When do you need a back pressure regulator?

An ordinary pressure regulator keeps the outlet pressure constant. A back pressure regulator (BPR) does something fundamentally different: it keeps the inlet pressure constant. That sounds like a subtle difference, but for many processes it is exactly what you need.

⏱ approx. 4 minutes reading time

The difference between a pressure regulator and a back pressure regulator

In a standard pressure regulator, gas flows from high pressure to low pressure. The regulator lowers the pressure at the outlet and keeps it constant, regardless of what happens upstream. This is what you use to connect a gas cylinder to a process.

A back pressure regulator works the other way round: it sits on the outlet side of a system and keeps the pressure upstream of the regulator constant. Gas can only escape once the inlet pressure exceeds a set value. In this way, it acts as a kind of pressure threshold.

Standard pressure regulator

Keeps outlet pressure constant Located downstream of the source

Back pressure regulator

Keeps inlet pressure constant Located upstream of the outlet

Rule of thumb

Ask yourself: do I want to control the pressure after the regulator, or the pressure before the regulator?

Pressure after = standard pressure regulator. Pressure before = back pressure regulator.

Typical applications

Application 01

HPLC and GC-MS

With mass spectrometric detection, the pumping speed of the vacuum pump varies. A BPR at the column outlet keeps the pressure in the separation system stable, regardless of downstream fluctuations.

Application 02

Supercritical fluid chromatography (SFC)

CO₂ as the mobile phase must stay above its critical point. A BPR at the end of the column ensures that the system pressure always remains above the critical pressure of CO₂ (73 bar).

Application 03

Fuel cell (PEMFC)

A constant system pressure must be maintained on both the anode side (H₂) and the cathode side (air/O₂) for optimal membrane performance. A BPR at the outlet of each channel takes care of this.

Application 04

Reactors and autoclaves

In chemical reactors operating under pressure, a BPR keeps the reactor pressure constant while gas or liquid can be discharged in a controlled manner. Essential for safe and reproducible reaction conditions.

Application 05

Flow measurement with variable resistance

VA meters (variable area meters) are sensitive to downstream pressure changes. A BPR downstream of the VA meter stabilises the outlet pressure and thus considerably improves measurement accuracy.

Application 06

Gas sampling systems

When taking gas samples for analysis, the sample pressure must be constant for reproducible results. A BPR on the sample line ensures a stable and predictable pressure at the sensor or analyser.

How does a back pressure regulator work?

A BPR contains a diaphragm or piston loaded by a spring. The spring is set to the desired set pressure. As long as the upstream pressure is lower than the set pressure, the valve remains closed. As soon as the upstream pressure exceeds the set pressure, the valve opens and lets gas through, just enough to keep the pressure at the set value.

Important: BPR vs. relief valve

A back pressure regulator is not a safety valve (relief valve). A relief valve only opens in the event of overpressure as an emergency measure and is not intended for continuous process control. A BPR is a precision regulator for continuous pressure control and has a much tighter tolerance and a longer service life with regular use.

Selection criteria: what should you look for?

1. Set pressure range: Determine the desired upstream pressure and check that the BPR covers this range. BPRs are available from a few mbar to hundreds of bar. Choose a model whose set pressure falls in the middle of its range for the best stability.
2. Flow range (Cv value): The Cv value indicates how much gas the BPR can pass at a given pressure differential. Too small a Cv = pressure build-up; too large a Cv = unstable control. Calculate the required Cv based on your maximum flow rate and pressure differential.
3. Material compatibility: Choose the material based on the medium. 316L stainless steel for most gases and liquids. PTFE seals for corrosive media. Hastelloy for aggressive chemicals. Also check the O-ring materials.
4. Temperature range: BPRs in laboratories typically operate at room temperature. At elevated temperatures (reactors, autoclaves), choose a model certified for the process temperature. Pay attention to the temperature dependence of the set pressure.
5. Dead volume and response time: For analytical applications (HPLC, GC-MS), a low dead volume is essential to prevent band broadening. Choose a BPR specifically designed for analytical applications with minimal internal volume.

Do you need a BPR? Checklist

1. You want to keep the pressure upstream of a point in your system constant, not downstream of it
2. You work with a medium that must remain above a minimum pressure (e.g. supercritical CO₂, fuel cell)
3. Downstream pressure fluctuations in your system affect the reproducibility of your measurement or process
4. You use a VA meter or other flow meter that is sensitive to variable outlet pressure
5. You have a reactor, autoclave or pressure vessel that must operate at a fixed working pressure
6. You want to control the system pressure of a fuel cell or electrolyser accurately

Two or more yeses? Then you almost certainly need a BPR.

A back pressure regulator is not an expensive or complex component, but leaving it out in the wrong place costs you measurement accuracy, process reproducibility and sometimes safety. Used in the right place, it is one of the simplest ways to make your system more stable.