A sample receiver and a constant pressure cylinder are two different containers for holding a collected sample, and the difference is whether the sample is kept under pressure. A receiver accumulates composite grabs in a vessel at or near ambient conditions, which is fine for stable liquids. A constant pressure cylinder uses a floating piston and a precharge to hold the sample at line pressure, keeping volatile or gas bearing product single phase so nothing flashes off before the lab sees it. This page contrasts the two and explains when each is required.
Sample Receiver vs CP Cylinder in one line: A sample receiver is a container that collects composite grabs at ambient or near-line conditions, suitable for stable liquids that will not lose light ends. A constant-pressure cylinder is a floating-piston vessel precharged on one side that keeps the sample at pressure, holding volatile or gas-bearing product single-phase so it does not flash before laboratory analysis.
A sample receiver is essentially a holding vessel that a composite sampler injects grabs into over a batch. It accumulates the doses until the batch is complete and the collected sample is drawn off for analysis. Many receivers operate at or close to ambient pressure, and they are well suited to stable liquids, crudes and products that will sit in a container without meaningfully changing, so the sample the lab receives is the same sample the sampler collected. The receiver's job is straightforward: gather the grabs, mix them, and hold them until they are needed.
A constant pressure cylinder is a fundamentally different container built around a floating piston that divides the cylinder into two chambers. One side receives the sample; the other is precharged with a gas or fluid to a pressure at or above the line pressure. As sample enters, the piston floats to accommodate it while the precharge holds the sample side at pressure the whole time. The defining feature is that the sample is never allowed to drop below its bubble point, so it stays as a single liquid phase from the moment of capture through transport to the laboratory.
The contrast is essentially about pressure retention. A receiver holds a sample; a constant pressure cylinder holds a sample at pressure. That single difference determines which volatile or dissolved components survive in the sample. For a stable liquid the extra machinery of a floating piston and precharge buys nothing, so a receiver is the simpler, cheaper, adequate choice. For a live, volatile, or gas bearing fluid, the pressure retention is the whole point, and only a constant pressure cylinder preserves the sample the analysis depends on.
Many custody fluids carry dissolved gas or light ends that stay in solution only as long as the fluid remains at pressure. Drop the pressure below the fluid's bubble point and those light components come out of solution as a vapor phase. Once that happens in a sample container, the sample has split into liquid and gas, and any portion the laboratory analyzes no longer has the composition the pipeline had. The lightest, most valuable components are exactly the ones that flash off first, so the loss is not random; it strips the sample of the very fractions that matter most for characterizing the product.
An ambient receiver cannot prevent this for a volatile fluid, because as soon as the sample sees a pressure below its bubble point it starts to flash and lose light ends. That is why receivers are matched to stable products that have no significant light ends to lose. A constant pressure cylinder solves the problem by never letting the pressure fall: the precharge on the piston's back side keeps the sample side above the bubble point continuously, so no vapor phase ever forms, the light ends stay dissolved, and the sample delivered to the lab is genuinely representative of the live fluid in the line.
Single phase retention also protects the analysis on the way in and out. When the cylinder is filled, the floating piston lets sample enter without the sample side ever depressurizing, and when the lab discharges it, the precharge can be used to push the sample out while keeping it above its bubble point right up to the instrument. The whole design exists to maintain one condition, pressure above the bubble point, from capture to analysis, because breaking that condition even briefly permanently changes the sample and cannot be undone.
The choice follows the fluid. If the product is a stable liquid with no meaningful light ends, a stabilized crude or a refined product that will not change in an open vessel, a sample receiver is the right container: simpler, cheaper, and fully adequate. If the product is live, volatile, or carries dissolved gas, a crude with appreciable light ends, a natural gas liquid, or any fluid whose composition depends on staying pressurized, a constant pressure cylinder is required, because only it keeps the sample single phase and preserves the components the analysis is meant to measure. Choosing a receiver for a volatile fluid guarantees a sample that has already lost what mattered before the lab opens it.
When lab results look wrong, the container is one of the things to examine, and the symptom points the way. A volatile product whose analysis comes back consistently light in the light ends, lower gravity light fractions than the stream should carry, is a classic sign the sample flashed, which means the wrong container was used or a constant pressure cylinder lost its charge. A cylinder that reads correctly on some samples and light on others points to intermittent loss of precharge or a leaking piston. Diagnosis is to confirm the fluid's volatility warranted a constant pressure cylinder in the first place, then to verify the cylinder actually held pressure from fill through delivery.
A constant pressure cylinder only does its job if it stays charged, and precharge pressure is a monitorable quantity. A cloud SCADA platform such as Merobix reads the sample system's pressures and, on instrumented setups, the cylinder precharge from the field devices, so operators can confirm a sample was actually held single phase rather than discovering a lost charge only when the lab reports flashed off light ends. Trending sampler and cylinder pressures across a fleet lets the team catch a cylinder that has bled down or a fill that occurred below the bubble point, so a defective single phase sample is caught in the field rather than accepted as a true analysis. The container mechanism works in the field, but the assurance that the sample stayed single phase from capture to lab lives in the monitored data.
A sample receiver collects composite grabs in a vessel at or near ambient conditions and suits stable liquids that will not change in an open container. A constant-pressure cylinder uses a floating piston and a precharge to keep the sample at pressure, holding volatile or gas-bearing product single-phase. The key difference is pressure retention: the receiver holds a sample, the cylinder holds a sample above its bubble point.
You need a constant-pressure cylinder whenever the fluid carries dissolved gas or light ends that stay in solution only under pressure, such as live crude, natural gas liquids, or any product whose composition depends on remaining pressurized. In those cases an ambient receiver would let the light ends flash off, giving the lab a sample that no longer matches the line. For stable liquids with no significant light ends, a receiver is adequate.
A floating piston divides the cylinder, with the back side precharged to at least line pressure. As sample enters, the piston floats to make room while the precharge keeps the sample side above the fluid's bubble point the entire time, so no vapor ever forms and the light ends stay dissolved. Because the pressure never falls below the bubble point from fill through transport to analysis, the sample delivered to the lab is genuinely representative of the live fluid.
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