Introduction: SIL ratings describe how reliably a safety loop performs its job, so a Coriolis meter's certification only makes sense alongside the loop it serves.
When a plant team reviews instruments for an emergency shutdown or interlock loop, the datasheet line "SIL 2/3" is often read as a badge the meter either carries or lacks. That reading causes real mistakes. It hides the fact that a safety integrity level is a target for the whole safety function, shared among the sensor, the logic solver, and the final element. It also encourages teams to treat hazardous area approval as if it arrived on the same certificate. Three questions tend to trip up those reviews: what SIL 2 and SIL 3 actually mean for a Coriolis flowmeter, what NE 107 status reporting adds to a loop, and why explosion protection stays a separate, configuration-specific question.
IEC 61508 defines safety integrity levels as target measures of how reliably a safety function performs when it is called on. In low-demand mode, SIL 2 corresponds to roughly one dangerous failure in somewhere between 100 and 1,000 demands, and SIL 3 to somewhere between 1,000 and 10,000. Those numbers belong to the safety function, which in most plants means the entire loop: the flowmeter that detects the condition, the logic solver that decides, and the valve or other final element that acts. A Coriolis meter contributes to that budget through its functional safety assessment, which documents the failure rates, safe failure fraction, hardware fault tolerance, and diagnostic coverage used in the loop calculation. The KROHNE OPTIMASS 6400 is published with SIL 2/3 coverage up to DN200 / 8″ and proof test coverage up to 82 percent, so it can be counted inside loops targeting those levels within the published size and configuration limits.
The most common mistake in instrument reviews is to buy a meter described as SIL 3 and assume the loop is now SIL 3. It is not. The achieved level is calculated from every element in the chain, and the weakest element sets the ceiling. A meter with strong diagnostic coverage paired with a final element that has no functional safety data will pull the loop well below its target. That is why selection in a safety instrumented system starts from the safety function — what has to happen, how fast, and how often — and works backward to the devices. A meter's certified capability is an input to the calculation, never the conclusion.
Dangerous failures come in two useful categories: those the device detects itself, and those that stay hidden until something checks for them. Undetected failures accumulate between proof tests, so the loop's calculated reliability depends on how much of that hidden population a periodic test actually exposes. Proof test coverage expresses that as a percentage. A published figure of up to 82 percent means the specified proof test procedure reveals most, though not all, dangerous undetected failure modes, and the remainder stays in the reliability calculation for the full test interval. Higher coverage supports longer intervals between tests and less production interruption, and because it follows the test procedure and the specific hardware, the figure that applies to one size or configuration should be matched to the ordered variant.
NE 107 is a NAMUR recommendation that standardizes how a field device reports its own condition. Instead of a proprietary error code, the device publishes one of a small set of status categories that any control system or maintenance team reads the same way: failure, function check, out of specification, and maintenance required, alongside a normal state. Failure means the device cannot perform its measurement function. Function check means a simulation or test is running and the output should not be read as a process value. Out of specification means the measurement is outside the limits the device can guarantee. Maintenance required means the device still works, but a condition is developing that will need attention. For a safety instrumented loop, that plain-language status matters most between proof tests. The reliability calculation behind a SIL claim assumes the device is healthy unless it has failed, and slowly developing degradation is what erodes that assumption. NE 107 flags drift, coating or build-up, temperature stress in the electronics, and other emerging conditions while the measurement is still running, so the plant can act inside a planned window instead of discovering the problem at the moment the loop is demanded. On a meter such as the OPTIMASS 6400, these status signals can be read over HART or through Bluetooth at the pipe, so the technician and the control room see the same category. Diagnostics keep the loop's assumptions current between tests; the SIL figure itself still comes from certified failure data.
Hazardous area approval answers a different question from functional safety. The ATEX Directive 2014/34/EU, and comparable schemes such as IECEx, deal with ignition risk: equipment installed where an explosive atmosphere may be present has to avoid sparks, hot surfaces, and other ignition sources. That is why hazardous area certificates specify equipment groups and categories, the zones where the equipment may be used, the gas or dust group it suits, and the temperature class it stays below. None of those entries describe how often a device fails dangerously. SIL, by contrast, comes from a functional safety assessment and describes failure rates, diagnostic coverage, and hardware fault tolerance. A meter can hold full explosion protection without a SIL capability that suits a particular loop, and a meter can be published with SIL coverage while the approval for the exact hazardous area configuration sits in a separate entry on the certificate. In practice both checks come back to the same thing: the exact configuration ordered. Size, wetted materials, sensor and electronics variant, and housing all shift which approvals and which SIL figures apply. The OPTIMASS 6400 is published with explosion protection and SIL 2/3 coverage up to DN200 / 8″, and both statements follow the selected device configuration rather than the model name alone.
SIL 2 and SIL 3 are targets for a safety function, and a Coriolis flowmeter earns its place in that function by publishing the failure data, diagnostic coverage, and proof test coverage an integrator needs for the loop calculation. Published coverage up to DN200 / 8″ and proof test coverage up to 82 percent on a meter such as the KROHNE OPTIMASS 6400 show what the device can contribute inside the size and configuration limits stated for it. NE 107 status reporting keeps the loop's assumptions current between proof tests, and hazardous area approval remains a separate question answered by the approval documents for the exact configuration. Reading those three items independently is what turns a datasheet line into a defensible loop design.
A：It means the manufacturer publishes functional safety data for the device — failure rates, diagnostic coverage, hardware fault tolerance, and proof test coverage — so the meter can be counted inside a loop that has to reach SIL 2 or SIL 3. The level itself is achieved by the whole loop, not by the meter alone. Coverage also follows the ordered variant: this model family is published with SIL 2/3 coverage up to DN200 / 8″, with the applicable figure tied to size and configuration.
A：NE 107 gives the meter a standard way to report its condition as failure, function check, out of specification, or maintenance required. In a safety loop, the last two categories are the useful ones: they flag drift or developing degradation while the measurement is still running, so the problem can be handled in a planned window rather than discovered during a demand. The status keeps the loop's reliability assumptions current between proof tests, while the SIL figure itself still comes from the certified failure data.
A：No. SIL and hazardous area approval are separate questions with separate documents. The hazardous area certificate for the exact configuration — gas or dust group, zone, temperature class, housing, and electronics variant — decides where the meter may be installed. A SIL rating published for a model family does not automatically cover every size, material, or electronics option, so the approval and the SIL figures both need to be matched to the specific device that will be delivered.
Directive - 2014/34 - EN - ATEX directive
