Description
The EFMH is an ATEX and IECEx-approved radio transmission module for strain gauge bridges. It measures the bridge signal from a load cell, force, pressure, or torque transducer and transmits it cyclically to X24 or T24 receivers – from the hazardous area, wirelessly.
The marking is I M2 Ex ib I and II 2G Ex ib IIC T4 Gb at Ta = −20 to +50 °C. This means the device is approved for Zone 1 and Zone 2. The electronics are housed in an IP67-rated enclosure with a battery holder. Only two Energizer L91 AA size lithium cells are permitted – this condition is stated in the approval, and cell replacement must not take place in an explosive atmosphere.
The input operates in a four-wire system with 5 V bridge excitation and accepts bridge resistances from 85 to 5000 Ω, up to four 350-Ω bridges in parallel. The measurement is bidirectional, thus detecting both tension and compression. Zero point drift a maximum of 4 ppm/°C, gain drift a maximum of 5 ppm/°C, non-linearity before linearization a maximum of 25 ppm of the measuring range. Calibration and linearization are performed in engineering units using the free Toolkit software.
The radio link operates on the license-free 2.4 GHz DSSS band with 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 use a T24-ANTA antenna; against a receiver with an integrated antenna, it is around 600 m – measured in open field, transmitter on a 3-m mast, receiver 1.5 m above the ground. Battery life can be up to five years depending on the operating mode.
Advantages 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 housing, 76.6 × 59 × 31 mm plus 25 mm gland
- Bridge excitation 5 V DC, 85 to 5000 Ω, four-wire system
- Sensitivity ±3.1 mV/V, version with 20 mV/V available
- Zero point drift max. 4 ppm/°C, gain drift max. 5 ppm/°C
- Power 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
- Works with X24 and T24 receivers
Important for system planning: X24 devices belong in the hazardous area, T24 devices in the safe area – T24 modules must not be used in the Ex Zone. The EFMI ATEX Wireless Handheld Display X24-HD serves as the receiver in the zone; all T24 receivers can be used in the safe area. The housing maintains IP67 only with cable diameters of 3.0 to 6.5 mm in the cable gland. Further Ex devices can be found under Explosion Protection ATEX & IECEx.