For a laboratory checking H2S, NH3 and CO in hydrogen fuel to ISO 14687, preparing the calibration points by dynamic dilution costs about 2,990 € per year, against about 8,990 € per year with ready-made low-level cylinders: about two thirds less, diluter included, with a payback of about 20 months. Dilution also reaches the 4 ppb sulfur and 100 ppb ammonia targets that no certified cylinder covers, with a calculated uncertainty for every point. For CO alone at 200 ppb, a certified cylinder remains a perfectly good choice.
Why is ISO 14687 calibration so hard with cylinders alone?
ISO 14687 (and its Chinese counterpart GB/T 37244) sets maximum impurity levels for hydrogen used in fuel cells. Many of these limits sit in the low ppb range. To measure them reliably, the analyzer needs reference gases close to the sample level, ideally several points that bracket it, rather than a single calibration point far above it.
Three analytes show the three situations a hydrogen quality lab meets:
- Carbon monoxide (CO), target 200 ppb. Stable, and certified cylinders exist at the target level.
- Ammonia (NH3), target 100 ppb. Difficult to handle. No certified cylinder at 100 ppb; the lowest available is in the same decade.
- Total sulfur, target 4 ppb. Far below the lowest certified cylinder. H2S is used as the representative sulfur molecule.
| Analyte in hydrogen | ISO 14687 target | Lowest certified cylinder | Gap |
|---|---|---|---|
| CO | 200 ppb | 200 ppb | x1 |
| NH3 | 100 ppb | 500 ppb | x5 |
| Total sulfur (as H2S) | 4 ppb | 100 ppb | x25 |
Table 1. ISO 14687 target vs lowest certified cylinder used in this comparison
With a x25 gap, a sulfur result calibrated on a 100 ppb cylinder is closer to an indication than to a measurement.
What do certified cylinders offer at low concentrations?
The lower the concentration, the less a certified cylinder gives you: wider uncertainty, a wider production tolerance (how far the delivered value can be from the value you ordered), a shorter certificate validity and a higher price.
| Mixture | Concentration | Expanded uncertainty | Production tolerance | Certificate validity | Price |
|---|---|---|---|---|---|
| H2S / H2 | 1 ppm | 5% | 20% | 36 months | 991 € |
| H2S / H2 | 200 ppb | 10% | 50% | 12 months | 991 € |
| H2S / H2 | 100 ppb | 10% | 20% | 3 months | 1,144 € |
| NH3 / H2 | 50 ppm | 5% | 10% | 36 months | 586 € |
| NH3 / H2 | 1 ppm | 5% | 10% | 6 months | 586 € |
| NH3 / H2 | 500 ppb | 5% | 20% | 6 months | 696 € |
| CO / H2 | 50 ppm | 0.5% | 5% | 60 months | 426 € |
| CO / H2 | 200 ppb | 2% | 5% | 24 months | 569 € |
Table 2. Typical certified cylinders in a hydrogen matrix (indicative 2026 prices)
A 100 ppb H2S cylinder costs more than a 1 ppm one, has twice its uncertainty, and must be replaced every three months instead of every three years.
How much does each approach cost per year?
We compared two ways of running the same calibration programme for the three analytes over two years, expressed as a yearly cost.
Assumptions
- Full calibration of each analyzer every 3 months, with a weekly performance check in between.
- Calibration points: H2S at 1,000, 200 and 100 ppb; NH3 at 1,000 and 500 ppb; CO at 200 ppb. This is the minimum with cylinders.
- Each cylinder is charged over its certified life: a 36-month cylinder counts for one third of its price per year.
- One dedicated pressure regulator per cylinder (400 €, used for 10 years, so 40 € per year).
- Diluter: about 13,000 €, depreciated over 7 years (1,860 € per year), with a verification every 2 years (400 € per year).
- Gas consumption is not a limiting factor. Stock management and ordering time are not counted.
| Cylinders only | Dynamic dilution | |
|---|---|---|
| H2S | 1 ppm (330 €) + 200 ppb (991 €) + 100 ppb (4,576 €) = 5,897 € | 1 ppm parent cylinder: 330 € |
| NH3 | 1 ppm (1,172 €) + 500 ppb (1,392 €) = 2,564 € | 50 ppm parent cylinder: 195 € |
| CO | 200 ppb: 285 € | 50 ppm parent cylinder: 85 € |
| Regulators | 6 x 40 € = 240 € | 3 x 40 € = 120 € |
| Diluter | None | 1,860 € depreciation + 400 € verification = 2,260 € |
| Total per year | about 8,990 € | about 2,990 € |
Table 3. Yearly cost, three analytes, ISO 14687 programme
The diluter is included in the 2,990 €. The saving, about 6,000 € per year, comes mostly from the short-lived low-level cylinders: the 100 ppb H2S cylinder alone costs about 4,576 € per year because it is replaced every quarter.
How long until the diluter pays for itself?
Leaving out the depreciation, running the dilution setup costs about 1,130 € per year (three parent cylinders, three regulators and the verification), against about 8,990 € per year for cylinders only. That is about 7,860 € saved each year, so a diluter of about 13,000 € pays for itself in about 20 months.
What about lead time and cylinder management?
Some costs are hard to put a number on, but every lab manager knows them.
