Watson Smith Type 68 P/I Transmitter, Pressure to 4-20 mA

A two-wire pneumatic pressure to current transmitter

The Watson Smith Type 68 is a two-wire pressure-to-current (P/I) transmitter that converts pneumatic pressure into a 4 to 20 mA signal for data loggers, computers and microprocessors. It uses non-critical power supplies, can be mounted on a DIN rail or directly on the pipe, and is available weatherproof to IP65. The datasheet targets paper converting and processing, wood and pulp, steel mills, plastics and rubber, textile machinery and wire handling, wherever a pneumatic pressure must be read as an electrical signal, for example dancer arm control, winder systems, roller tension and roller force control.

We found this datasheet only as a copy issued as Watson Smith Instrumentation literature (dated 10/06) and hosted by a UK instrument supplier; we did not find the product in current manufacturer literature, so treat it as an older, legacy line and confirm current availability, any replacement model and lead time before you rely on it.

Technical specifications

Ranges and ordering codes

Standard and weatherproof models are 6801 00 and 6801 10 (3 to 15 psig), 6815 00 and 6815 10 (0 to 150 psi), 6821 10 (0.2 to 1 bar, weatherproof only), 6835 00 and 6835 10 (0 to 10 bar) and 6836 00 (0 to 7 bar, standard only). Other pressure ranges between 50 mbar and 10 bar can be ordered; consult the supplier for availability. A model with temperature compensation from minus 20 to plus 40 degrees C for natural gas applications is available to special order, and all models include a DIN rail clip and mounting plate.

Electrical connection is a Klippon terminal block for cable up to 2.5 square millimetres on the standard version, or a 16 mm square connector to DIN 43650 for cable up to 6 mm diameter on the weatherproof version. Range and zero are adjustable by at least plus or minus 10 percent of full scale.

Selection and installation

Use dry, non-corrosive air or gas; the units are not suitable for continuous liquid exposure. Overpressure capability is at least 100 percent with negligible calibration error, except the 10 bar models, which have a 13.5 bar maximum. The loop needs at least 9 V across the unit, and the maximum load resistance is 750 ohm at a 24 V supply. Confirm the range, standard or weatherproof version and the availability of this older model before ordering. Each unit is supplied with an individual test certificate.

How the Watson Smith Type 68 works

The Type 68 turns an air pressure into an electrical current. Air from the process or the control loop enters the 1/8 NPT port and acts on a sensing element in the transducer. The electronics convert the movement into a current that rises from 4 mA at the bottom of the range to 20 mA at the top. Only two wires are needed, because the same pair carries both the power and the signal. A logger, a computer or a controller then reads the current as pressure.

Where the Watson Smith Type 68 is used

• Dancer arm control on web lines
• Winder systems
• Roller tension and roller force control
• Paper, pulp, steel and plastics plants
• Textile machinery and wire handling

Selection checklist for the Watson Smith Type 68

• Pressure range in psi or bar
• Model code, such as 6801, 6815, 6821, 6835 or 6836
• Standard IP54 or weatherproof IP65 version
• DIN rail or pipe mounting
• Supply of 9 to 30 V
• Load resistance in the loop
• Overpressure margin
• Current availability of this legacy line

Installation, commissioning and maintenance notes for the Type 68

Connect the air line to the 1/8 NPT port with clean, dry air. Wire the two terminals in series with a supply and a load, and check polarity. Apply zero pressure and full-scale pressure and note the output. Adjust zero and span if needed. Keep the air free of oil and water.

Requesting a quotation for the Watson Smith Type 68

Please send the pressure range, the model code, the protection level 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 68 output and supply

The output is 4 to 20 mA from a supply of 9 to 30 V. The maximum load is 750 ohm at 24 V.

Type 68 accuracy

Setting error is within plus or minus 0.2 percent of full scale. Hysteresis is typically plus or minus 0.15 percent.

Type 68 response

The output reaches 95 percent of a step in under 10 ms.

Type 68 construction

The casing is extruded aluminium with zinc die-cast end plates. The unit weighs 200 g.

Type 68 pressure ranges

Models cover 3 to 15 psig, 0 to 150 psi, 0.2 to 1 bar, 0 to 10 bar and 0 to 7 bar.

Transmitter records for the Type 68

Record the model, the range and the zero and span readings after commissioning.

Choosing the right Type 68 model

Choose the range so the normal pressure sits in the middle of the span.

Documents and quotation

The datasheet is attached for download. 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. Related items: Pressure Instrumentation.