The most common breather on an atmospheric tank is a pressure-vacuum vent, and the simplest and oldest version of it works by nothing more than gravity acting on a weighted disc. A weight-loaded pressure-vacuum vent, sometimes called a dead-weight or pallet vent, uses the mass of a movable pallet to hold the vent shut until tank pressure or vacuum is enough to lift it. This guide explains how the weighted pallet mechanism works, how adding or removing weights changes the set points, and where the weight-loaded design fits alongside spring-loaded and pilot-operated vents.
Weight-Loaded Pressure-Vacuum Vent in one line: A weight-loaded pressure-vacuum vent uses gravity acting on a weighted pallet, or disc, to keep the vent closed until tank pressure or vacuum builds enough to lift it. On the pressure side, out-breathing lifts the pressure pallet once the pressure under it exceeds the pallet's weight per unit area; on the vacuum side, in-breathing lifts a separate vacuum pallet. The set points are adjusted by adding or removing calibrated weights on the pallets - more weight raises the point at which the vent opens, and less weight lowers it.
Inside a weight-loaded PV vent sit one or more pallets - flat, disc-like assemblies that rest by gravity over a seat, sealing the vent closed. There are typically two: a pressure pallet that seals the out-breathing path and a vacuum pallet that seals the in-breathing path. Each pallet simply lies on its seat, held down by its own weight, and the weight spread over the pallet's area defines a threshold pressure. As long as the tank pressure stays below that threshold, the pallet stays seated and the vent is shut.
When the tank out-breathes - during a fill or a warm-up - pressure builds beneath the pressure pallet. Once that pressure, acting over the pallet area, produces an upward force that exceeds the pallet's weight, the pallet lifts off its seat and vapor flows out. When enough vapor has escaped and the pressure falls back below the threshold, the pallet settles back down and reseals. The vacuum side works the same way in reverse: when the tank in-breathes and its internal pressure drops below atmospheric, the pressure difference lifts the vacuum pallet, air flows in, and the pallet reseats once the vacuum is relieved.
Because the mechanism is just a weight and a seat, the set point is expressed as a pressure, most naturally in inches or ounces of water column - very small pressures, since atmospheric tanks operate close to ambient. The pallet's opening pressure is essentially its weight divided by its area. This simplicity is the design's great virtue: there are no springs to relax, no pilots to fail, and the opening pressure is a direct, physical property of the pallet and its load, which makes the vent's behavior easy to understand and easy to verify.
Setting a weight-loaded vent is a matter of choosing how much mass sits on each pallet. Manufacturers supply the pallets with a base weight and a set of calibrated add-on weights, and the pressure or vacuum set point is changed by adding or removing those weights. Adding weight increases the force holding the pallet down, so the tank must reach a higher pressure or vacuum before the pallet lifts - the set point goes up. Removing weight does the opposite, lowering the pressure or vacuum at which the vent begins to relieve. The pressure and vacuum sides are adjusted independently, since each has its own pallet and its own weights.
This gives the weight-loaded vent a straightforward, mechanical adjustment path, but it also bounds what it can do. Because the set point comes from gravity on a pallet of fixed area, weight-loaded vents are practical only at the low pressures typical of atmospheric tanks; reaching a higher set point would need impractically heavy pallets. The adjustment is also granular - you can only add the discrete weights supplied - rather than continuously tunable. For most breather duty on low-pressure tanks, that is entirely adequate, and the direct weight-to-pressure relationship makes the set point transparent and repeatable.
A practical consequence of the weighted design is orientation and installation sensitivity. Because the pallets rely on gravity to seat, a weight-loaded vent must be mounted upright as intended, and it can be affected by fouling, sticking, or ice that changes how freely the pallet lifts and reseats. When a weight-loaded vent misbehaves, the causes are usually physical and visible - a stuck pallet, a fouled seat, missing or added weights that changed the set point - which fits the design's overall character as a simple, inspectable device.
The weight-loaded pallet is one of three common ways to set a PV vent, and each suits different conditions. Spring-loaded vents replace the pallet's gravity load with a spring, which lets them reach somewhat higher and more precisely tuned set points and makes them less dependent on mounting orientation, at the cost of a spring that can weaken or corrode over time. Pilot-operated vents use a small pilot that senses tank pressure and controls a main valve, giving very tight sealing right up to the set point and the ability to handle higher pressures and larger flows, at the cost of more complexity and a pilot that itself must be maintained.
Against those, the weight-loaded vent is the simplest and often the most economical choice for straightforward low-pressure breathing duty. Its opening pressure is a plain function of weight and area, with nothing to drift, and it is easy for a technician to inspect, understand, and reset by checking the pallets and weights. The trade-offs are its limited pressure range, its sensitivity to fouling and orientation, and a tendency to leak slightly near the set point compared with a well-designed pilot-operated valve, which matters more when conserving vapor is a priority.
Whichever design is fitted, its job is to hold the tank pressure and vacuum within a narrow band, and a cloud SCADA such as Merobix monitors that band by trending the tank pressure and vacuum the vent controls. Merobix reads those signals from the field over Modbus, DNP3, OPC UA, and MQTT, so a weight-loaded vent whose pallet has stuck - letting pressure climb during a fill or vacuum deepen during a draw - shows up as a pressure trend that no longer respects the expected set points. For an operator watching many tanks, that continuous view of pressure against the vent's setting is how a simple mechanical breather's health is confirmed without climbing every tank.
You add or remove calibrated weights on the pallet. Adding weight raises the pressure or vacuum needed to lift the pallet, so the vent opens at a higher set point; removing weight lowers it. The pressure and vacuum sides have separate pallets and are adjusted independently, and the set point is essentially the pallet weight divided by its area, expressed in small units like inches or ounces of water column.
A weight-loaded vent holds its pallet down with gravity, while a spring-loaded vent uses a spring. The weight-loaded design is simple, transparent, and easy to inspect but is limited to low pressures, sensitive to mounting orientation, and adjustable only in discrete weight steps. A spring-loaded vent can reach somewhat higher, more finely tuned set points and is less orientation-dependent, but relies on a spring that can weaken or corrode over time.
Because their set point comes from the weight of a pallet spread over a fixed area, reaching a higher opening pressure would require an impractically heavy pallet. For the very low pressures typical of atmospheric storage tanks, a modest pallet and a few add-on weights are enough, which is why weight-loaded vents are common breathers on low-pressure tanks and are not used where higher set pressures are required.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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