Choosing a Cv value, without guessing.
Cv is the number that decides how much pressure a coupling costs you at your flow — and it is the number most often copied from a competitor's datasheet rather than calculated. This brief defines Cv properly, shows the quadratic relationship that makes small flow increases expensive, explains why dash size and bore are not Cv, and works three selections end to end on real OCP UQD ratings from Cv 0.25 to 2.50.
DL-006 · Published 2026-09-06 · 8 min read
A rack has about 18 psi of total pressure budget, and roughly 56% of that disappears inside the server modules — cold plates, board-level lines and the couplings on the branch. Which means the coupling selection is not a rounding error in the budget. If a full-server branch is pushed through a part with the wrong Cv, the coupling alone can consume most of the rack's allowance before the cold plate has taken its share.
Getting this right takes one formula and a clear idea of what you are buying. The formula is the same one used for valves, orifices and hoses; the clarity is about what the OCP size tiers guarantee and what they leave to the vendor.
What Cv actually means.
One sentence, and it is worth memorising: Cv is the flow of water at 60 °F, in US gallons per minute, that passes through the device at a pressure drop of exactly 1 psi.
Everything else follows from that definition. Flow is proportional to Cv and to the square root of pressure drop, corrected for the fluid:
Q = Cv × √(ΔP / SG) — Q in GPM, ΔP in psi, SG the specific gravity of the coolant relative to water.
Three consequences matter in a liquid cooling loop. First, Cv is a measured hydraulic property of the complete internal path — bore, valve, spring, seal and the machining between them — not a dimension you can read off a drawing. Second, the relationship is quadratic: doubling flow through the same part quadruples pressure drop. Third, the metric equivalent is not interchangeable with Cv. Kv is the same idea in m³/h at 1 bar, and Kv ≈ 0.86 × Cv. A specification written in Kv and read as Cv understates the flow by about 14%, which is the sort of error that survives a design review and surfaces on a test bench.
Fluid properties enter through the specific gravity term, and they are small but not negligible. Water sits at SG 1.00; a 25% propylene glycol mix sits near 1.02. Going from water to that mix raises the pressure drop across the same part by roughly 2% at the same flow — not enough to change a selection, easily enough to change a pressure budget that is being held to two decimal places.
Flow and pressure drop move on a square law.
The second column below is why a coupling that looks fine at part load can be the thing that fails a thermal validation at full load.
| Flow through the part | ΔP at Cv 0.80 (SG 1.00) | ΔP at Cv 1.22 (SG 1.00) | ΔP at Cv 2.50 (SG 1.00) |
|---|---|---|---|
| 0.55 GPM | 0.47 psi | 0.20 psi | 0.05 psi |
| 1.00 GPM | 1.56 psi | 0.67 psi | 0.16 psi |
| 1.70 GPM | 4.52 psi | 1.94 psi | 0.46 psi |
| 2.50 GPM | 9.77 psi | 4.20 psi | 1.00 psi |
| 3.17 GPM | 15.70 psi | 6.76 psi | 1.61 psi |
| 4.70 GPM | 34.52 psi | 14.85 psi | 3.53 psi |
Computed from Q = Cv × √(ΔP / SG) at SG 1.00. Add about 2% for a 25% propylene glycol mix. One coupling sits in each direction on a branch, so double the figure for a supply-and-return pair.
Read the 3.17 GPM row across. That is a full-server branch at the 1.2 L/min/kW planning baseline: 12 L/min, about 3.2 GPM. On a Cv 2.50 dash-08 part it costs 1.61 psi. On a Cv 0.80 part — still a perfectly compliant dash-04 coupling — it costs 15.7 psi, which is nearly the entire rack budget through one fitting. The dash-04 part is not defective; it is being used at four times the flow its tier was rated for.
Dash size, bore and Cv are not the same number.
The OCP interface fixes the outside; the vendor's internals decide the Cv. That split is where most selection errors live.
| Size tier | Nominal interface | Minimum flow bore ØN | Minimum Cv | Rated flow | ΔP at rated flow, at the Cv floor |
|---|---|---|---|---|---|
| UQD02 | 1/8" | 3.63 mm | 0.25 | 0.55 GPM | 4.84 psi |
| UQD04 | 1/4" | 7.14 mm | 0.80 | 1.7 GPM | 4.52 psi |
| UQD06 | 3/8" | 9.47 mm | 1.60 | 3.0 GPM | 3.52 psi |
| UQD08 | 1/2" | 10.75 mm | 2.50 | 4.7 GPM | 3.53 psi |
The last column is the useful part of the table, and it is not published as such. Every OCP tier pairs a Cv floor with a rated flow so that the tier lands between roughly 3.5 and 4.8 psi at its rating with water. The standard is internally consistent: whichever size you pick, the coupling costs about the same pressure at the flow that size was designed to pass. Deviate from the pairing — a big flow through a small tier, or a high Cv claimed on a tiny bore — and the consistency is what you lose.
