Fischer DS21 ATEX Industrial DP Switch – thermal oil flow monitor

Fischer DS21 is explosion proof hazardous area approved for Zone 1, 2 (gas explosion) and zone 22 (dry dust explosion) highly rugged, industrial differential pressure switch, specifically designed for flow monitoring and control of thermal oil and hot water. It is designed to meet DIN 32 727 and VdTUV compliance of flow control.

DS21 comes with adjustable limit contacts and dial indicator.


Fischer DS21 – ATEX approved DP Switch for thermal oil, hot water flow control

pdf-imageexplosion proof (zone 1, 2, 22) DP switch, for thermal oil, hot water flow control acc. DIN 32 727, for thermal oil, hot water, rugged & industrial design
Make: Fischer Messtechnik (Germany)
Model: DS21


Fischer DS21 is highly rugged, industrial grade explosion proof differential pressure switch, custom designed for thermal oil, hot water flow monitoring application. It can be used as flow-operation safety device in heat carrier oil plants, thermal oil application according to DIN 32 727 and hot water plants according to VdTUV data sheet flow 100.

It can be installed in explosion – hazardous area zone 1 and zone 2 (hazardous gas, explosion proof) as well as zone 22 (hazardous dry dust) if they are connected to certified intrinsically safe circuits.


  • EC type approval for application in explosion hazardous areas
    TUV 06 ATEX 2964

    • marking for usage in zone 1 and 2:  Ex II 2G EEx ib c IIC T6
    • zone 22:  Ex II 3D c T70° C IP65
  • Type approval acc. to DIN 32 727 in conjunction with differential pressure devices as flow operation safety device for heat transfer plants
    DIN record No. 1B012 / 07
  • Structural testing acc. to VdTUV datasheet flow 100 in conjunction with differential pressure devices as flow control and flow limiting device in hot water plants.
    Component marking: TUV SW/SB 07-020
  • EC type approval acc. to directive 97/23/EC (pressure equipment directive (PED)) as equipment accessory with safety function.
    Certificate No. 07 202 5435Z 0063 / 2/2
  • Type test according to the German Lloyd directives for marine applications.
    Certificate No. 93 828-88 HH Quality mark

The flow operation safety devices consists of differential pressure device, eg. orifice plate, differential pressure switch and adequate shut-off valves.

Fischer DS21 - ATEX Industrial DP Switch - thermal oil, water
Fischer DS21 ATEX, Industrial DP Switch – thermal oil, water

 Working Principle:

DS21 is based on rugged and uncomplicated diaphragm movement. It is suitable for overpressure, partial vacuum and differential pressure measurements.

The forces of springs on each side of diaphragm is balanced in a state of equilibrium. The applied pressure or differential pressure will creates an unbalanced force at the diaphragm. This force moves the diaphragm system against the force of the springs for the measuring range until a new equilibrium is reached. When subjected to excessive pressure, the diaphragm rests on metal supporting plates.

A centre-mounted tappet transfers the motion of the diaphragm system to the motion work and to the actuating elements of the microswitches.


Key Features:

  • High sensitivity
  • High repeatability of switching points
  • Long service life
  • Robust Design, over pressure protected

Key Specifications:

  • Application: flow switch / control – Thermal oil DIN  32 727 or hot water Flow 100
  • Measuring range: 0 ~ 250 mbar to 0 ~ 6 bar
  • Nominal pressure: 25 bar
  • Max. static operating pressure: 6 ~ 16 bar
  • Max. pressure load: one sided overpressure protected up to nominal pressure on (+) and (-) side of diaphragm, partial vacuum protected
  • Accuracy: ± 2.5% of full scale
  • Operating ambient temperature: -10° C to +70° C
  • Operating media temperature: +70° C
  • Protection class: IP54 acc. to DIN EN 60529
  • Mounting: Vertical
  • Zero adjustment: located on dial
  • Contact output: 1 or 2 microswitches, 1-channel change-over contacts
  • Pressure connection: Thread G 1/4 female, cutting ring connection for 6,8,10,12 mm diameter tube of brass, zinced steel or chrome nickel steel
  • Measuring system: diaphragm, of reinforced Viton

Application areas:

  • Thermal oil flow monitor and control
  • Hot water flow monitoring and control

Industrial areas:

  • Building heating system
  • Power plants, boiler application
  • Clean room and laboratory rooms