Single-Stage vs Two-Stage Gas Regulators: Which Do You Need?
Quick answer: if your regulator sits on a high-pressure gas cylinder and you want the delivery pressure to stay put from a full cylinder to a nearly empty one, buy a two-stage regulator. A single-stage regulator is the right tool when the inlet pressure is already steady, such as a liquid cylinder, a pipeline or a point-of-use line, or when you need very high delivery pressure. Below we explain why, using Harris’s published numbers, and point you to the matching models. For the full selection process, see our Regulator Buying Guide.
How a single-stage regulator works
A single-stage regulator drops the cylinder pressure to your delivery pressure in one step. As the cylinder empties and the inlet pressure falls, the delivery pressure creeps up. Harris calls this the supply pressure effect. On a draining cylinder you have to readjust the knob from time to time.
How a two-stage regulator works
A two-stage regulator does the job in two steps inside one body. The first stage is preset at the factory and is not adjustable; it reduces cylinder pressure to a steady intermediate pressure. The second stage, the one you adjust, only ever sees that near-constant intermediate pressure, so your delivery pressure stays steady from a full cylinder to a nearly empty one. That is why gas chromatograph and analyzer users want two-stage regulators, and why most equipment manufacturers recommend them.
The numbers: how much does delivery pressure drift?
Harris publishes the supply pressure effect for each model as the change in delivery pressure for every 100 psig drop in inlet pressure.
| Harris model | Type | Delivery change per 100 psig inlet drop | Flow coefficient (Cv) |
|---|---|---|---|
| GP402 | Two-stage | 0.04 psig | 0.15 |
| HP702 | Two-stage | 0.04 psig | 0.15 |
| HP722C | Two-stage | 0.05 psig | 0.06 |
| HP742 | Two-stage | 0.05 psig | 0.06 |
| HP405 | Single-stage (liquid cylinders) | 0.9 psig | 0.37 |
| HP721 / HP741 (not stocked) | Single-stage cylinder | 1.8 psig | 0.08 |
| HP8700 | Single-stage high pressure | 3.3 psig | 0.08 |
Worked example. A cylinder drains from 2000 psig to 200 psig, an 1800 psig drop:
- On a two-stage HP722C or HP742 (0.05 psig per 100), delivery pressure changes by only about 0.9 psi.
- On a single-stage HP721 or HP741 (1.8 psig per 100), it changes by about 32 psi.
If your instrument is set at 60 psig, that is the difference between a setting that holds and one you must watch.
When a single-stage regulator is the right choice
- Liquid cylinders (dewars). A liquid cylinder’s gas-use pressure is fairly constant, so a single stage is enough. The Harris HP405 is built for this: see the HP405 CGA 320 for CO2, HP405 CGA 540 for oxygen and HP405 CGA 580 for inert gas. Never connect a regulator to the liquid-withdrawal valve.
- Pipelines and point-of-use lines. Downstream of a two-stage cylinder regulator or a central supply, the inlet is already steady. Harris makes line regulators for this (GP403, HP703, HP723C and HP743). We do not list them online, so contact us for a quote.
- Very high delivery pressure. Harris gives 500 psi as the normal maximum delivery pressure for two-stage regulators. Above that, the single-stage HP8700 0-1500 psi and HP8700 0-2500 psi take over for high-pressure testing and charging accumulators.
- Short jobs you will watch and adjust by hand.
When a two-stage regulator is the right choice
- Gas chromatography carrier, fuel and detector gases
- Process analyzers, calibration standards and EPA protocol gases
- CO2 incubators and other unattended equipment
- Any high-pressure cylinder where you want to set the pressure once
All four of our two-stage series come in the common CGA connections. Pick the series by gas purity:
| Need | Two-stage series |
|---|---|
| Industrial-grade gas, up to grade 4.0, budget | Harris GP402 economy regulators |
| High purity up to grade 5.0, GC, purging, incubators | Harris HP702 general purpose regulators |
| Grade 5.0 ultra high purity, GC, analyzers | Harris HP722C high purity regulators |
| Grade 6.0 and higher, corrosive gases or harsh surroundings | Harris HP742 stainless steel regulators |
| CO2 incubators | CO2 incubator regulator or the two-cylinder CO2 manifold |
Reading a Harris model number
The last digit of a Harris specialty gas model tells you the type: 1 is a single-stage cylinder regulator, 2 is a two-stage cylinder regulator, and 3 is a single-stage line regulator. So the HP741 is single-stage, the HP742 is two-stage, and the HP743 is a line regulator, all in the same 316L stainless family.
Two-stage does not mean more flow
Staging is about pressure stability, not flow. Regulators control pressure, not flow; use a flowmeter or metering valve to set a flow rate. To compare flow capacity, look at Cv: the GP402 and HP702 (Cv 0.15) have larger flow passages that suit purging and blanketing, while the HP722C and HP742 (Cv 0.06) trade flow for low internal volume and cleanliness, which suits GC and calibration gases. The single-stage HP405 has the highest Cv of our range at 0.37.
Every regulator also shows some droop, a fall in outlet pressure as flow increases. If pressure drops a lot when flow starts, check that the cylinder is not nearly empty and consider a higher-Cv model.
Frequently asked questions
What is the difference between a single-stage and a two-stage regulator?
A single-stage regulator reduces cylinder pressure in one step, so its delivery pressure rises as the cylinder empties. A two-stage regulator has a factory-preset first stage feeding an adjustable second stage, so delivery stays steady from a full to a nearly empty cylinder.
How much does delivery pressure change on a two-stage regulator?
Very little. Harris rates its two-stage GP402 and HP702 at 0.04 psig, and its HP722C and HP742 at 0.05 psig, for every 100 psig drop in cylinder pressure. A full 1800 psig drop moves an HP742 by only about 0.9 psi.
Why does my delivery pressure rise as the cylinder empties?
That is the supply pressure effect of a single-stage regulator. As inlet pressure falls, delivery pressure creeps up, by about 1.8 psig per 100 psig drop on a Harris HP741. It is normal for single-stage regulators. Use a two-stage regulator if you need a steady setting.
Can I use a single-stage regulator on a liquid cylinder?
Yes. A liquid cylinder supplies gas at a fairly constant pressure, so a single-stage regulator such as the Harris HP405 works well on its gas-use valve. Never connect a regulator to the liquid-withdrawal valve.
What is the highest delivery pressure for a two-stage regulator?
Harris gives 500 psi as the normal maximum delivery pressure for two-stage regulators. The HP722C and HP742 are built up to 0-500 psig. Above that, use a high-pressure single-stage regulator such as the Harris HP8700, available up to 0-6000 psig.
Does a two-stage regulator give more flow?
No. Staging improves pressure stability, not flow. Flow capacity depends on the regulator flow coefficient (Cv): 0.15 for the Harris GP402 and HP702, 0.06 for the HP722C and HP742, and 0.37 for the single-stage HP405. Use a flowmeter or metering valve to set flow.
Sources
- Harris Products Group, Specialty Gas Equipment Catalog, North America, P/N 9505646 Rev E (2021): single- vs two-stage explanation, supply pressure effect (“Pressure Regulation, psig/100 psig”) and Cv for each model, model numbering.
- Harris The Proper Selection of a Regulator (2018).
- LabRegulator Application Help page (manufacturer recommendation of two-stage regulators).