Fischer NK21 – Conductive level switch, dirty liquids, multi-point, corrosion-resistant

Fischer NK21 is accurate, robust liquid level switch with LED indicator. With its adjustable probe length, corrosion-resistant electrodes, it is suitable for conductive media, contaminated liquid eg. sewage, effluent discharge etc.


Fischer NK21 – Conductive liquid level switch monitor, for contaminated media, adjustable probe length

pdf-imageAccurate, liquid level switch with LED indicator, for conductive liquid, contaminated liquids
Make: Fischer Messtechnik (Germany)
Model: NK21


Fischer NK21 is accurate level control switch and monitor for conductive liquid, suitable for contaminated media. It has adjustable probe length with low assembly cost. It has LED progress indicator.

Fischer NK21 - liquid level switch for conductive liquids, with LED indicator
Fischer NK21 – liquid level switch for conductive liquids, with LED indicator

Working Principle:

Fischer Nk21 is level control switch suitable for level detection in containers with electrically conductive media. It works on principle of conductivity measurement and is suitable for numerous measurement parameters.

NK21 has three measuring probes (electrodes) and common probe (com), that can also be connected to the container if required. The probe length can be optionally shortened and therefore adapted to the process.

At the electrodes, there is a low AC voltage. An AC current flows when electrodes comes in contact with conductive medium. This current is analysed by the integrated electronics. The threshold limit (resistance range of the medium) can be adjusted in 10 steps by the user.

Three photoMOS contacts are available as output signal, the switching function of which can be set onsite. The contacts are electrically isolated from the measuring circuit. The switching status is indicated by light emitting diodes.

Another light emitting diode indicates the operating status of the device.


Key Features:

  • Corrosion resistant
  • Suitable for contaminated, dirty media eg. sewage, effluent, alcohol discharge
  • Robust and rugged design
  • Adjustable probe length and switching function
  • Affordable cost
  • Integrated electronic module
  • High interference immunity
  • Electric isolation
  • LED progress indicator

Key Specifications:

  • Permissible ambient temperature: -10° C to +70° C
  • Permissible media temperature: +60° C
  • Max. Operating pressure: 16 bar
  • Connection thread: G 1 1/2 in
  • Ingress protection: IP68
  • Probe length: 2000 mm (other length available on request)
  • Measuring frequency: 120 Hz
  • Measuring voltage: max. 5 V AC (at the electrodes rods)
  • Sensitivity: 5 – 60 kOhm (adjustable in steps)
  • Hysteresis: 1.5 kOhm
  • Minimum conductivity of media: 2 µS/cm
  • Operating voltage: 24 V AC / DC +/- 15%
  • Output: 3 photoMOS relays
  • Contact function: break contact / make contact (configurable at site)
  • Reference potential: non-floating or floating
  • Housing: polyoxymethylene (POM)
  • Material, electrodes: stainless steel 1.4404
  • Electrode coating: Kynar® shrink-fit tubing


Application areas:

  • Monitoring of minimum or maximum filling levels of vessels
  • Protection of pumps against dry running

Industrial areas:

  • Power plants
  • Marine, shipping, ship building
  • Water, effluent treatment system