GasMix™ · Instrument guide
Choosing the right MFC range for your instrument
The full-scale flow range of each flow controller sets the span of concentrations you can actually produce and how accurately. Here is how to size it correctly the first time.
01What we are talking about
A GasMix™ instrument builds a gas concentration by metering, in real time, the flow of each channel through a mass flow controller (MFC). The full-scale flow range i.e. the maximum flow the MFC can measure and regulate is essentially the "size" of that flow controller. This is what we call its range.
An MFC rated at 100 mL/min and one rated at 500 mL/min do not deliver the same thing: they cover different flow spans. Selecting the range means choosing the flow window each channel will work in and, in turn, the concentration window your instrument can produce accurately.
The one idea to rememberA range is not chosen "as large as possible, for headroom." It is chosen so that your working points fall in the zone where the MFC is most accurate: between 10 % and 100 % of full scale.
02The golden rule: aim for 10 – 100 % of full scale
On GasMix™ instruments, the highest accuracy is achieved between 10 % and 100 % of full scale, with a repeatability better than 0.1 % and an accuracy of ± 1 % of setpoint across that band. This is the working window to design for. An MFC can still operate down to 2 % of full scale, but with a higher uncertainty. Anyway our AlySoft software provides you with uncertainty calculcation, wherever you are on the range of the MFC.
Working zone of an MFC range = 100 mL/min · example
Same rule across the rangeAll GasMix™ instruments operate from 2 % to 100 % of full scale, with the highest accuracy from 10 % upward. Aim your working points at the upper part of the scale on every model.
03The method, step by step
The GasMix™ software automatically computes each channel's flow from your parent-cylinder concentration and the controller's range. Our job — when ordering, or before a new application — is to make sure those computed flows land within the operating range of the MFC.
1. List your extreme working points
Identify the lowest and highest concentration you want to generate, together with the concentration of your source gas (parent cylinder). These two extremes are what constrain the choice.
2. Convert them into flows
For a dilution, the source-gas flow is: Q_source = (C_target / C_parent) × Q_total. The most dilute point needs the smallest flow; the most concentrated point needs the largest.
3. Size one controller per channel
A dilution uses two flow controllers : one for the source gas, one for the diluent. And the dilution ratio comes from the ratio of the two flows. For each channel, choose the range whose full scale holds that channel's highest flow, so its points sit high on the scale. A well-chosen pair reaches even deep dilutions: a small source range running near its low end alongside a large diluent range near full scale. The calculator below sizes both for you.
04Range calculator
A dilution uses two mass flow controllers : one for the source gas and one for the diluent. The dilution ratio is set by the ratio of their two flows. Enter your application and the tool sizes both. Indicative results; to be confirmed by Alytech for your specific gas and instrument model.
Size your two channels
Source gas diluted in a carrier — the source channel and the diluent channel. Flow units are up to you — just keep them consistent (e.g. mL/min).
05A worked example
You have a cylinder of NO at 1000 ppm and you want to generate a calibration series from 10 to 200 ppm, at a fixed total flow of 1000 mL/min. A dilution uses two channels — the source gas and the diluent — so two flow controllers.
Converting to channel flows
Each channel gets one controller, sized to its own highest flow:
| Channel | Flow | Controller | Highest point |
|---|---|---|---|
| Source gas | 10 – 200 mL/min | 200 mL/min FS | 100 % FS |
| Diluent | ≈ 800–990 mL/min | 1000 mL/min FS | 99 % FS |
Two controllers — a 200 mL/min source range and a 1000 mL/min diluent range — cover the whole series. The most dilute point (10 ppm → 10 mL/min) sits at 5 % of the source range, well within the operating range; raising the total flow would lift it into the optimal zone if needed. For a much deeper dilution, the source controller is simply a smaller range running near its low end while the diluent runs near full scale — still two controllers.
06Two common mistakes
Oversizing "for headroom"
The most frequent mistake. A single, very large range pushes your low points far down the scale, out of the optimal 10–100 % zone. The apparent headroom costs you accuracy on your most dilute points — where a second, smaller range would have kept them optimal.
Undersizing and saturating
Too small a range saturates beyond 100 % FS on your high points: the instrument physically cannot deliver the requested flow, and your highest concentrations become unreachable.
Good sizing lives between the two, keeping every point high on its scale. When one application spans several decades, the answer is not to stretch a single MFC but to configure it with several ranges — or to use multi-stage (cascade) dilution.
07Your checklist before locking the range
- I have listed my lowest and highest target concentration, plus the parent-cylinder concentration.
- I have converted those extremes into flows on each channel.
- My highest flow stays ≤ 100 % FS of the chosen range.
- My lowest flow stays ≥ 2 % FS.
Follow these and every concentration you request lands in the zone where the instrument holds its best accuracy, while the software computes and reports the final uncertainty of each point for full traceability.
References: operating range 2–100 % FS on all instruments, highest accuracy from 10 % upward; repeatability < 0.1 %; ISO 6145-7 compliant.
Case-by-case: available flow ranges depend on the gas, the instrument model and the configuration. For any specific application, have the configuration validated by AlyTech — gasmix@alytech.fr.