H2 Detector Always Reads 4% Instead of 1% During Calibration

Hi all,
Hoping someone here has run into this before.
We’re using a GD200-H2 portable H2 gas detector (range 0–4 Vol%), and when we try to calibrate it with a 1% H2 cylinder (balance Nitrogen), the reading doesn’t stay at 1%.
Instead, it keeps climbing all the way up to 4%, then alarms (red light + beeping).
So basically, it should read 1%, but it just maxes out.
Details:
1. Pump-type portable detector (1–4 Vol%)
2. Flow rate: < 1.5 LPM
3. Pressure: < 1 bar (per manual)
4. Display unit: Vol%
Also wondering — is it possible to switch from vol% to ppm on this device? Any ideas appreciated.

AA wrote:This usually points to a flow/pressure issue rather than the detector itself. If your flow is anywhere near 1.5 LPM, that might already be too high. A lot of these detectors are calibrated for much lower, controlled flow rates.

AA wrote:Yeah, and I’d also ask how long you're applying the gas?
If you're not giving it enough time to stabilize, the reading can overshoot like that. Pretty common with pump-type detectors.

AA wrote:One thing that stands out, you mentioned “less than 1 bar.” That might actually be the problem. The detector usually expects a stable pressure, not just “below a limit.” If the pressure is fluctuating, readings can go all over the place.

Agree with the others. I’d double check the full setup: regulator, tubing, leaks, etc. In my experience, when a detector pegs at full scale during calibration, it’s often something in the setup rather than the detector failing.

Got it. For reference, our flow is under 1.5 LPM and pressure is under 1 bar as suggested in the manual.

AA wrote:Thanks for the details. Based on your situation, please verify the following:

The pressure must be stable at 1 Bar; it may not fall below 1 Bar.
The flow rate should be stable at 750 ml/min, not close to or exceeding 1.5 LPM;
The gas application time must exceed 90 seconds to ensure proper stabilization.
If all the above conditions are confirmed and the issue persists, please arrange a call with our technician for further assistance.

Regarding your question about units: switching is only available between ppm and mg/m3. It is not possible to switch between %vol and ppm on this detector.

That helps a lot, thanks. We’ll try again with a stable 1 bar supply, lower the flow closer to 750 ml/min, and ensure we apply gas for over 90 seconds.