Boiler Water Dissolved Oxygen Measurement: Trace (ppb) DO Monitoring
Verifying that oxygen at the deaerator outlet remains below target depends as much on the sample conditioning line as on sensor selection.
DO in Boiler Feedwater: Four Decisions
- Range: ppb level. Pyxis ST-774 0.1–2,000 ppb; Hamilton VisiTrace 0–2000 ppb; OxyGold G 1 ppb – 40 ppm.
- Sampling point: Deaerator storage tank, downstream of a sample cooler (not the feed pump discharge).
- Temperature: Sample 4–49°C (Pyxis); optical Hamilton sensors do not output DO readings above 85°C.
- Technology: Optical (maintenance: optical cap) or amperometric (maintenance: membrane, electrolyte, cathode).
What Oxygen Does in Boiler Feedwater
Feedwater is generally neutral and contains dissolved oxygen. Iron dissolves at the anode on the metal surface while oxygen is reduced at the cathode; the resulting ferrous hydroxide reacts with oxygen to form ferric hydroxide. This electrochemical corrosion accelerates as water temperature rises. Inside the boiler, when the protective iron oxide film is partially disrupted, a local cell forms and pitting corrosion progresses; chloride and sulfate ions intensify this effect in the presence of oxygen.
Economizer tubes are typically carbon steel, which is why inlet oxygen must be kept as low as possible. The deaerator itself can also experience oxygen and erosion corrosion at nozzles, tray connections and internal walls. When deaerator performance degrades, feedwater oxygen rises above the specified level and copper-nickel heater tubes can be damaged. Engineering references list dosing an oxygen scavenger at the deaerator inlet and controlling feedwater pH among the measures that reduce corrosion.
This mechanism leads to two conclusions. First, measurement must be taken at the deaerator outlet, because that is where deaerator performance is verified. Second, the measuring range must be in ppb: while the deaerator and oxygen scavenger are operating, concentrations are very low and a standard mg/L sensor cannot resolve them.
Source: Handbook of Water Treatment, boiler water chapter (oxygen corrosion, deaerator corrosion, oxygen scavengers). No numerical limit value is given here; the target value depends on your boiler pressure and plant/manufacturer specifications.
Trace-Range DO Sensors
| Sensor | Technology | Range | Condition | Output | Notes |
|---|---|---|---|---|---|
| Pyxis ST-774 (IK-2000 GUARDIAN) | Optical, fluorescence quenching | 0.1 – 2,000 ppb; plus/minus 0.3 ppb | Sample 4–49°C; 7.25–30 psi; 100–500 mL/min | Panel: 6× 4-20 mA, Modbus RTU/TCP | 316L body and flow cell; DCC-2 replaceable cartridge; field zero calibration with Sulfite-ZERO |
| Hamilton VisiTrace RS485 / mA | Optical | 0 – 2000 ppb | -20 to 140°C (no DO reading above 85°C); -1 to 12 bar; autoclave, CIP, SIP | RS485 Modbus RTU; mA: 2-wire 4-20 mA, HART (ATEX/IECEx) | L1 cap; brochure specifies weekly stability below 1% at 100 ppb, accuracy at 25°C plus/minus 0.5 ppb or 2% (whichever is greater); power plant and brewing applications |
| Hamilton OxyGold G | Amperometric | 1 ppb – 40 ppm | 0–130°C; 0–12 bar; autoclave, CIP, SIP | VP6 (120 and 225 mm); Arc version | Hamilton specifies boiler feedwater and microelectronics as typical applications; cathode-only replacement |
| Hamilton OxyGold B | Amperometric | 8 ppb – 40 ppm | 0–100°C; 0–12 bar; CIP | VP6 (120 and 225 mm) | Developed for CO2-containing media; water deaeration listed among typical applications |
Values not shown in the table (such as a separate pressure rating for ST-774) are omitted because they are not stated in the source document. Sources: Pyxis GUARDIAN datasheet, Hamilton Process Analytics Catalog 2026, Hamilton L30011-04 brochure.
Sampling and Panel Installation
| Topic | Manufacturer requirement or reference note |
|---|---|
| Sampling point | Deaerator storage tank; not the feed pump discharge (Pyxis GUARDIAN) |
| Cooling | Panel connects downstream of a sample cooler; sample 4–49°C |
| Pressure and flow | 7.25–30 psi; 100–500 mL/min |
| Panel | 600×800×335 mm, approximately 25 kg; panel IP-65, sensors IP-67; 304/316L stainless steel SwageLok fittings |
| Air contact | Engineering references state that oxygen samples must be taken from a flowing line without contact with air; hot samples are cooled inline first |
| Temperature limit (Hamilton optical) | VisiFerm and VisiTrace do not output DO readings above 85°C |
GUARDIAN has four configurations: IK-2000 includes only ST-774, IK-2010 adds sulfite + pH + ORP (ST-765SS-SO3), IK-2020 adds fluorescein (ST-525SS-T, 0.00–60.0 ppb), and IK-2030 adds conductivity (ST-724, 0.02–1,000 µS/cm). The panel layout is identical across all tiers, so adding sensors later does not require replacing the panel hardware. The display is 7 inches and the 4 GB memory stores approximately 1 million records.
