A hydrogen sulphide channel specified as 0-1000 ppm sounds more capable than the same channel at 0-100 ppm. It covers ten times the concentration, so it must be better. It is not, and the reason is arithmetic.
Accuracy quoted against full scale
Most industrial gas sensors state accuracy as a percentage of full scale, written ±2% FS or similar. That figure is a fixed number of units, not a fixed proportion of the reading.
Take a cell at ±2% FS:
| Range | Accuracy stated | Actual uncertainty |
|---|---|---|
| 0-100 ppm | ±2% FS | ±2 ppm anywhere on the scale |
| 0-1000 ppm | ±2% FS | ±20 ppm anywhere on the scale |
Now place a first-stage alarm at 10 ppm, which is a common occupational exposure limit for hydrogen sulphide. On the 0-100 ppm range, the uncertainty is ±2 ppm, or 20% of the alarm point. On the 0-1000 ppm range, it is ±20 ppm, twice the alarm value itself. The instrument will read zero while the atmosphere is at the threshold.
This is the whole argument in one table. A wider range does not measure more; it measures the same thing less precisely, and it does so at the low end, which is where the alarm sits and where the interesting part of the measurement happens.
Where accuracy is quoted against reading
Some instruments state accuracy as a percentage of the reading or of the measured value. That figure scales with concentration, so a wide range behaves better under that convention. The two conventions are not interchangeable, and a datasheet that does not say which one it uses is telling you something about its author.
When comparing two instruments, convert both to the same basis before comparing. A ±2% FS specification on a narrow range often beats a ±1% reading specification on a wide one, at the concentrations you care about.
The sensing element has limits too
Range is not only a measurement decision. An electrochemical cell exposed to a concentration far above its rated range can be permanently degraded. The membrane saturates, the electrolyte is consumed faster than designed, and the cell comes back reading low for the rest of its life.
A detector rated to 100 ppm installed where a process can release 5000 ppm is not protecting anything after the first event. It reports a number, and the number is wrong from then on.
A wider range protects the sensor in that situation. The trade-off is the accuracy loss described above. The correct answer is usually two ranges, not one compromise: a low-range channel for leak detection and a high-range channel, or a sample dilution system, for the process excursion.
Three steps to a defensible range
1. Find the target concentration. This is the alarm setpoint, or the exposure limit, or the typical process concentration. It is the number you must measure accurately.
2. Multiply by two to three. The range should sit comfortably above the target so that normal excursions do not saturate it, and no further. A factor of ten adds nothing except error.
3. Check the over-range condition. Ask what happens if the process dumps its full inventory into the sample point. If the answer is that the sensor is destroyed, either the range is wrong or the installation needs protection: a dilution probe, a sintered restrictor, or an automatic shut-off valve on the sample line.
The cases where a wide range is correct
Emergency response instruments and multi-gas survey meters often carry wide ranges, and that is the right call. A fire crew does not know what they are walking into, so a channel that covers 0-1000 ppm and survives 2000 ppm is more useful than a precise one that dies on contact.
Leak survey instruments for gas distribution also use %LEL ranges that extend well beyond the alarm point, because the operator is sweeping an area rather than monitoring a fixed source.
The distinction is between a monitoring instrument, which knows its target and should be precise, and a survey instrument, which does not know and should be robust. Specifying a survey range on a monitoring application is where the accuracy gets wasted.
What to write in a specification
State the range, the accuracy and the basis of that accuracy in the same line, for example: 0-100 ppm H2S, ±2% FS. Then state the alarm setpoints, so that the supplier can check the two against each other. A specification that gives ranges without setpoints leaves the most important relationship unexamined.
Ranges and accuracy figures vary by sensor and by configuration. Confirm against the datasheet for the specific module before design freeze; this page explains the trade-off, not a product’s limits.