Gas quality, dew point and flow measurement across the whole plant
Running an electrolyser means measuring gas quality, pressure, flow, level and leaks. We start by setting out what has to be measured where, then confirm range, accuracy, signal type and the certification required in your country before finding a model.
WaterTreatmentCirculationCell / stackDryingAnalysisCompressionWhat this part does and what decides it
The safety limit is reached on the oxygen line, not the hydrogen line. The indicator is hydrogen in oxygen (H₂-in-O₂) and the ceiling is 2 percent. That is not a figure we set: it is the limit Korean and international safety regulation both work to. Sizing the measuring range from the rated point is not enough, because two effects stack. Permeation through the membrane is nearly independent of current while oxygen production scales with it, so at lower load the same permeation is diluted into less oxygen and the ratio rises. On top of that, raising the pressure widens the hydrogen partial pressure difference across the membrane and increases the permeation itself. Measured on a 23-cell HXS-2: at ambient pressure, 0.39 percent at rated load and 0.66 percent at 40 percent load; at 6 barg, the same two points are 0.98 and 1.75 percent. Set range and alarms from the lowest load and the highest pressure you will actually run, together. The method decides the running cost. Principles with no consumable cell need periodic calibration and nothing else; electrochemical cells have a service life, so replacement and a shorter calibration interval come with them. If trace oxygen at ppm level pushes you to an electrochemical unit, put that upkeep in the budget from the start. What each principle measures, and where they divide, is set out under "What each measuring principle actually measures" below.
Over the 2,400 hour continuous run on a 23-cell HXS-2, hydrogen in oxygen stayed below 2 percent for most of the run, with two brief excursions above it. That run was at 0.3 M KOH, 40 °C and ambient pressure. The sweep across load and pressure is a separate dataset: 49 points from ambient to 6 barg at 0.1 M KOH and 43–45 °C, and the four figures above are its four corners. The electrolyte concentration differs between the two datasets, so read that alongside them when you place them side by side. We do not have a load sweep at the 10 barg rating. Oxygen in hydrogen reached a maximum of 0.10 percent on the 2,400 hour run, and 0.10 percent again over the 294 hour window at 10 barg. If you are adding instrumentation to one of our systems, we work out with you how it ties into the leak detection, E-STOP and interlocks already there.
The Measuring principle column says what the instrument physically responds to. For the same oxygen concentration, each principle is strong over a different range and carries different consumables and calibration intervals. Read this column first and it becomes clear why some models belong at trace level and others at percent level. There are two families. Principles that consume the target gas reach lower, but the sensor has a service life. Principles that do not consume it last longer and hold calibration longer, but lose resolution as the concentration falls. The two flow meters are a different matter altogether: they measure how much is flowing, not what it is made of. How far down a given model reaches varies within a principle. That is what the Range column in the table records.
If the instrument goes into a hazardous area it has to be rated for that area. A single line such as Ex db IIB+H₂ T6 Gb in a catalogue is four separate things written end to end. The db at the front is the protection method. Flameproof (the Ex d family) contains an ignition inside a strong enclosure so it cannot spread; intrinsic safety (Ex ia, Ex ib) keeps the energy in the circuit below what it takes to ignite anything in the first place. Analysers with a display and mains power are usually flameproof; small transmitters with no display are usually intrinsically safe. The IIB or IIC in the middle is the gas group. The further along it goes the easier the gas is to ignite, and hydrogen sits in the most demanding group, IIC. A unit certified only to IIB cannot be used in a hydrogen area. Some markings add hydrogen on its own as IIB+H₂, so do not rule a unit out on the letters IIB alone. T6 is the temperature class. It says the surface stays below the ignition temperature of the gas, and T1 through T6 guarantees progressively lower surface temperatures. The Gb at the end is the equipment protection level: Ga corresponds to Zone 0, Gb to Zone 1, Gc to Zone 2. The certification schemes differ by country: ATEX in Europe, IECEx internationally, KCs in Korea. The same unit may still need a separate KCs certificate to be installed in Korea, so tell us the country of installation first. Below is only what we have confirmed against manufacturer documentation. A model left out is not a model without a hazardous-area version; it is one we have not confirmed. Note also that the hazardous-area version is usually a separate model number rather than an option on the same one. Get the part number wrong on the order and a general-purpose unit arrives.
