SM4418 bubbler level sensor

SM4418 is a water level measurement system based on the bubbler principle, and it replaces the existing SM4416. The system is connected to a compressed air source with a pressure regulator, based on a compressor or compressed air cylinder, and a measurement tube or orifice. The measurement tube is installed between the bubbler system and the point of measurement.

The water in the measurement tube is purged at commissioning time. This can also be done manually by pressing a button in the cabinet door or from remote from the IED/Datalogger.

A measurement is performed regularly by injecting a small burst of compressed air into the measurement tube. If the pressure in the measurement tube is higher than the water level, air will egress from the end of the measurement tube and push out all water from the measurement tube. This will handle variation of the water level in the reservoir.

The pressure in the measurement tube is measured, and this pressure is the same as the water level in the reservoir. The pressure must stabilize before a measurement can be performed. The time between the burst of compressed air and the measurement depends on the water level, how much compressed air is used in the burst and length of the measurement tube.

The pressure in the compressor or in compressed air cylinders is reduced by a pressure regulator that is adjusted to a value that is a little higher than the maximum water level. This is done once at commissioning time of the system.

The amount of compressed air in each burst can be adjusted by the rotameter at commissioning of the system. This makes it possible to optimize the bubbler system for operation from solar panels and using compressed air cylinders. The amount of compressed air in the burst is reduced to the minimum level needed by the measurement. This approach uses significantly less compressed air compared to a constant flow bubbler system.

The pressure in the measurement tube can be read from a manometer inside the cabinet.

The bubbler system is controlled by digital outputs from a datalogger. Three or four digital outputs are used, and they are equipped with relays. Supported IED/Datalogger are:

  • Scanmatic SM5041
  • Scanmatic SM5049
  • Scanmatic SM5059
  • Sutron 9210B
  • Sutron Expert2


SM4418 is a flexible platform that can be equipped with several types of pressure sensors used for different measurement ranges and accuracy:

  • Ott PLS: gauge sensor with 0.05 % accuracy and a SDI-12 interface.
  • GE PTX5032: gauge sensor with 0.04 % accuracy and a 4 – 20 mA interface.
  • GE DPS8000: Absolute sensor with 0.02 % or 0.01 % accuracy and a RS232 interface.
  • Paroscientific Digiquartz 6000: gauge sensor with 0.01 % accuracy and a RS232 interface.


The pressure sensor is connected to an interface on the IED/datalogger e.g. SDI-12, 4 – 20 mA or RS232.

SM4418 can use several types of measurement tubes with length up to 500 m. Measurement tubes with heating (Elvestad tube) and without heating are supported. The heater is used to prevent the water from freezing in the measurement tube. Scanmatic recommend using the Elvestad tube (6 mm internal diameter and 25 mm external diameter) since this is a robust copper tube with internal heating. The heater will only be activated if the water freezes in the measurement tube. This will not happen if the bubbler system is continuously in operation.

The measurement tube must be installed with a continuous downslope to prevent creation of a water trap in the measurement tube. A water trap will disturb the measurements. The measurement tube must be protected against icing, and it should be installed in protective tubing bolted to rock or buried in bottom sediments in the reservoir.

The measurement tube is terminated in a bubble chamber to prevent that it is blocked by sediments.

SM4418 support the use of a compressor or compressed air cylinders. The maximum air pressure in the measurement tube is 10 Bar. The pressure tank in the compressor or the compressed air cylinder are equipped with pressure sensor connected to the datalogger analog input (4 – 20 mA). The system supports one or two compressed air cylinders.

The system is equipped with a humidity absorption system to remove humidity in the compressed air from the compressor. This is not necessary for compressed air cylinders using dry air.

Key parameters:

  • Measurement range: 0 – 100 m
  • Accuracy better than +/- 1 cm
  • Temperature range: -40 OC – 50 OC
  • Compressed air source
    • Compressor
    • One or two compressed air cylinders
  • Power supply: +12V DC/+24 V DC
  • Measurement tube length: 0 – 500 m
  • Size: 400x600x200 mm

Nils Lofstad

Energy & Environment
+47 91 64 79 22