The trap is the gap between the bore and the Cv. A dash-04 plug has a minimum flow bore of 7.14 mm, but published UQDB04 class parts quote a 5.7 mm flow diameter with a Cv of 1.22. Nothing is wrong: the valve, spring and seal sit in the flow path, so the effective flow diameter is smaller than the bore, and the metering happens at the seat rather than in the bore. Two vendors can hold identical interface dimensions and identical bores and still differ by 50% in Cv, because the difference is in the geometry you cannot see from the outside. That is the reason to ask for a measured Cv on the batch rather than a catalogue number.
Three selections, worked end to end.
All three use the same two lines of arithmetic. The judgement is in what pressure you are willing to spend.
1. A cold-plate service line at 1.5 GPM
Flow 1.5 GPM, coolant SG 1.02, and a target of no more than about 1.5 psi per coupling. Required Cv = 1.5 ÷ √(1.5 / 1.02) = 1.5 ÷ 1.213 = 1.24. The dash-04 floor of 0.80 does not reach it — at 1.5 GPM the floor-grade part drops (1.5 ÷ 0.80)² × 1.02 = 3.59 psi, more than twice the target. A class part at Cv 1.22 lands at 1.54 psi. Across a supply-and-return pair that is 3.1 psi instead of 7.2 psi, which is 4 psi of pump head recovered by paying attention to one datasheet line.
2. A full-server branch at 12 L/min
12 L/min is 3.17 GPM. A dash-08 part at the 2.50 floor drops 1.64 psi on the glycol mix. A dash-06 at its 1.60 floor drops 4.00 psi — and 3.17 GPM is already past the dash-06 rated flow of 3.0 GPM, so the part is outside the flow class it was qualified at. A dash-04 at the 0.80 floor drops 16.0 psi and fails the exercise outright. The selection rule writes itself: full-server branches take the dash-08 tier, and the dash-04 tier belongs on cold-plate and sub-branch service below its 1.7 GPM rating.
3. Checking the result against the manifold budget
Here is the counter-intuitive part. If you halve the branch loop's pressure drop by choosing a better Cv — say from 8 psi to 4 psi on the cold plate, its board lines and its two couplings — the manifold allowance also halves, because the LBNL rule caps manifold supply plus return at 1/40 of the IT loop it feeds: 8 psi buys a 0.2 psi manifold budget, and 4 psi buys 0.1 psi. Choosing a higher Cv is still right, because it lowers total rack pressure drop and lets the pump run easier. What it does not do is earn the manifold more headroom; the allowance scales with the loop it belongs to.
How to specify Cv without over-specifying it.
Four lines on a drawing do the whole job. Over-specifying is a real cost: the 1U envelope on the 44.45 mm pitch caps how much internal volume a part can have.
- State the flow, not just the size — "3.2 GPM design flow" tells the supplier more than "dash-08", because it fixes the rating you are buying against.
- Set a Cv floor with margin — the OCP minimum for the tier plus your own headroom, for example Cv ≥ 1.2 on a dash-04 branch that flows 1.5 GPM.
- Ask for the measured value — a batch Cv check on a flow bench, recorded per batch, not a catalogue claim. Our dash-04 sample record holds flow within 1.8% of rating.
- Ask for the flow curve — ΔP against Q over the working range, so you can check the part at your actual operating point rather than at its rating.
- Do not buy Cv you cannot use — beyond roughly twice the flow the branch will ever pass, extra Cv costs envelope, weight and mate force (53–71 N across the UQDB class, 62 N at dash-04) and buys nothing an operator can feel.
The mate-force point is worth stating plainly, because it is the cost of internal volume. A part with a large flow area has a larger valve face and a stiffer spring, and the tray or technician has to push against that force. In a 1U envelope with a gloved hand, an over-specified coupling converts a hydraulic win into a service problem — and in blind-mate positions, an over-stiff pair raises the alignment load the rack has to carry. Specify the Cv the loop needs, then stop.
Five ways a Cv calculation goes wrong.
Check these before you release the drawing
Three questions this brief answers most.
What does Cv mean on a quick disconnect datasheet?
Is dash size the same as Cv?
How much Cv do I actually need?
Send the flow and the pressure you can spend.
We will come back with the size class, the Cv and the branch pressure drop in writing — 48-hour turnaround, MOQ from 50 pieces, flow records in the carton.