The same measurement is read in different places depending on the model. That is what the Display column records. A screen on the instrument is the simplest case. Where the sensor and the screen are one body it is integrated with the sensor; where it is built to drop into a cabinet door it is a panel-mount type. Either way somebody reads the number on site. A model without a screen only puts the value out as a current. That is the 4-20 mA signal, and the number is visible only if something at the other end receives it: a PLC, a recorder, or the manufacturer's own separate readout. Miss this and the instrument is fine while nobody can see what it reads. Not all 4-20 mA wiring is the same. A two-wire loop carries power and signal on the same pair; a three-wire unit takes its power separately. Connectors are standardised too, so a model using a DIN 43650 connector needs the matching socket ordered with it. Instruments with a wide span often add auto-ranging: the unit changes its own measuring range as the concentration moves. On a bare 4-20 mA link that means the same 20 mA is 1000 ppm at one moment and 100% at another. Decide whether you also want the signal that identifies the current range.
This is where instrumentation most often goes wrong against upper-level control. Whether the command reached the unit, whether the unit accepted it, whether it is running, whether it finished, whether it failed: each of these has to be a separate state. Without that separation you get a screen that reads normal while nothing is actually happening.
The same quantity comes in different units from different instruments. This table alone has oxygen-in-hydrogen reported in percent and in ppm. If the tag units are not matched and the 4-20 mA scaling is carried over as is, the reading is out by a factor of ten thousand, and it often sits in a plausible range long enough that nobody notices. If you also bring pressure in, settle barg against bara for the same reason.
The options and the models we handle
Start here and we adjust to your conditions. We can usually source what is not listed.
Continuous hydrogen concentration on the oxygen line. That is where the limit is reached first, so unattended operation starts here. The H2 in O2 group below is where these sit.
Quote this option→Oxygen in hydrogen and dew point. These are the two values behind a stated purity target. Read the O2 in H2 and Dew point groups below together.
Quote this option→Hydrogen and oxygen flow measured separately and compared with the theoretical rate from current. This is what gives you Faraday efficiency. The Gas flow group below is where these sit.
Quote this option→Pressure, temperature and level are needed to run a stack too, but we do not supply them. Where each one goes is set out in the instrumentation table in the HXS-2 stack integration guide.
Oxygen-in-hydrogen and hydrogen-in-oxygen are measured by different methods, and where you read the value decides the rest. On oxygen in hydrogen the order of magnitude of your target is what selects the model, so settle first whether you are managing it in ppm, as a fuel cell or refuelling specification requires, or whether percent is enough for a purity check. Thermo-paramagnetic is strong across the percent range; electrochemical reaches ppm and, on some models, below. The Range column is that order of magnitude. Hazardous-area rating is not a column here: only the models we could confirm against manufacturer documentation are listed in the table under "How to read a hazardous-area marking" above.
