Watson Smith Type 422 I.S. Fail Freeze I/P Converter

An intrinsically safe I/P converter that holds its output when the signal is lost

The Watson Smith Type 422 I.S. is an ATEX intrinsically safe current-to-pressure (I/P) converter with advanced electronic control and fail-freeze operation: if the 4 to 20 mA input signal fails, the output pressure is retained at its previous value, with a drift rate of 0.2 percent of span or less in five minutes. Other features are minimum vibration effects, high accuracy and a captured bleed as standard on the captured-bleed versions.

It is a two-wire device with a typical terminal voltage below 11 V at 20 mA and a 30 mA maximum overload current, and it is unaffected by reverse current. The cover is glass-reinforced polyamide and the body zinc die-cast, passivated and epoxy painted, with NBR diaphragms and an integral surface mounting bracket.

Technical specifications

Models and certification

The Norgren sheet lists four models: AC301PJ1 (G1/4, 3 to 15 psi), AC301PK1 (1/4 NPT, 3 to 15 psi), AC301BJ1 (G1/4, 0.2 to 1 bar) and AC301BK1 (1/4 NPT, 0.2 to 1 bar), each 0.98 kg. The option selector AC301 followed by three characters sets the pressure range (P or B), the port (J for G1/4, K for 1/4 NPT) and the special notes: 0 standard exhaust, 1 captured bleed, 2 junction box and captured bleed. Special-order options include alternative pressure units, M20 or 1/2 inch conduit entry and a 50 mm pipe mounting bracket.

Certification printed in the sheet is intrinsically safe: ATEX Sira 03ATEX2007X, marking II 1G Ex ia IIC T4 Ga for Ta minus 40 to plus 80 degrees C, and IECEx SIR 11.0095X with the same marking. Entity parameters are Ui 28 V dc, Ii 110 mA, Pi 0.8 W, Ci 5 nF and Li 0.24 mH. Install only within these limits and with a suitable intrinsically safe supply or barrier.

Selection and installation

The converter needs oil-free dry air filtered to 5 micron, at least 0.7 bar above the maximum output and not above 4 bar gauge, and the air must be dry enough to avoid ice formation below 2 degrees C. It works in any orientation, with an orientation effect below 0.2 percent. IP65 applies with piped exhaust, so pipe the exhaust away in wet locations. Response is slower than a general-purpose I/P at below 6 seconds for a 10 to 90 percent change into a 0.5 l volume, which suits positioners and slow loops but should be checked for fast control. There are no user serviceable parts.

How the Watson Smith Type 422 works

The converter takes a 4 to 20 mA signal and turns it into an air pressure. Electronics drive the pressure up or down to match the signal. If the signal is lost, the converter holds the last pressure. The output then drifts only very slowly.

Where the Type 422 is used

• Control valve positioners
• Pneumatic actuators
• Process control loops
• Plants with vibration
• Loops that must hold the last position

Selection checklist for the Type 422

• Output range, psi or bar
• Port thread, NPT or G
• Supply pressure
• Flow needed
• Fail freeze behaviour
• Bleed type
• Ambient temperature
• Connector type

Installation, commissioning and maintenance notes for the Type 422

Mount the converter on its bracket. Connect filtered supply air. Wire the two-wire loop with the right polarity. Pipe the exhaust away if the area is wet. Check the output at 4 and 20 mA.

Requesting a quotation for the Watson Smith Type 422

Please send the output range, the port thread and the quantity. Seven Star LLC serves industrial customers in Oman and the GCC and replies with a quotation based on the information provided.

Type 422 converter type

It is a current to pressure converter with fail freeze operation.

Type 422 models

The models are AC301PJ1, AC301PK1, AC301BJ1 and AC301BK1.

Type 422 input signal

The input is 4 to 20 mA on two wires.

Type 422 terminal voltage

The terminal voltage is typically below 11 V at 20 mA.

Type 422 output ranges

Outputs are 3 to 15 psi or 0.2 to 1 bar.

Type 422 supply pressure

The maximum supply pressure is 4 bar gauge.

Type 422 flow capacity

The flow capacity is up to 250 Nl/min.

Type 422 linearity

Linearity is 0.5 percent of span or better.

Type 422 response time

The response time is below 6 seconds.

Type 422 failure mode

The output is held with drift of 0.2 percent of span or less in 5 minutes.

Type 422 temperature

The temperature range is minus 10 to plus 70 degrees C.

Type 422 ports

Ports are 1/4 NPT or G1/4.

Type 422 materials

The body is zinc die-cast and the cover is glass-reinforced polyamide.

Type 422 weight

Each model weighs 0.98 kg.

Converter records for the Type 422

Record the model code, the calibration check and the supply pressure.

Choosing the right Type 422 model

Choose the model from the output range and the port thread.

Documents and quotation

The Norgren datasheet is attached. Seven Star LLC is an independent supplier in Muscat. Send the model or order code, quantity and operating conditions and we will request a quotation; availability depends on the model. See also Pressure Instrumentation.