The capacitive boiler level probe is a continuous level transmitter that outputs an analogue 4–20 mA signal proportional to the water level in steam boilers, feedwater tanks and deaerators. The signal feeds a PLC, level controller or VFD for modulating boiler feedwater control (feed valve or pump speed), keeping the level steady instead of the on/off cycling of conventional electrodes. ISE supplies genuine probes from European brands such as ViRA and Gestra, rated to 32 bar and 239 °C.
Why does a boiler need continuous level measurement?
- Low water: risk of dry firing and overheating of tubes – serious boiler damage.
- High water: carryover of water with steam, wet steam and water hammer.
- On/off feed control: level and pressure swing, frequent pump starts and shorter pump life.
A 4–20 mA continuous level probe gives smooth feedwater control, stable steam pressure and a % level display. Boilers should still have local level indication such as a magnetic level gauge and an independent low-water alarm.
How a capacitive level probe works
The PTFE-coated probe rod and the boiler wall (or protection tube) form a capacitor; as the water level changes, capacitance changes. The electronics convert this to 4 mA at the set minimum level and 20 mA at the set maximum level, and display level as a percentage. Three push buttons (SET, MIN, MAX) and LEDs make field setup quick.
Advantages
- Continuous level measurement with standard 4–20 mA for PLC, controllers and VFDs.
- PN40 – 32 bar g and up to 239 °C, suitable for most industrial boilers.
- PTFE-coated probe rod, 300–1500 mm length to suit the measuring range.
- 4 mA / 20 mA points set by push buttons – no calibration equipment needed.
- Status and fault LEDs: above/below range, incorrect setting.
- 230 VAC or optional 24 VDC supply; ½” BSP thread for quick installation.
Capacitive boiler level probe specifications
| Parameter | Value |
|---|---|
| Measuring principle | Capacitive |
| Control characteristic | Continuous (modulating) level control |
| Output | 4–20 mA analogue |
| Connection | ½” BSP thread |
| Probe length | 300 – 1500 mm |
| Nominal pressure | PN40 |
| Max. operating pressure | 32 bar g |
| Max. operating temperature | 239 °C |
| Power supply | 230 VAC (+5 % / −10 %), 50/60 Hz; optional 24 VDC |
| Ambient temperature | 0 – 70 °C |
| Current loop | 2-wire ANSI/ISA Type IV (internal loop supply) or Type II (external 24 VAC/DC) |
| Max. loop load | 500 Ω |
Please specify probe length and supply voltage when ordering.
Installation
Two installation options, minimum tube diameter DN100:
External level chamber
Internal protection tube
- The level chamber or protection tube must be ≥ DN100 with balance holes so the level inside equals boiler level.
- Never remove the probe under pressure – depressurise to 0 bar g first.
- Disconnect cables before removing the probe.
Setting the 4–20 mA range
- Lower point (4 mA): bring the level to the lower point → hold SET 6 s until the left red LED lights → hold MIN 2 s.
- Upper point (20 mA): bring the level to the upper point → hold SET 6 s → hold MAX 2 s.
- Green LED on, red LEDs off: normal. All three LEDs flashing: incorrect setting (often swapped points) – set again.
- Green flashing + left red on: level below lower point; green flashing + right red on: level above upper point.
Boiler feedwater control applications
1. Modulating feedwater valve via a controller
2. Feed pump speed control by VFD
3. Feed pump control by PLC
Also used for feedwater tanks, deaerators, condensate tanks and other conductive or non-conductive liquids.
Maintenance
- Every 12 months remove, inspect and clean the probe rod with a cloth or soft brush.
- No chemicals or hard cleaners – they damage the PTFE coating.
- Re-check alarm points and valve/pump operation after maintenance.
FAQ
Capacitive probe vs on/off level electrodes?
Electrodes only switch at fixed points to start/stop the pump. A capacitive probe measures continuously (4–20 mA) for modulating valve or VFD pump control.
Can it replace a DP drum level system?
For small and medium fire-tube boilers, the capacitive probe is compact and economical. High-pressure water-tube boilers should use a DP boiler drum level measurement system.
Related products
- Boiler drum level measurement system
- Magnetic level gauge
- Liquid level controller relay
- Boiler TDS sensor
- Automatic TDS blowdown
Where to buy capacitive boiler level probes?
EQUIPMENT INDUSTRIAL SOLUTION CO., LTD (ISE) – Ho Chi Minh City, Vietnam – supplies genuine ViRA and Gestra boiler level probes with installation advice, range setup and feedwater control integration.
Contact us: Sales +84 909 617 185 – Technical +84 979 117 185 – Email: [email protected]
