SHAW AcuDew 4-20 mA and Modbus RTU Dewpoint Transmitter
Overview
The SHAW AcuDew is a compact, high-performance dewpoint transmitter from Shaw Moisture Meters for continuous trace moisture measurement in process gases and dry compressed air. It is a two-wire, loop-powered 4-20 mA instrument built around the SHAW ultra high capacitance aluminium oxide sensor, with the SHAW AutoCal feature to maintain accuracy between laboratory calibrations. Ranges cover an overall span from -120 to +20 °C dewpoint, accuracy is ±2 °C dewpoint, and a Modbus RTU version (AcuDew-M) adds RS485 digital communication alongside the analog output.
Seven Star LLC supplies the SHAW AcuDew and its sensor holders, displays and sample systems to gas processing plants, power stations, air separation and nitrogen users and compressed air operators in Oman and the GCC. Our Muscat team provides engineering support for range selection, sample conditioning and integration with PLC and DCS systems.
Measurement Principle
The SHAW AcuDew sensor is a thin-film aluminium oxide capacitor. In general, water molecules from the gas diffuse into the pores of the oxide layer and change the capacitance of the element in relation to the water vapour pressure, which the transmitter electronics convert into a linear 4-20 mA signal over the chosen range. SHAW sensors saturate at their design maximum, which is the basis of AutoCal: when the sensor is exposed to air wetter than its range, it reaches equilibrium quickly, and the user can run a field calibration or span check by turning the potentiometer on the transmitter body.
Response times quoted by SHAW are less than 120 seconds from -20 to -60 °C (wet to dry) and less than 20 seconds from -100 to -20 °C (dry to wet). Measurement is flow independent, though a sample flow of 2 to 5 litres per minute is ideal and 25 litres per minute is the maximum. The microprocessor provides self-diagnostics: on open circuit or under range the output drives to 20.5 mA, and on short circuit or over range it drives to 20.75 mA, so a control system can tell a fault from a genuine wet reading.
Engineering Design of the SHAW AcuDew
The transmitter has a 316 stainless steel probe and wetted parts with an aluminium body, a 34 mm diameter, a 30 mm across-flats hex and a weight of 215 g including the connector. It screws into the process with a 3/4 in UNF (16 tpi) thread sealed by an integral Viton O-ring, and SHAW states it has been tested to withstand pressures up to 35,000 kPa (350 bar) for direct installation into pipelines; the operating pressure range is 1 kPa to 35,000 kPa absolute. The sensor is protected by a 50 micron sintered 316 stainless steel filter.
Electrical connection of the SHAW AcuDew is by an M12 five-pin connector rated IP66 and NEMA 4X when mated, with 2 m of cable supplied: two-wire for the analog transmitter and four-wire for the Modbus version. The electrical data gives an operating voltage of 12 to 28 VDC with reverse polarity protection, a maximum series resistance of 40 x (supply voltage – 7) ohms and a 10 second warm-up time. The SHAW AcuDew 4-20 mA signal can be carried up to 1,000 m to a control system. Output can be configured in °C or °F dewpoint, ppm(V), ppb(v), g/m³ or lb/MMSCF, and Modbus RTU runs at 4,800, 9,600, 19,200 or 38,400 baud.
Key Features
Main features of the SHAW AcuDew:
- Two-wire loop-powered 4-20 mA dewpoint transmitter, with optional Modbus RTU over RS485.
- Ranges from -120 to +20 °C dewpoint, including a Gold range down to 0.1 ppb.
- Accuracy of ±2 °C dewpoint, NPL traceable from -90 to +20 °C, repeatability better than ±0.3 °C.
- AutoCal field calibration and span check using the potentiometer on the transmitter body.
- Rated for direct pipeline installation up to 350 bar absolute.
- Rugged 316 stainless steel wetted parts and an IP66 / NEMA 4X M12 connection.
- Open and short circuit detection with defined fault currents.
- Compatible with the AcuLoop, AcuVu and AcuTrak displays and the SU4 and SWS sample systems.
