SHT 8824 – active water sensor with 4–20 mA current loop

The active water sensor with 4–20 mA current loop for PLC, building management systems and controllers

The Schabus SHT 8824 is an active water sensor that operates according to the switching principle of a controlled constant current sink – fully in line with the industrial standard “4–20 mA current loop”. In the dry state it draws approx. 4 mA; on contact with water the current consumption rises to approx. 20 mA. This clear change in current can be evaluated unambiguously and interference-free by PLCs, building management systems (BMS) and other controllers with an analogue current input.

The sensor is supplied with operating voltage directly via the signal line and therefore requires only two wires. It is so sensitive that it even reliably detects “normal” distilled water according to VDE 0510-5 in DIY-store quality. Two contacts made of V4A stainless steel, potting in Wepuran® and an aluminium-reinforced mounting hole make it a robust floor sensor.

The SHT 8824 is intended for regularly dry environments with only occasional water occurrence. It is not suitable for direct connection to Schabus water detectors.

Precise wet/dry detection via current signal

One standard, one sensor: The SHT 8824 delivers a clear 4–20 mA signal for further evaluation.

How the controlled constant current sink works

The input of your controller supplies the voltage and measures the current – the sensor only controls its current consumption.

• When dry the sensor draws approx. 4 mA; on contact with water the current consumption rises to approx. 20 mA
• An analogue current interface requires an active input for a passive 2-wire sensor
• The input measures the current and has a voltage source; the sensor is merely a controlled current sink
• As soon as the water has disappeared again, the loop current returns to the dry value
• The level of the wet current depends on the conductivity of the liquid and the operating voltage: the higher the conductivity, the sooner the wet current is reached
• Only completely ion-free ultra-pure water cannot be reliably detected for physical reasons – a few free ions must be present

What’s in the box

The sensor is supplied with the material for mounting.

• SHT 8824 active water sensor
• Mounting material (1 screw, 1 wall plug)
• Operating instructions

How the SHT 8824 is designed

The SHT 8824 is a compact, fully potted sensor with a permanently connected supply cable.

Mounting and connection

Read the operating instructions carefully before mounting and commissioning.

Mounting the sensor

1. Screw the sensor firmly in place with the screw and wall plug via the integrated aluminium mounting hole.
2. Place the two V4A stainless steel contacts at the lowest point to be monitored. A defined installation position is not required, as the sensor reacts directly to liquid contact.
3. If the cable is too short, it can be extended to a maximum of 20 m, e.g. with LIYY cable (article 300780). Ensure good connections and a clean cable route.

Connecting the sensor

1. Connect the sensor to an active current input (2-wire interface): brown wire to plus (Ub), white wire to minus (0 V).
2. Provide an operating voltage of 10 to 30 V DC (typ. 24 V DC, SELV/PELV). The load may be a maximum of 500 Ω at Ub < 24 V DC and a maximum of 700 Ω at Ub ≥ 24 V DC.
3. Evaluate the current signal in the controller: approx. 4 mA means “dry”, approx. 20 mA means “wet”. A current below 3 mA indicates a sensor failure or cable break.

Current values in operation

The actual current values depend on the conductivity of the liquid and the operating voltage.

• Dry: 4 mA at Ub = 24 V, actual 3.6 to 4.2 mA
• Wet: 19 to 20 mA at Ub = 24 V, actual 14.1 to 20.3 mA
• Highest current: max. 21 mA at Ub = 30 V DC
• Lowest current: min. 3.6 mA at Ub = 10 V DC (sensor dry)

Manufacturer’s notes

• Read the operating instructions carefully before mounting and commissioning and keep them
• Do not open the device; it contains no parts that can be serviced by the user
• Packaging material is not a toy and must be kept away from children
• Operate only with operating voltage according to SELV/PELV
• Avoid moisture (splash or rain water), dust and direct sunlight on the device
• Clean only with a dry linen cloth; if heavily soiled, slightly dampened; do not use solvent-based cleaning agents
• Intended for residential areas, commercial and business premises as well as small businesses
• The manufacturer is not liable for damage or loss resulting from the fact that no alarm signal is issued despite changes in water level
• The CE declaration is available for download on the manufacturer’s website

Statutory warranty, guarantee and disposal

The device is covered by the statutory warranty of at least 2 years in the EU. The manufacturer Elektrotechnik Schabus also offers a 5-year guarantee that can be registered online. Repairs and spare parts are available directly from the manufacturer. In the event of a defect, please contact the manufacturer; within Germany you will receive a free return label by e-mail.

The device does not belong in household waste. The crossed-out wheeled bin symbol reminds you to use the take-back systems for waste electrical and electronic equipment (WEEE No. DE91394868).

Why the SHT 8824 is suitable

Areas of application

• Home automation with analogue current input
• PLC systems with 4–20 mA input and digital inputs on PLCs
• Building management systems (BMS) and SCADA systems
• Controllers that evaluate an analogue current input
• Data centres and server rooms
• Industrial and production halls
• Cooling water and air-conditioning systems
• Pharmaceutical and food production

The Schabus SHT 8824 delivers a standards-compliant current signal wherever 4–20 mA is the standard – for precise differentiation between wet and dry.

Further active water sensors from Schabus

• SHT 8800 (Art. No. 200362) – active water sensor with analogue voltage output, same design, 10–30 V DC, 3-wire connection, 4 m cable 3 × 0.14 mm² LIYY
• SHT 8815 (Art. No. 200370) – active water sensor as unipolar resistor, same design, 10–30 V DC, operation with external pull-up/pull-down resistors of 10–100 kΩ, ideal for digital logic inputs, 2-wire connection, 4 m cable 2 × 0.14 mm² LIYY

Frequently asked questions