Flow measurement in a Zone 1 or Zone 2 hazardous area presents a design choice that shapes the entire instrument loop: either contain any potential explosion within an enclosure strong enough to withstand it (explosion-proof / Ex d), or limit the electrical energy in the circuit to a level incapable of igniting the atmosphere in the first place (intrinsically safe / Ex i). For flow instrumentation—electromagnetic, Coriolis, vortex, turbine, and ultrasonic flow meters—the decision between IS and Ex d determines cable type, field wiring practices, maintenance procedures, and whether a technician can calibrate the instrument live without a hot-work permit.
This article compares the two protection concepts as they apply specifically to flow metering loops, covering installation cost, operational flexibility, and compatibility with modern two-wire and four-wire flow transmitters.
How explosion-proof (Ex d) protection works
An Ex d enclosure is designed to contain an internal explosion. If flammable gas enters the housing and is ignited by a spark or hot surface inside—the electronics are not sealed from the atmosphere—the resulting flame front and pressure wave are contained within the enclosure. The housing is machined with flamepaths: precisely toleranced gaps (typically 0.1–0.4 mm) between the lid and body or the shaft and bearing. As hot gases escape through the flamepath, they cool below the auto-ignition temperature of the external atmosphere before reaching it. The enclosure walls are thick—typically 5–15 mm of cast aluminum or stainless steel—to withstand the explosion pressure without deformation. For a Coriolis flow meter with remote transmitter, both the sensor junction box and the transmitter housing must be Ex d-certified, and the interconnecting cable must use armored glands that maintain the flamepath integrity at each entry point.
How intrinsic safety (Ex i) protection works
Intrinsic safety takes the opposite approach: limit the electrical energy in the hazardous-area circuit to below the minimum ignition energy (MIE) of the gas group. For hydrogen-air mixtures (IIC), the MIE is approximately 20 µJ—roughly the energy of a static discharge you feel when touching a doorknob after walking across carpet. An IS barrier or galvanic isolator, mounted in the safe area, clamps the voltage (typically to 28 V or less via Zener diodes) and limits the current (typically to 93 mA or less via series resistance) that can reach the field instrument, even under two simultaneous fault conditions (Ex ia) or one fault (Ex ib). If the barrier fails, it fails safe—the current is interrupted, not passed through. The field instrument itself is designed with minimal capacitance and inductance, and its internal energy-storage components (capacitors, inductors) are potted or otherwise prevented from discharging into the hazardous atmosphere.
Cable and installation cost comparison
Ex d installations require armored cable with explosion-proof cable glands at every enclosure entry—typically €30–€80 per gland, installed by a certified technician who must document the gland type, cable type, and torque setting for each entry. The cable armor provides mechanical protection and ensures the flamepath is not compromised. IS installations use standard unarmored instrument cable with ordinary IP-rated cable glands, because there is no flamepath to maintain. The IS barrier or isolator is DIN-rail-mounted in the safe-area cabinet—a clean, accessible location for wiring and testing. On a typical flow metering installation with 50 meters of cable between the safe-area panel and the field instrument, the installed cost difference—armored cable + Ex d glands + certification documentation vs. standard cable + IS isolator—works out to roughly €200–€500 in favor of IS, per instrument loop. This difference multiplies across a plant with 200 flow measurement points.
Live-maintenance: the operational differentiator
The most consequential operational difference is live maintenance. An IS circuit can be disconnected, re-terminated, and calibrated while the plant is running and the hazardous atmosphere is present—no gas test, no hot-work permit, no plant shutdown. The technician opens the IS instrument housing, connects a HART communicator or laptop, and performs zero-span adjustment or diagnostic interrogation on the live loop. An Ex d enclosure cannot be opened while the circuit is energized in a hazardous area without a gas-clearance certificate and a cold-work permit. In practice, this means an IS flow meter can be maintained during normal operations in 30 minutes, while the same task on an Ex d flow meter requires a planned shutdown window or a gas-freeing procedure that can take 4–8 hours to coordinate. For continuous-process plants—refineries, chemical reactors, pharmaceutical API production—this difference alone drives IS adoption for field instrumentation, even when the component cost favors Ex d.
Power limits and the two-wire transmitter advantage
IS imposes a hard ceiling on available power at the field instrument. A typical IS barrier delivers 24 V, 93 mA into a IIC load—roughly 2.2 W available. This is sufficient for a modern two-wire, loop-powered flow transmitter (Coriolis, electromagnetic, or vortex) that consumes 0.5–1.5 W. It is not sufficient for four-wire flow transmitters with local backlit displays, heater elements, or integrated ultrasonic transducer drivers that require 5–15 W. For these higher-power devices, Ex d is the only practical option—or an Ex e (increased safety) enclosure for the transmitter combined with IS for the sensor circuit (a "hybrid" approach common on large-line-size Coriolis meters with remote transmitters). The trend toward low-power electronics is steadily expanding IS applicability: many flow transmitters that required Ex d enclosures a decade ago now operate comfortably within IS power budgets.
Gas group and zone compatibility
| Protection Method | Zone Suitability | Gas Group Capability | Typical Flow Instrument Application |
|---|---|---|---|
| Ex ia (IS, two faults) | Zone 0, 1, 2 | IIC (hydrogen, acetylene) | Sensor circuit, simple transmitter |
| Ex ib (IS, one fault) | Zone 1, 2 | IIB/IIC | Loop-powered flow transmitter |
| Ex d (flameproof) | Zone 1, 2 | IIB/IIC | 4-wire flow transmitter, remote display |
| Ex e (increased safety) | Zone 1, 2 | IIA/IIB/IIC | Terminal box, junction enclosure |
MTL 949X Series: IS power for FOUNDATION Fieldbus and 4–20 mA flow loops
For flow instrumentation on a FOUNDATION Fieldbus or conventional 4–20 mA HART loop, the MTL 949X Series galvanic isolator provides intrinsically safe power and signal isolation in a single DIN-rail module. The unit delivers field power, HART transparency for diagnostic access, and Ex ia certification for Zone 0 deployment—allowing the same IS infrastructure to serve flow meters in the most hazardous plant locations.
Decision framework for hazardous-area flow instrumentation
- Choose Intrinsic Safety (Ex i) if
- Your flow transmitters are two-wire loop-powered, you operate a continuous process where live maintenance without shutdown is essential, the instrument count is high enough that cable and gland savings multiply to a meaningful project difference, or your site already has IS barriers installed for pressure and temperature loops—consistency reduces spares and training.
- Choose Explosion-Proof (Ex d) if
- Your flow instrument requires more than 2 W of power (4-wire transmitter, built-in heater, ultrasonic drivers), the instrument is a direct replacement for an existing Ex d installation where the cable infrastructure is already in place, or the instrument is a mechanical-only device (turbine meter with passive magnetic pickup) where the electrical circuit is inherently low-risk and Ex d provides an adequate protection level without adding IS barriers.
Browse our full range of intrinsically safe barriers and isolators for hazardous-area instrument loops, and explore flow meter options with IS and Ex d certifications for Zone 1 and Zone 2 deployment.
