SAMSON Type 3725 Electropneumatic Positioner
Overview
The SAMSON Type 3725 is a compact, single-acting electropneumatic positioner built for attachment to pneumatic globe and rotary control valves. It receives a 4 to 20 mA set point from the control system, compares that signal with the actual travel or opening angle of the valve, and issues a matching signal pressure to the actuator. The device is self-calibrating and adapts itself automatically to the valve and actuator it is mounted on, which keeps commissioning short and removes most of the manual zero and span work that older analog positioners required.
Seven Star LLC supplies the SAMSON Type 3725 to refineries, gas plants, utilities and process facilities across the Sultanate of Oman and the wider GCC. As a trusted supplier with engineering support in Muscat, we help instrumentation teams select the right attachment kit, pneumatic connecting rail and pressure gauge option so that each positioner arrives ready to mount on the intended valve.
Operating Principle
Inside the SAMSON Type 3725, the valve position is picked up by a lever connected to a magnet. When the lever moves, the direction of the magnetic field changes, and a non-contact magnetoresistive sensor detects that change. The electronics then calculate the current position of the actuator stem or the opening angle and pass the value through an A/D converter to the microcontroller. Because the sensing is contactless, there are no wiper tracks or gear trains that wear out, and the measurement is unaffected by environmental effects and steam hammering.
A PD control algorithm in the microcontroller compares the measured position with the 4 to 20 mA reference variable. When a deviation exists, the activation of the analog i/p converter is changed, and the downstream air capacity booster either pressurizes or vents the actuator until the valve reaches the commanded position. Supply air is fed to the booster and to an internal pressure regulator, while a fixed restriction and an optional volume restriction allow the response to be tuned for small, fast actuators.
Engineering Design of the SAMSON Type 3725
The housing of the SAMSON Type 3725 is made of polyphthalamide (PPA), with external parts in stainless steel 1.4571 and 1.4301, a black polyamide M20x1.5 cable gland and a high-density polyethylene vent plug. The complete unit weighs approximately 0.5 kg, which makes it a light option for small and medium valves where a heavy die-cast positioner would be unnecessary. The cover carries a polycarbonate window, and the LCD reading direction can be rotated by 180 degrees so the display stays legible in any mounting position.
Mechanically, the positioner attaches directly to the SAMSON Type 3277 actuator (120 to 750 cm²) over a connection block, to linear valves according to IEC 60534-6-1 (NAMUR), to rotary actuators according to VDI/VDE 3845, to the Type 3278 rotary actuator and to the Type 3372 actuator used on Series V2001 valves. On the Type 3277-5 actuator the signal pressure is routed through an internal bore in the yoke; for fail-open versions and diaphragm areas of 175 cm² or larger, ready-made external piping is used. Pneumatic connecting rails are available with G¼ or ¼ NPT threads, with or without a 6 bar pressure gauge.
Key Features
The SAMSON Type 3725 combines simple operation with the diagnostics-free reliability that many throttling loops need:
- Self-calibrating start-up adapts the positioner automatically to the valve travel and actuator size.
- Non-contact magnetoresistive position sensing eliminates wear on the travel pick-up.
- Three capacitive keys and a single-level menu let technicians read and change every parameter on site.
- Split-range operation is possible with 4 to 11.9 mA and 12.1 to 20 mA signal ranges.
- Three characteristics are available for globe valves and nine characteristics for rotary valves.
- A tight-closing function can be activated and the zero point is monitored continuously.
- All parameters are stored in non-volatile EEPROM, so settings survive a power failure.
- Explosion-protected versions are certified for intrinsic safety under ATEX, CSA and CCC Ex schemes.
Industrial Applications
The SAMSON Type 3725 is a practical choice for steam, water, air and utility control valves where a robust 4 to 20 mA positioner is needed without fieldbus or advanced diagnostics. Typical duties include heat exchanger temperature control, boiler feedwater and deaerator level loops, cooling water flow control and pressure reduction stations in power and desalination plants. The resistance of the SAMSON Type 3725 to steam hammering and its light housing make it well suited to compact Series 240 and Series V2001 valves in HVAC, district cooling and food and beverage processing.