- Lead time. The more complex the mixture and the lower the concentration, the less predictable the delivery. If a new cylinder arrives early, its validity overlaps the old one and money is wasted. If it arrives late, which happens often, the lab has no valid certified gas for a while.
- Number of orders. A 100 ppb H2S cylinder valid 3 months means four orders a year, each with its own delivery risk. A 1 ppm parent valid 36 months means one order every three years.
- Management time. Tracking expiry dates, placing orders, chasing late deliveries and handling returns all take time that is not spent on analysis.
- Storage and safety. More cylinders means more space, more regulators, more rental and more pressurised gas on site.
Is a diluted gas as accurate as a certified cylinder?
A gas prepared by dilution is a secondary standard: its uncertainty combines the uncertainty of the parent cylinder with that of the calibrated mass flow controllers. GasMix AlySoft calculates this expanded uncertainty for every point it generates.
Because the parent cylinder is more concentrated, it is certified more tightly and stays valid longer. The diluted point therefore keeps an uncertainty close to the parent's, and it is generated at the value you programmed, not at a value that may differ by 20 to 50% from the one you ordered.
| Analyte | Parent cylinder | Point generated | Expanded uncertainty | Cylinder alternative at the lowest level |
|---|---|---|---|---|
| H2S | 1 ppm | 100 ppb | ± 5.4 ppb (5.4%) | 100 ppb, 10%, tolerance 20% |
| H2S | 1 ppm | 2 ppb | ± 0.11 ppb (5.5%) | Not available |
| NH3 | 50 ppm | 100 ppb | ± 5.5 ppb (5.5%) | 500 ppb, 5%, tolerance 20% |
| NH3 | 50 ppm | 50 ppb | ± 2.7 ppb (5.5%) | Not available |
| CO | 50 ppm | 200 ppb | ± 4.5 ppb (2.2%) | 200 ppb, 2%, tolerance 5% |
Table 4. Calibration points calculated by AlySoft (AIOLOS III, two channels, hydrogen as diluent)
For CO, the two approaches are equivalent. For NH3 and H2S, dilution gives points at and below the ISO 14687 target, with an uncertainty comparable to or better than the closest cylinder (5.5% vs 5% for NH3, 5.5% vs 10% for H2S), which sits 5 to 25 times higher.
When is a certified cylinder still the right choice?
Cylinders and dilution are not opposites: a diluter always starts from a certified cylinder. The question is whether you buy each level separately, or one concentrated cylinder per analyte. Five questions decide it:
| Question | Ready-made cylinders fit when... | Dynamic dilution fits when... |
|---|---|---|
| Target concentration | the target is sold as a certified level (CO at 200 ppb) | the target is below the lowest certified cylinder (NH3 at 100 ppb, H2S at 4 ppb) |
| Calibration points | one point is enough | you need several points that bracket the sample |
| Number of analytes | one or two, checked occasionally | three or more in a routine programme |
| Required uncertainty | the certified uncertainty at that level is acceptable | low-level cylinders are too wide or too far from the target |
| Rhythm | occasional, project-based checks | weekly checks and quarterly calibrations |
Table 5. Cylinders or dynamic dilution: decision guide
Does this only apply to hydrogen fuel?
No. The same reasoning holds for many trace analyses: whenever the method asks for a level below the lowest stable certified cylinder, or for several calibration points, one concentrated parent cylinder and a diluter cost less and get closer to the sample than a set of low-level cylinders. Sulfur compounds, ammonia and other reactive species at ppb levels are typical cases.
How do you set this up in a hydrogen quality lab?
- A two-channel GasMix diluter, such as AIOLOS III or ZEPHYR II, with hydrogen as the diluent.
- One concentrated certified parent cylinder per analyte.
- AlySoft sequences that generate each calibration point automatically and report its uncertainty. The simulation mode lets you check the dilution plan before ordering any gas.
For measured data on H2S, COS, SO2 and NH3 in hydrogen at ppb levels, see our application note with PAC.
Frequently asked questions
Why are there no certified H2S cylinders at 4 ppb?
At ppb levels, sulfur compounds adsorb on cylinder walls, so producers cannot guarantee a stable value for long. Certified H2S in hydrogen is typically available down to about 100 ppb, with a 3-month validity.
Is a diluted gas traceable?
Yes. It is traceable through the certificate of the parent cylinder and the calibration of the mass flow controllers. GasMix diluters follow ISO 6145-7, the reference method for dynamic dilution with thermal mass flow controllers.
How much can dynamic dilution save on ISO 14687 calibration?
In our three-analyte example, about two thirds: roughly 2,990 € per year instead of 8,990 €, diluter included. The diluter pays for itself in about 20 months, before counting storage, ordering time and late deliveries.
Does ISO 14687 require multipoint calibration?
ISO 14687 sets impurity limits, not a calibration method. Good analytical practice calls for several points close to the sample level when you measure near the detection limit, which is the case for all three analytes here.
Should I stop buying calibration cylinders?
No. A diluter still needs certified cylinders, just fewer of them and at higher, more stable concentrations: one cylinder, many standards.
Planning an ISO 14687 calibration programme?
Send us your analytes, target levels and analyzer, and we will propose a dilution plan with the parent cylinders and calibration points.
Ask our team