| Model | Method | Range | Display | Quote this |
|---|---|---|---|---|
| Thermal conductivity | Selectable from 0–1% up to 0–100% | Built-in touchscreen | Quote this→ | |
MesskonzeptFTC 320↗ | Thermal conductivity | Selectable from 0–0.8% up to 0–100% | Built in | Quote this→ |
PanametricsXMTCpro↗ | Thermal conductivity | Order specific, per gas pair | With or without, by part number | Quote this→ |
Teledyne Analytical Instruments2000 · 2020↗ | Thermal conductivity | Order specific, per gas pair | Built in | Quote this→ |
HORIBATCA-5000↗ | Thermal conductivity | Selectable from 0–10% up to 0–100% | Built-in colour touch panel | Quote this→ |
ArchigasTCD3000↗ | Thermal conductivity | 0 to 5% | None, 4-20 mA three-wire | Quote this→ |
KNRH2-EC-OEM | Solid-polymer electrochemical | 0 to 4% H₂ | None, UART only | Quote this→ |
| Model | Method | Range | Display | Quote this |
|---|---|---|---|---|
Michell InstrumentsXTP601↗ | Thermo-paramagnetic | Selectable from 0–0.5% up to 0–50% | Built in | Quote this→ |
Teledyne Analytical Instruments3000MA↗ | Magnetodynamic (dumbbell) | Three user-selectable ranges down to 0–1% | Built in, auto-ranging | Quote this→ |
HORIBAVA-5000↗ | Magnetopneumatic | Order specific, per module | 5.7 inch touch screen, built in | Quote this→ |
Southland SensingOMD-507↗ | Electrochemical, fuel-cell type | 0 to 10 ppm through 0 to 100%, selectable | Backlit, auto-ranging | Quote this→ |
Panametricsoxy.IQ↗ | Electrochemical, fuel-cell type | 0 to 1000 ppm | Built in | Quote this→ |
Analytical IndustriesGPR-1900↗ | Electrochemical, fuel-cell type | ppm level, 0.05 ppm sensitivity | Panel-mount type | Quote this→ |
Analytical IndustriesGPR-1500↗ | Electrochemical, fuel-cell type | ppm level | Integrated with sensor | Quote this→ |
KNRO2-OEM | Luminescence optical | 0 to 25% O₂ | None, UART only | Quote this→ |
| Model | Range | Display | Quote this |
|---|---|---|---|
| Dew point −100 to +20 °C, accuracy ±2 °C | None, 4-20 mA two-wire | Quote this→ | |
STORK InstrumentsDEWCom II↗ | Dew point −100 to +20 °C, accuracy ±2 °C | None, 4-20 mA, DIN 43650 connector | Quote this→ |
| Model | Method | Range | Display | Quote this |
|---|---|---|---|---|
Alicat ScientificPortable mass flow meter↗ | Laminar differential pressure mass flow | Order specific, per gas | Built-in backlit display | Quote this→ |
BronkhorstEL-FLOW Select↗ | Thermal mass flow | 0.014 mln/min to 1,670 ln/min, order specific | None; separate readout module optional | Quote this→ |
You can buy direct from the manufacturer. Coming through us means we carry the international payment, the customs clearance and the minimum order, and you deal with a domestic invoice.
What to settle before a quotation
Only what has come up more than once in real enquiries.
It does, and raising the pressure raises it again. Measured: at ambient pressure, 0.39 percent at rated load and 0.66 percent at 40 percent load; at 6 barg, the same two points are 0.98 and 1.75 percent. Read the multiple rather than the absolute value. Dropping to 60 percent load multiplies it by 1.25 to 1.43 and dropping to 40 percent by 1.67 to 1.94, and those multiples hold across pressures. How much headroom is left is set by pressure. At ambient there is room to spare at 60 percent load against the 2 percent control limit; at 6 barg and 40 percent load only 0.25 percentage points remain. Minimum load under pressure is therefore a function of pressure. For deeper turndown we recommend a minimum load limit, depressurising at low load and a hard interlock on the oxygen side.
That value is a purity indicator more than a safety one. The limit is reached first on hydrogen in oxygen. Whether you measure both or only the safety indicator depends on unattended operation and the installation conditions.
Partly. If hydrogen in oxygen exceeds 2 percent, stop and inspect. That line is set by regulation, so it is not raised for operating convenience. Electrolyte replacement is not decided on this value alone; we look at it together with the cell voltage trend at the same current and the spread between cells.
If you want to read the markings yourself, see "How to read a hazardous-area marking" above; the reference table of the models we have confirmed is there too. Protection method, gas group, temperature class and equipment protection level are run together into one line, and hydrogen is at the demanding end, so look for IIC or IIB+H₂. Note also that the hazardous-area version is usually a separate model number rather than an option on the same one, so check the part number on the order once more before it goes out.
For monitoring, yes. For a safety trip, no. A network link can be delayed or drop, and a safety trip cannot be late, so wire it directly. Treat a stop command sent over the network as the secondary path. That is why you should check how many volt-free contacts a unit has when you choose it.
Once these are set, selection and quotation happen in one step. If they are not, we start there.
Fill in only what you know. We ask about the rest.
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