Industrial Applications
The SHAW AcuDew suits continuous moisture monitoring on compressed air dryer outlets, instrument air headers, nitrogen generation packages, natural gas and fuel gas lines, hydrogen-cooled generators, and plastics and pharmaceutical drying processes. In Oman, typical installations include instrument air systems at oil and gas production stations, fuel gas skids at power stations in Barka, Sohar and Salalah, and nitrogen supply in petrochemical and LNG plants, where a stable 4-20 mA or Modbus signal must reach the DCS for alarming on dryer breakthrough.
Seven Star LLC supplies the SHAW AcuDew complete with the SHAW sensor holder (AcuDew-SEH with 1/4 in, 1/8 in or 6 mm fittings) and display options. For portable verification of the same points, see the SHAW SADP-P automatic dewpoint meter; alternative fixed transmitters on this site include the Easidew PRO I.S. intrinsically safe moisture transmitter and the CS Instruments FA 510/515 dew point sensor.
Technical Specifications
Data for the SHAW AcuDew is taken from the SHAW AcuDew datasheet, issue 1.5:
| Parameter | Specification |
|---|---|
| Model | AcuDew (4-20 mA) and AcuDew-M (4-20 mA with Modbus RTU) |
| Sensing element | SHAW ultra high capacitance aluminium oxide sensor |
| Ranges | ZD -100 to +20 °C; HD -65 to +20 °C; Go -120 to -20 °C; P -100 to 0 °C; S -100 to -20 °C; G -80 to 0 °C; R -80 to -20 °C; B -80 to +20 °C dewpoint; MP, LP (ppm), GH (g/m³), LB (lb/MMSCF) |
| Accuracy | ±2 °C dewpoint, NPL traceable -90 to +20 °C |
| Repeatability | better than ±0.3 °C dewpoint |
| Output | 4-20 mA linear; Modbus RTU RS485 on AcuDew-M |
| Operating voltage | 12 to 28 VDC, reverse polarity protected |
| Fault outputs | 20.5 mA open circuit/under range; 20.75 mA short circuit/over range |
| Operating temperature | -20 to +60 °C |
| Operating pressure | 1 kPa to 35,000 kPa (350 bar) absolute |
| Sample flow | 2 to 5 l/min ideal, 25 l/min maximum |
| Wetted parts | 316 stainless steel; 50 micron sintered 316 SS filter |
| Process connection | 3/4 in UNF (16 tpi) with Viton O-ring |
| Electrical connection | M12 five-pin, IP66 / NEMA 4X, 2 m cable supplied |
| EMC | BS EN 61326 |
| Weight | 215 g including connector |
How to Order
To order the SHAW AcuDew, specify AcuDew or AcuDew-M with the range code, for example AcuDew-M-G, and add the sensor holder or display you need. The SHAW AcuDew datasheet can be downloaded from this page. Seven Star LLC will confirm availability and lead time from Muscat and support installation and commissioning across Oman and the GCC.
- Datasheet SHAW AcuDew Modbus RTU Dewpoint Transmitter Datasheet (Iss 1.5) Download
SHAW dewpoint meters and how they are used
The SADP is a portable hygrometer for spot checks. A small sample flow passes over the sensor in the instrument head, which is kept dry between tests, and the reading is taken at the sample point. Typical checks are instrument air after a dryer, nitrogen and other industrial gases, natural gas, and welding or cryogenic gases. SHAW also makes the hand-held SDHmini and the intrinsically safe SDHmini-Ex, the fixed Superdew 3 hygrometer and the AcuDew and SDT-Ex dewpoint transmitters for continuous measurement.Ordering a SHAW dewpoint meter: what we need
A SHAW meter is defined by its sensor range. State the driest and wettest dewpoint you expect, the gas, the line pressure at the sample point and the display units wanted (°C or °F dewpoint, ppm). For a replacement or a spare sensor, send the model and serial number from the instrument label. We supply to the GCC and provide the manufacturer's documents on request.Browse SHAW Instruments products by category: Gas Analyzer Systems.
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