Calibration, Maintenance and Troubleshooting
On the Pyxis ST-774, the zero point is calibrated in the field with the Sulfite-ZERO calibration kit, and the pre-loaded replaceable cartridge (DCC-2) is renewed as a unit. On Hamilton optical sensors, maintenance consists of cap replacement; on the amperometric OxyGold G, the OxyGold Membrane Kit, Oxylyte G electrolyte and a spare cathode are used, and spare sensors can be pre-polarized on a Polarization Module. See Hamilton oxygen sensor accessories for details.
| Symptom | Possible cause | Action |
|---|---|---|
| DO reads high although deaerator operates normally | Air ingress or fitting leak in sample line; sample taken from pump discharge | Move sampling point to deaerator storage tank, inspect fittings, flush line with flowing sample |
| No reading or frozen reading | Sample temperature above 85°C on Hamilton optical sensor | Check sample cooler operation |
| Zero point drift | Cap or cartridge aging, or zero calibration overdue | Perform zero calibration with Sulfite-ZERO on ST-774; follow Hamilton calibration procedure on Hamilton sensors |
| Panel operating outside manufacturer conditions | Sample flow outside 100–500 mL/min or pressure outside 7.25–30 psi (Pyxis) | Bring sample flow and pressure into manufacturer range |
| New amperometric sensor takes a long time to settle | Polarization time | Pre-polarize using a Polarization Module |
For other boiler house parameters, see boiler, condensate and steam generator automation and boiler water conductivity. For feedwater silica, see silica measurement.
Frequently Asked Questions on Dissolved Oxygen in Boiler Feedwater
Why trace DO is required, sensor selection, sampling point, sample temperature and sulfite monitoring.
Why is dissolved oxygen measured at the ppb level in boiler feedwater?
In feedwater, oxygen drives a corrosion cell on iron surfaces, and its effect accelerates as temperature and pressure increase. For this reason the target is in ppb rather than mg/L, and measurement is performed with a trace-range sensor. Engineering references state that when deaerator performance drops, oxygen in the feedwater threatens economizer and heater tubes, and an oxygen scavenger can be dosed into the deaerator inlet.
Which dissolved oxygen sensor is used for boiler feedwater?
A trace-range sensor is required: Pyxis ST-774 (0.1–2,000 ppb, part of the IK-2000 GUARDIAN panel), Hamilton VisiTrace (0–2000 ppb, optical) or Hamilton OxyGold G (1 ppb – 40 ppm, amperometric; Hamilton specifies boiler feedwater as a typical application). Standard mg/L sensors (such as Pyxis ST-772, 0.004–20 mg/L) are intended for wastewater and pools.
Why must the sample be cooled?
The Pyxis GUARDIAN panel must be connected downstream of a sample cooler and the sample temperature must remain within 4–49°C. Hamilton VisiFerm and VisiTrace sensors do not provide DO readings above 85°C; even though the sensor withstands sterilization up to 140°C, the sample must be cooled below 85°C for measurement.
Should the sample be taken from the feed pump discharge or the deaerator storage tank?
For the Pyxis GUARDIAN, the manufacturer requires the panel to be fed from the deaerator storage tank; it must not be fed from the feed pump discharge. Sample pressure must be 7.25–30 psi and sample flow 100–500 mL/min.
Optical or amperometric?
An optical sensor requires no polarization and no membrane maintenance, consumes no oxygen, and measures more accurately in low-flow, low-DO conditions (Hamilton). The amperometric OxyGold G offers trace measurement under high pressure (12 bar) and sterilization up to 130°C and allows replacing only the cathode. Selection depends on maintenance preference and existing process connections; see optical dissolved oxygen sensors for details.
How is oxygen scavenger (sulfite) dosing verified?
The Pyxis IK-2010 GUARDIAN tier adds the ST-765SS-SO3 sulfite + pH + ORP sensor to the ST-774; sulfite is read over a 0.00–100.00 ppm range. This allows both residual oxygen and oxygen scavenger in the feedwater to be monitored from the same panel.
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