Description
The EFMH is an ATEX and IECEx approved radio transmitter module for strain gauge bridges. It measures the bridge signal of a load cell or a force, pressure or torque transducer and transmits it cyclically to the receivers of the wireless system — out of the hazardous area, without cable.
The marking is I M2 Ex ib I and II 2G Ex ib IIC T4 Gb at Ta = −20 to +50 °C. The device is therefore approved for zone 1 and zone 2. The electronics sit in an IP67 sealed enclosure with battery holder. Only two Energizer L91 lithium cells in size AA are permitted — this condition is stated in the approval, and cells must not be changed in an explosive atmosphere.
The input works in four-wire technology with 5 V bridge excitation and accepts bridge resistances from 85 to 5000 Ω, up to four 350 Ω bridges in parallel. Measurement is bidirectional, covering tension and compression. Zero drift max. 4 ppm/°C, gain drift max. 5 ppm/°C, non-linearity before linearization max. 25 ppm of range. Calibration and linearization in engineering units are done with the free Toolkit software.
The radio link uses the licence-free 2.4 GHz band with DSSS at 10 mW and 15 channels; the transmit power is adjustable from 0 to 100 %. The range of up to 800 m applies when both sides carry an equivalent external antenna; to a receiver with an integrated antenna it is about 600 m — measured in open field, transmitter on a 3 m mast, receiver 1.5 m above ground. Battery life reaches up to five years depending on the operating mode.
Benefits at a glance
- ATEX and IECEx: I M2 Ex ib I and II 2G Ex ib IIC T4 Gb, Ta −20 to +50 °C
- Approved for zone 1 and zone 2
- Intrinsically safe electronics in IP67 enclosure, 76.6 × 59 × 31 mm plus 25 mm cable gland
- Bridge excitation 5 V DC, 85 to 5000 Ω, four-wire technology
- Sensitivity ±3.1 mV/V, 20 mV/V version available
- Zero drift max. 4 ppm/°C, gain drift max. 5 ppm/°C
- Supply 2.1 to 3.6 V DC from two Energizer L91 AA cells
- Battery life up to 5 years depending on transmission interval
- 2.4 GHz DSSS, 10 mW, up to 800 m in open field
Important for system planning: only the Ex devices EFMH and EFMI belong in the hazardous area; the EFMH is compatible with existing T24 systems, but their devices may only be used in the safe area. The receiver in the zone is the EFMI ATEX wireless handheld display; in the safe area all receivers of the wireless system can be used. The enclosure keeps IP67 only with cable diameters of 3.0 to 6.5 mm in the gland. Further Ex devices can be found under Explosion protection ATEX & IECEx.