How to Choose a Regulator for High Purity and Specialty Gases
A regulator can only deliver gas as clean as its wetted parts allow. Put grade 5.0 helium through a regulator with an elastomer diaphragm, or chlorine through a brass body, and you lose the purity you paid for, or the regulator. This guide walks through how Harris matches its specialty gas regulators to gas purity and gas type, so you can choose with confidence. For the full step-by-step selector, see our Regulator Buying Guide.
Step 1: Match the regulator to the gas grade
Gas purity is written as a grade: the number of nines. Grade 4.0 is 99.99%, grade 5.0 is 99.999% and grade 6.0 is 99.9999%. Harris builds a regulator tier for each:
| Gas grade | Harris series | Construction | Inboard helium leak rate |
|---|---|---|---|
| Up to 4.0 (industrial) | GP402 economy | Brass body, neoprene diaphragm | Not specified |
| Up to 5.0 | HP702 | Plated brass, 2-1/8″ 302 stainless diaphragm | 1×10-8 cc/sec |
| 5.0 ultra high purity (UHP) | HP722C | Plated brass barstock, 1-11/16″ 316L stainless diaphragm | 1×10-9 cc/sec |
| 6.0 and higher, plus corrosive gases | HP742 | 316L stainless barstock body, 316L diaphragm | 1×10-9 cc/sec |
Harris states plainly that the GP402 is not recommended for high purity service. From the HP702 up, the diaphragm is stainless steel and the seats are PTFE, with no neoprene in the gas path.
Step 2: Understand what the materials do
- Diaphragm: this is the largest wetted surface. Neoprene is fine for industrial gas, but high purity regulators use stainless steel diaphragms (302 on the HP702, 316L on the HP722C and HP742).
- Body: a brass barstock body is acceptable for ultra high purity work; a stainless body does not make the gas itself cleaner. Stainless steel bodies are needed for corrosive gases and harsh external environments, such as corrosive vapors in a fume hood.
- Seats and seals: PTFE on all four two-stage series.
- Inlet filter: a 10 micron filter protects the seat on each model; it is sintered stainless steel on the HP742.
Step 3: Apply the rules for your gas type
Inert carrier gases: nitrogen, argon, helium
For GC carrier gas, match the tier to the grade: the HP702 CGA 580 for gas up to grade 5.0, the HP722C CGA 580 as the standard UHP choice, and the HP742 CGA 580 for grade 6.0 or calibration standards with low-ppm reactive components.
Oxygen and nitrous oxide
Harris recommends the HP702 or the HP722 / HP722C with a CGA 540 inlet for oxygen: see the HP702 CGA 540 and HP722C CGA 540. Keep an oxygen regulator dedicated to oxygen and completely free of oil and grease, inspect threads before connecting, and open the cylinder valve slowly. Nitrous oxide (CGA 326) is also an oxidizer and needs the same oxygen-clean care.
Hydrogen and other flammable gases
Use a CGA 350 regulator in the tier that matches your purity: HP702, HP722C or HP742. Add a flashback arrestor and check valve, vent the relief valve and purge outlets to a safe location, and leak-test with soap solution, never with a flame.
Corrosive and toxic gases
Chlorine, sulfur dioxide and nitric oxide (CGA 660), hydrogen chloride and hydrogen sulfide (CGA 330) and ammonia (CGA 240 or 705) attack brass. Use the 316L stainless HP742 CGA 660, or order the HP742 with your gas specified for other connections; Harris also makes an electroless nickel HP701EN for ammonia. Fit a cross-purge assembly, purge with an inert gas at every startup and shutdown, and never “crack” the cylinder valve of a toxic, corrosive or flammable gas to clear it. Corrosive and toxic gases are often housed in a gas cabinet.
Carbon dioxide
CO2 cools sharply as it expands. Below about 60 psig delivery it can form dry-ice snow inside the regulator, which can plug it or hold the seat open. For research-grade CO2 choose the HP722C CGA 320, or the HP742 CGA 320 for corrosive mixtures or harsh surroundings. For high, continuous CO2 flow, ask us about the Harris HP705 electrically heated regulator.
Calibration standards, EPA protocol and specialty mixtures
These gases often contain low-ppm reactive components. Harris lists EPA protocol, emissions monitoring and process analyzer gases among the uses of both the HP722C and HP742; choose the HP742 when components are reactive or corrosive. Confirm the mixture’s CGA with your supplier and see our CGA Charts.
Step 4: Choose the outlet valve
An outlet valve lets you isolate the instrument without touching the regulator setting.
