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Watson Smith - Controlling Pressure Worldwide
Type 421 Failsafe Compact Electronic Converter Current to Pressure (I/P)
Manifold or Rail Mount


 Advanced electronic control
 High density rail or manifold mount
 Failsafe operation (output pressure falls to minimum
    on power failure)
 Vibration immune

General Description

The 421 proportional I/P converter uses advanced electronic control to achieve outstanding performance. It is highly reliable with long life, is free from the effects of vibration and its high density mounting capability makes it ideal for control room applications,

The manifold system, usually surface mounted, allows several converters to be connected to a single air supply and is available in different lengths.

Type 421 Failsafe Electronic Converter

Functional Symbol

Technical Data
*Output Signal *0.2-1bar (3-15psig); minimum outlet pressure less than 15mbar (0.2psig)
*Air Supply Oil free, dry air, filtered to 5 microns; 1.5-3.5bar (20-50psig) or at least 0.7bar above maximum output pressure
*Flow Capacity Up to 150Nl/min (5scfm)
*Air Consumption 0.2 l/min typical low pressure (0.007scfm)
*Response Time 5 seconds (10 to 90% or 90 to 10% of output pressure into a 5 litre load
*Total Error +/-0.25% of span (typical, independent error includes the combined effect of non-linearity, hysteresis, deadzone and repeatability
*Temperature Effect Typically less than 1% span for span and zero between 0°C and 50°C
*Supply Sensistivity Maximum of +/-2% of outlet pressure at extremes of supply range
*Connections 1/8" NPT female
*Operating Temperature -10°C to +60°C
*IP Rating IP40
*Vibration The unit possesses a high degree of immunity
*Electromagnetic Compatibility Compliant and CE marked in accordance with the EC E.M.C. directive. Tested to standards: BS EN50082-2: 1995, BS EN50081-2: 1994
*Material of Construction Anodised natural aluminium
*Mass 600g
*Mounting Position The instrument can be mounted in any orientation. A rail clip is provided with each instrument for TS32 (EN50035)/TS35 (EN50022) rail
*Electrical Signal 4-20mA (two wire); load presents 10 volts (+/-0.5V) constant voltage drop to the current source
*Failure Mode Signal falls to below 15mbar (0.2psig) when input signal fails
*Connections Two part quick release terminal block with capacity up to 2.5mm2 cable

*Materials Aluminium extrusion, clear anodised, nitrile 'O' ring seals
*Port Sizes Supply and output 1/4" NPT
*Proof Pressure 10bar (150psig); pressure should be regulated to converter requirements. WARNING: maximum pressure should not be exceeded for safety reasons.
Note: It is recommended that an appropriate filter-regulator be fitted to supply all converters

Ordering Information


Output Pressure

Order Code

Options to specific order:

Alternative pressure ranges
Surface mounting bracket
Manifold kit for up to 20 converters






Number of Ports

Order Code









Adapter kit (1 required per converter)



All instruments are tested on the Watson Smith Automatic Testing System and an individual test certificate is provided at no extra charge. Each unit is tested for linearity, hysteresis, total error, settling error, over pressure, air consumption, response time, calibration, insulation, start-up current, supply sensitivity and voltage load.


Type 421 Brochure

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Watson Smith

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