In the oil and gas sector, the intrinsically safe versions of the SAMSON Type 3725 are used on auxiliary control valves in gas processing, tank farms and utility systems inside hazardous areas. Seven Star LLC supports these projects across Muscat, Sohar and Salalah, helping engineers confirm hazardous-area certificates, attachment hardware and supply air quality before the positioners are shipped to site.
Technical Specifications
The following data for the SAMSON Type 3725 is taken from SAMSON data sheet T 8394 EN.
| Parameter | Specification |
|---|---|
| Model | SAMSON Type 3725 Electropneumatic Positioner |
| Action | Single-acting |
| Travel | 5 to 30 mm (direct attachment to Type 3277); 5 to 50 mm (IEC 60534-6 NAMUR) |
| Opening angle | 24 to 100° (rotary actuators) |
| Reference variable | 4 to 20 mA, two-wire; split range 4 to 11.9 mA / 12.1 to 20 mA |
| Minimum current / load impedance | 3.8 mA / max. 6.3 V (315 Ω at 20 mA) |
| Supply air | 1.4 to 7 bar (20 to 105 psi), quality to ISO 8573-1 |
| Signal pressure | 0 bar up to supply pressure, can be limited to approx. 2.3 bar |
| Hysteresis / sensitivity | ≤0.3 % / ≤0.1 % |
| Air consumption | ≤100 ln/h (supply up to 6 bar, signal pressure 0.6 bar) |
| Air output (supply / exhaust at Δp 6 bar) | 8.5 m³n/h / 14.0 m³n/h |
| Ambient temperature | –20 to +80 °C; –25 to +80 °C with metal cable gland |
| Degree of protection | IP66 |
| Housing material | Polyphthalamide (PPA) |
| Weight | Approx. 0.5 kg |
| Explosion protection | ATEX II 2 G Ex ia IIC T4 Gb; CSA c/us; CCC Ex |
How to Order
Send Seven Star LLC your valve size, actuator type and hazardous-area requirements to receive a formal RFQ response for the SAMSON Type 3725. Our instrumentation division in Muscat confirms lead time at quotation, can share the technical datasheet for download, and coordinates attachment kits and connecting rails so the positioner matches your SAMSON or third-party actuator. Related equipment we supply includes the SAMSON Type 3271 pneumatic diaphragm actuator, the SAMSON Type 3241 globe control valve and the Samson 3730-3 smart positioner. Contact us for fast quotations and delivery across Oman and the GCC.
- Datasheet SAMSON Type 3725 Electropneumatic Positioner Datasheet (T 8394 EN) Download
Samson valve, actuator and positioner types
Samson builds a control valve from modules. The Type 3241 globe valve is the general-service body of Series 240, and the Type 3251 is the Series 250 body for higher pressure ratings. Either one is combined with a Type 3271 pneumatic diaphragm actuator or a Type 3277, whose yoke allows the positioner to be attached directly with a concealed travel linkage. The positioners listed are the Type 3725 and the Type 3730 family: the 3730-1 as the basic electropneumatic version, sold in its current generation as TROVIS 3730-1, and the 3730-3 with HART communication and valve diagnostics. Samson also makes PROFIBUS PA and FOUNDATION fieldbus variants in the same family, plus self-operated pressure and temperature regulators.Where Samson valves are used
Samson control valves throttle steam, gas and liquids in chemical and petrochemical units, refineries, gas processing, power stations, water and desalination plants, and food production. Many reach site as part of a skid or package, so the plant instrument team often meets them first when a positioner or actuator needs replacing. We supply these items to end users and contractors throughout the GCC.Ordering Samson parts: what we need
Every Samson device has a nameplate with the type, a configuration ID (Var-ID) and a serial number. A photo of that plate is the surest route to an exact match. Without it, please send:- Valve: type, DN or NPS, pressure rating, body and trim material, Kvs or Cv, plug and seat style, packing and bonnet type
- Actuator: type, effective area in cm², bench range, fail action and travel
- Positioner: full type code including Ex protection, communication and options such as limit switches or position transmitter
Browse Samson products by category: Control Valves, Valve Positioner, Valve Actuator.
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