A weld purge monitor tells you how much oxygen is left in the purge gas, so you start welding on a reading instead of a timer. This guide covers what level to purge to, how to choose a monitor that can actually confirm that level, and how to use it on the job.
Why purge by reading, not by time
Purging for a set time is not a measurement. Turbulence, a poor seal, pipe geometry or a blocked vent can leave enough oxygen to discolour the root (“sugaring”), cut corrosion resistance, and in bad cases weaken the weld. A monitor at the exhaust shows when the purge has actually reached your procedure’s limit — and stops you wasting argon purging longer than needed.
What oxygen level do you need?
Your welding procedure or governing code sets the limit. Two published reference points for stainless steel:
- The Australian Stainless Steel Development Association recommends below 50 ppm oxygen while the metal is above about 250°C, to hold the root to no more than a pale straw tint (AS/NZS 1554.6 and AWS D18.1 level 3).
- A 2019 study on TIG-welded AISI 304 pipe (DYNA, vol. 86) found acceptable heat tint below 50 ppm, the best result at 10 ppm, and tensile fractures in the weld bead at 100 ppm and above.
Titanium, duplex and other reactive or high-alloy materials usually need tighter limits. Check the procedure before choosing a monitor.
Resolution: the spec that decides the monitor
A monitor can only confirm a limit it can resolve. One that reads in 100 ppm steps shows 0 or 100 — it cannot tell you whether you are at 40 ppm or 90 ppm. So match the monitor’s resolution to the lowest limit you work to.
| Your procedure’s limit | Monitor resolution needed | Aquasol monitor |
|---|---|---|
| 100 ppm or higher | 100 ppm or finer | POM-100B (100 ppm) |
| Below 100 ppm (e.g. 50 or 10 ppm) | Finer than the limit | POM-5B (5 ppm) |
If you work to both, the POM-5B covers both. A POM-100B can also be upgraded to a 5 ppm sensor through factory service.
Other features worth checking
- Internal sampling pump — draws gas continuously from the exhaust so the reading keeps up with the purge.
- Data logging — for QA records. The POM-100B and POM-5B log through the POM Connect app and export CSV or text files.
- Set-point alarms — a red/amber/green display and ppm alarm make it obvious when you are clear to weld.
- Calibration — both POM kits calibrate with ambient air and 99.999% argon and ship factory calibrated with a dated certificate.
- Durability — rechargeable battery and a dust- and water-resistant case for shop and field use.
How to use a purge monitor
- Seal the purge zone — with inflatable purge bladders, water-soluble dams or tape.
- Connect the monitor’s sample line to the purge exhaust (the white hose on Aquasol bladder systems).
- Start the purge and watch the reading fall.
- Weld when the reading is below your procedure’s limit, and keep monitoring through the root pass.
- Save the log if your QA plan requires a record.
Frequently asked questions
What oxygen level should I purge to for stainless steel?
Follow your welding procedure. As a reference point, the Australian Stainless Steel Development Association recommends below 50 ppm while the metal is above about 250°C to hold the root to a pale straw tint.
Can a 100 ppm monitor confirm a 50 ppm limit?
No. A monitor that reads in 100 ppm steps cannot tell 50 ppm from 100 ppm. Use a monitor with finer resolution, such as the 5 ppm Aquasol POM-5B.
Where do I connect the monitor?
At the purge exhaust. On Aquasol I-Purge and I-Purge X bladder systems the white hose is the exhaust connection for an oxygen monitor.
Can I record readings for QA?
Yes. The Aquasol POM-100B and POM-5B log readings through the POM Connect app and export them as CSV or text files.
Compare Aquasol weld purge monitors or request a quote — call 1-866-277-0661.