Boiler Water Conductivity Measurement: Automatic Blowdown and Sensor Selection
Conductivity is the continuously measurable indicator of dissolved solids concentration in boiler water; surface blowdown is controlled from this value.
How Conductivity Is Used in Boiler Water
- Why: Evaporation concentrates boiler water. As conductivity rises, carryover and foaming risks grow; keeping it too low wastes water and energy.
- How: The controller compares boiler water conductivity against the setpoint; when exceeded, it opens the surface blowdown via a proportional or motorized valve and fresh feedwater enters.
- Which sensor: Boiler-style contacting sensor, cell constant 1.0 (0–30,000 µS/cm) or 10.0 (0–300,000 µS/cm); 3/4" NPTM, 316 stainless steel + PEEK, rated for 16.7 bar and 205 °C.
- Caution: The measuring range narrows with temperature; at 100 °C it is 35.7 percent of the 25 °C range.
Surface Blowdown and Cycles of Concentration
A steam system is not a single water line, but three lines: feedwater entering the boiler, boiler water inside the boiler, and condensate returning from the process. As water evaporates and leaves the boiler, dissolved solids remain behind and concentration rises. If concentration exceeds the target band, solids are carried over with steam (carryover) and foaming risk increases; if it remains below target, excessive water and energy are consumed.
The solution is surface blowdown: a portion of the concentrated boiler water is discharged and replaced by fresh feedwater. Walchem controllers automate this via a proportional or motorized valve referenced to the conductivity (or TDS) in the boiler. Excessive blowdown is a direct energy loss: GE's industrial water reuse handbook explains that increasing cycles of concentration reduces heat lost through blowdown and delivers operating cost savings. Cycles of concentration is in practice the ratio of boiler water conductivity to feedwater conductivity, which is why two measurements are required: boiler water and feedwater.
The blowdown logic in a cooling tower is identical; see our cooling tower automation guide for details. The difference in a boiler is that temperature and pressure are much higher and the setpoint comes from the boiler manufacturer's water quality specification.
Three Separate Conductivity Measurements on Three Lines
| Line | Purpose | Sensor | Range | Control |
|---|---|---|---|---|
| Boiler water | Surface blowdown, carryover and foaming limit | Walchem contacting boiler sensor, 316 SS + PEEK, 3/4" NPTM | Cell constant 1.0: 0–30,000 µS/cm; for W100/W600 cell constant 10.0: 0–300,000 µS/cm | Blowdown valve relay; setpoint and alarm |
| Feedwater | Denominator of cycles of concentration; softener and deaerator performance | Pyxis ST-724 (top tier of the GUARDIAN panel) or low-cell-constant contacting sensor | ST-724: 0.02–1,000 µS/cm, cell constant 0.3, Hastelloy 2-electrode | Panel TDS display; cycles of concentration together with blowdown controller |
| Condensate return | Early detection of process-side contamination | Cell constant 0.01 or 0.1 contacting sensor (W100/W600 compatible) or Hefer C-2030 transmitter | 0.01: 0–0.3 mS; 0.1: 0–3 mS | Alarm and logging |
Boiler Contacting Conductivity Sensor: Selection Criteria
| Specification | Value (Walchem brochure) |
|---|---|
| Cell constant and range | 1.0: 0–30 mS (25 °C); for W100/W600 10.0: 0–300 mS |
| Temperature | 0–205 °C (32–401 °F) in typical boiler application |
| Pressure | 250 PSI (approximately 16.7 bar) |
| Material and connection | 316 stainless steel + PEEK, 3/4" NPTM; 6 inch pigtail cable or with J-box |
| Temperature element | Models with or without ATC via RTD (PT1000) |
| Controller compatibility | WebMaster: active sensor; W400, W600, W100: passive sensor |
Temperature multiplier. Ranges are specified at 25 °C. In Walchem's table, the range multiplier is 181.3% at 0 °C, 100% at 25 °C, 75.5% at 40 °C, 55.6% at 60 °C, 43.5% at 80 °C, 35.7% at 100 °C, 30.4% at 120 °C and 25.5% at 150 °C. Example: for a 0–30 mS sensor, the upper limit at 100 °C drops to 30 × 0.357 ≈ 10.7 mS. If your boiler water setpoint approaches this value, a more appropriate cell constant range should be selected instead of 1.0.
Fouling and maintenance. A contacting sensor is ideal in waters that do not leave oil or coating films (boiler, cooling tower, RO, oil-free condensate). The Walchem W900 manual requires recalibration after cleaning the electrode. Keeping calibration records tied to cleaning and mounting changes is essential for reliable blowdown control.