- Needle valve: the “-A” outlet on the GP402, HP702 and HP722C is a 1/4″ MNPT needle valve, good for throttling.
- Diaphragm valve: the “-A” outlet on the HP742 is a 1/4″ FNPT diaphragm valve. It is packless and leak-tight (Harris rates its diaphragm valves at about 2×10-8), which makes it the better choice for UHP and corrosive service.
Harris can also supply helium leak certification (inboard or outboard) on the HP722C and HP742; contact us if your quality system requires it.
Step 5: Install and purge like a pro
High purity gas is easily contaminated during a cylinder change. Simply flowing gas through a regulator leaves dead pockets of air. Harris recommends dilution (cycle) purging with a tee or cross purge assembly:
- Close the regulator, vent valve and instrument valve, and open the regulator outlet valve.
- Briefly open and close the cylinder valve to pressurize the inlet.
- Set the delivery pressure, then open the vent to bleed.
- Repeat this cycle 3 to 5 times.
Then leak-test: with the regulator pressurized, close the cylinder valve and watch the gauges. If the inlet gauge drops while the delivery gauge rises, the seat is leaking and the regulator should go for repair.
Step 6: Maintain it
Harris’s maintenance schedule, which follows CGA E-15, for non-corrosive service is a leak and functional test at startup, then monthly for the first one to two years and twice monthly in years two to five; after five years consider refurbishment or replacement. Mildly corrosive service needs tests twice monthly for the first year and a purge at every shutdown. Corrosive service needs an inert purge at every startup and shutdown and tests at every startup.
A note on the HP742 diaphragm
The North American Harris HP742 we sell uses a 316L stainless steel diaphragm, per Harris’s North American catalog and product page. The Hastelloy C276 diaphragm sometimes mentioned online belongs to Harris’s separate European HPI 742 line, a different, metric product.
Quick picks
| Application | Our pick |
|---|---|
| GC carrier gas up to grade 5.0 | Harris HP702 |
| UHP GC carrier gas, grade 5.0 | Harris HP722C |
| Grade 6.0 gas or reactive calibration standards | Harris HP742 |
| Research oxygen | Harris HP722C CGA 540 |
| Corrosive gas (CGA 660) | Harris HP742 CGA 660 |
| Not listed | Contact us or call 1-888-641-2267 |
Frequently asked questions
Do I need a stainless steel regulator for high purity gas?
Not always. A plated brass barstock regulator with a 316L stainless diaphragm and PTFE seats, such as the Harris HP722C, is built for grade 5.0 ultra high purity and GC work. Choose the 316L stainless Harris HP742 for grade 6.0 and higher, for corrosive gases, or for corrosive surroundings.
Which Harris regulator should I use for GC carrier gas?
Match it to the gas grade. The HP702 is rated for gases up to grade 5.0 and Harris lists it for GC carrier and detector gases. The HP722C is the standard choice for grade 5.0 UHP gas. Use the HP742 for grade 6.0 gas or calibration standards with low-ppm reactive components.
Which regulator should I use for corrosive gases like chlorine or ammonia?
Use the Harris HP742 316L stainless steel regulator with the correct CGA, such as 660 for chlorine or 240 or 705 for ammonia. Brass regulators are not suitable. Fit a cross purge and purge with inert gas at every startup and shutdown.
Which regulator should I use for oxygen?
Harris recommends the HP702 or HP722C with a CGA 540 inlet for oxygen. Keep the regulator dedicated to oxygen and completely free of oil and grease, and open the cylinder valve slowly.
How do I purge a high purity regulator?
Use dilution purging through a tee or cross purge: pressurize the regulator, set the delivery pressure, vent it, and repeat the cycle 3 to 5 times. Simply flowing gas through leaves dead pockets of air in the regulator.
Is the Harris HP742 diaphragm 316L stainless or Hastelloy?
The North American Harris HP742 uses a 1-11/16 inch 316L stainless steel diaphragm. The Hastelloy C276 diaphragm belongs to the separate European HPI 742 line.
Sources
- Harris Products Group, Specialty Gas Equipment Catalog, North America, P/N 9505646 Rev E (2021): purity tiers, materials, leak rates, outlet valves, purging procedure.
- Harris The Proper Selection of a Regulator (2018) and condensed Specialty Gas catalog (2018): gas-by-gas recommendations.
- Harris Important Safety and Operating Instructions for Regulators and Cylinders (9500303 Rev B) and Harris maintenance tech tip (CGA E-15 schedule).
- Harris North American HP742 product page; Harris HPI 722/742 European datasheet (2025).
- LabRegulator Application Help page (brass vs stainless).