates a UQD plug to a UQDB socket.
- The spec covers four sizes (02, 04, 06 and 08) for single-phase water and glycol cooling, with defined limits for flow, pressure, temperature, fluid loss per cycle, mating force and cycle life.
- It does not give numeric float values for blind-mate alignment, so designers must get those from the supplier.
UQD and UQDB: What the Terms Mean
The specification's scope is non-combustible, single-phase water or glycol liquid cooling of electronics. It defines three coupling arrangements:
- UQD: a handmate quick disconnect, coupled manually.
- UQDB: a blindmate quick disconnect, coupled remotely, for example when a tray slides into a rack and engages the manifold without a technician touching a hose.
- Hybrid: blindmate coupling of a UQD plug to a UQDB socket. This arrangement needs a floating "overdrive" device to compensate for axial and radial misalignment.
What the v2.0.0 Specification Defines
The figures below are for sizes 02, 04, 06 and 08, in that order, as stated in the specification.
- Rated flow: 2.27, 6.44, 11.36 and 17.79 L/min (0.6, 1.7, 3.0 and 4.7 GPM).
- Pressure: maximum operating pressure of 6.89 bar(g) (100 psi(g)) and minimum burst pressure of 20.68 bar(g) (300 psi(g)).
- Flow coefficient (Cv): minimum 0.30, 1.1, 2.2 and 3.6, and maximum 0.43, 1.6, 3.3 and 5.6.
- Temperature: operating range of 5 °C to 65 °C, and a non-operating range of -40 °C to 70 °C.
- Fluid loss: a maximum of 0.02, 0.03, 0.04 and 0.07 mL per coupling cycle.
- Cycle life: at least 5,000 coupling cycles.
- Coupling force (UQDB): a maximum of 55, 65, 90 and 135 N at zero pressure, rising with line pressure by 3.47, 9.62, 16.06 and 24.00 N per bar.
- Materials: the plug tip, shaft and guide area must be stainless steel (SUS 303, 304, 304L, 316 or 316L). Other wetted materials follow OCP coolant guidance.
- Life: a 5-year storage life from manufacture, followed by a 10-year service life.
The verification tests include a minimum of five samples, a seal washout test of 20 cycles in each direction at twice the rated flow at 60 °C and 6.9 bar(g), 72-hour temperature cycling at both ends of the operating and non-operating ranges, and a hydrostatic test at 6.9 bar(g) for at least three minutes with no visible leakage. Version 2 consolidates the earlier UQD and UQDB specifications, adds defined plug end conditions, lengths and maximum outer diameters, and raises the minimum Cv. It is described as backward compatible, but the document warns that mixed-version deployments need end-user evaluation.
Date note: the cover of the document gives an effective date of September 22, 2026, while its version table lists V2.0.0 as June 2026. Check the OCP page for the current revision. The source is the OCP UQD v2.0.0 design specification.
What the Specification Leaves Open
- Blind-mate float: the document describes a "radial allowance" for UQDB plug self-alignment but does not give a numeric float value, and the overdrive device for hybrid couplings is out of scope. One supplier, Amphenol, states approximately 1 mm of radial self-alignment for its UQDB series. That is a vendor claim, not a specification limit.
- Pressure drop: the pressure-drop coefficients are marked as planned for later revisions.
- Maximum flow: it is given only as a general 5 m/s velocity guideline, not as a flow figure.
- Cross-version compatibility: testing between v1 and v2 parts is out of scope.
MQD and MQDB: NVIDIA's Parallel Family
NVIDIA's MGX 1.2 announcement names "MQD" and "MQDB" liquid-cooling technologies without defining the terms. Parker's MQDB bulletin describes an "MQDB" series, which it expands as Multi Quick Disconnect Blind-mate, for in-tray and inner-manifold connections. The bulletin says the series was "developed to comply with NVIDIA specifications", which suggests it is the family NVIDIA means, though NVIDIA has not said so.
- Sizes: MQDB02, MQDB03, MQDB04 and MQDB1.
- Pressure and temperature: a maximum operating pressure of 100 psi with a 3-to-1 safety factor, and a working range of 17 °C to 65 °C.
- Connect force: 47.7, 47.9, 54.0 and 54.7 N across the four sizes.
- Materials: stainless steel coupling and plug with EPDM seals.
- Standards: the bulletin does not mention OCP or UQDB, and it gives no mating cycle rating or numeric float value.
Treat MQDB and UQDB as separate part families unless a supplier states otherwise. The size names are different, and we have no evidence that the parts are interchangeable.
Our Analysis: What This Means for Specifying Parts
The points below are our engineering assessment, built on the specification's own figures.
1. Mating force depends on line pressure
The UQDB coupling force rises with pressure. For a size 08 coupling, 135 N at zero pressure plus 24.00 N per bar at the full 6.89 bar adds about 165 N, which comes to roughly 300 N for one coupling. A tray with supply and return couplings would then need on the order of 600 N to seat while pressurized. This is illustrative arithmetic from the specification's figures, and the test conditions in the document govern the real values. The practical point is that the tray's latching, guide features and insertion-force budget should be designed for pressurized insertion, not only a dry fit.
2. The leak limit is a ceiling
A limit of 0.02 mL to 0.07 mL per cycle, held over the rated 5,000 cycles, gives an upper bound of 100 mL to 350 mL per coupling. Real parts lose much less. Parker lists spillage figures of 0.0004 to 0.0007 mL for its MQDB sizes, although the bulletin does not define how those are measured. The specification limit is what a part may do and still pass, so ask suppliers for measured results.
3. Float has to be compared with the tolerance stack
Because the specification gives no numeric float, the designer has to take the coupling's float from the supplier and compare it with the tray's tolerance stack. In an ORv3 rack, the manifold itself may deflect up to 1.0 mm in the depth direction under the OCP ORv3 blind mate manifold specification, which we summarize in our IcePack and IceCube article. Electrical power contacts and signal connectors on the same rear face add their own tolerance needs.
4. Questions to ask a coupling supplier
- Which specification and revision does the part conform to (UQD v1, UQD v2.0.0, or a customer specification such as NVIDIA's)?
- What is the measured coupling force at rated pressure, not only at zero pressure?
- What are the numeric radial and angular float values for blind-mate use?
- What are the measured fluid loss per cycle and the cycle-life test results?
- Which seal materials are used, and are they approved for the coolant in your system?
For the rack designs these couplings serve, see our articles on NVIDIA MGX 1.2 and AMD Helion. For the electrical side of an Open Rack, see ORv3 busbar power connectors and the ORv3 compliance checklist. More coverage is in our OCP server interconnects hub.
Last updated October 15, 2026. We will update this article as OCP and suppliers publish further UQD and MQDB details.