Which Controller Manages Blowdown?
| Controller | What it provides for boilers | Notes |
|---|---|---|
| Walchem Intuition-6 | Compact solution for single-boiler control; conductivity, pH and blowdown automation; 6 relay outputs | Contacting conductivity from 0–300 µS/cm to 0–300,000 µS/cm (4 cell constants) |
| Walchem Intuition-9 | Automatic boiler surface blowdown, chemical dosing pumps and LPR corrosion monitoring; 24 sensor inputs, 12 relays | Multi-boiler and cooling tower systems |
| Walchem WIND | Automatic blowdown and chemical dosing based on conductivity measurement | Cooling towers and steam boilers |
| Walchem W600 | Conductivity, pH, ORP and disinfectant; network connectivity | Contacting and inductive conductivity sensors |
| Hefer C-2030 | DIN-rail mounted conductivity transmitter, 2-wire 4-20 mA, 10–30 VDC | Boiler house control panels |
Feedwater Analyzer: GUARDIAN and ST-724
The Pyxis IK-2000 GUARDIAN combines up to four sensors in tiers on a single panel: dissolved oxygen, sulfite + pH + ORP, fluorescein and conductivity. The top tier, IK-2030, adds the ST-724 sensor for conductivity. With its Hastelloy 2-electrode cell design, the ST-724 measures over 0.02–1,000 µS/cm with a cell constant of 0.3; the panel interface includes a user-defined TDS conversion, making blowdown cycles of concentration measurable when used alongside an existing boiler blowdown controller.
The manufacturer requires the panel to be fed downstream of a sample cooler and from the deaerator storage tank (not from the feed pump discharge); sample temperature must be within 4–49 °C. Evaluating feedwater, boiler water and condensate measurements together makes it much faster to pinpoint the source of an excursion (softener breakthrough, deaerator issue or condensate contamination).
Conductivity Alone Is Not Enough
Conductivity gives the total ionic load of boiler water, but does not identify which ion is present. Hardness breakthrough (softener resin exhaustion), alkalinity balance, dissolved oxygen (deaerator performance) and pH are monitored separately. Related pages: boiler, condensate and steam generator water quality monitoring · dissolved oxygen · Walchem conductivity sensors.
For the same control principle in cooling towers, see our cooling tower automation guide.
Boiler Water Conductivity Measurement: Frequently Asked Questions
Blowdown control, cell constant, temperature multiplier and feedwater measurement.
Why is conductivity measured in boiler water?
As evaporation continues, the concentration of dissolved solids inside the boiler—and with it, conductivity—rises. If the concentration exceeds the target band, the risk of solids carryover with steam and foaming increases; if it stays below target, water and energy are wasted unnecessarily. Conductivity is the only continuously measurable indicator of this concentration and governs the surface blowdown valve.
What should the boiler water conductivity setpoint be?
There is no single number. The setpoint is determined by the boiler manufacturer and the water treatment program according to boiler type, operating pressure, feedwater quality and chemical treatment regime. On the sensor side, what matters is that the setpoint falls within the sensor's measuring range at operating temperature; check the temperature range multiplier table below for this purpose.
How is the cell constant selected for a boiler conductivity sensor?
Among Walchem boiler sensors, the cell constant 1.0 model covers 0–30,000 µS/cm at 25 °C. The cell constant 10.0 model offered for W100 and W600 controllers extends up to 0–300,000 µS/cm in boiler water. In very low-conductivity lines such as condensate and feedwater, smaller cell constants (0.01 and 0.1; covering 0–0.3 mS and 0–3 mS respectively) are used.
Why does the sensor's measuring range narrow in hot boiler water?
Conductivity increases with temperature. In Walchem's range multiplier table, the range at 25 °C is taken as 100%; it drops to 55.6% at 60 °C, 35.7% at 100 °C and 25.5% at 150 °C. In other words, the upper limit that a 0–30 mS sensor can read at 100 °C, normalized to 25 °C, is approximately 10.7 mS (30 × 0.357). Verify your setpoint against this narrowed range.
What is the difference between active and passive conductivity sensors?
WebMaster series controllers require an active sensor with electronics that convert the sensor signal into a voltage the controller can read. In contacting boiler sensors, W400, W600 and W100 series controllers operate with a passive sensor; cable wiring differs by controller series. With active sensors, extension cable length can reach up to 250 ft (approximately 76 m).
Should conductivity also be monitored in feedwater and condensate lines?
Yes, but with a different sensor. In the condensate return line, a conductivity excursion is an early sign of process-side contamination, which carries the problem into the boiler if it enters undetected. In feedwater, very low conductivity must be measured: the ST-724 sensor of the Pyxis GUARDIAN panel measures over 0.02–1,000 µS/cm and makes boiler cycles of concentration measurable.
Can the sensor be replaced without changing the existing blowdown controller?
In most cases, yes. Share the make and model of your existing instrument, its cell constant and process connection, and the equivalent with the same connection and measuring range can be identified. First, check whether the controller has an active or passive sensor input.
Send the make and model of your current instrument to [email protected] — our engineers will propose the equivalent with the same connection and measuring range.
Planning a project? Send us your flow rate, feed water source and target water quality to [email protected] — our engineers will reply with a technical proposal.