A tracer-gas detector is a weak-current instrument
A leak-detector development project shows the problem concretely. Helium or hydrogen is used as a tracer gas, and the instrument is effectively a mass spectrometer tuned to that gas: tracer is sprayed over the container under test, which is connected to the analyzer, and any leak produces a response that locates the leak and gives its rate. The method is extremely sensitive, and that sensitivity is exactly what makes the front end difficult — it begins with currents so small that leakage or a switching artifact can drown the signal.
The switch sits at the highest-impedance node
The pickup electrodes deliver very weak currents, usually across several channels because multiple pickoffs or test ports are scanned in turn. Each channel is switched into the chain before the op-amp, then the ADC and MCU. The relay therefore sits directly ahead of the amplifier, at the highest-impedance point in the whole system, and any weakness in its insulation becomes a parallel path that steals signal. The required relay insulation is on the order of 1014 Ω, which an open mechanical relay cannot reach; the HVFR-HI high-insulation series is built for this position.
The failure mode is worth tracing end to end. A relay with inadequate insulation lets the pickup current bleed away before the amplifier; the amplified signal is then too small, the ADC digitizes a wrong value, and the MCU reports a leak rate — or a false “no leak” — that does not match the part. The detector’s sensitivity is bounded by the insulation of its input switch, which is why a 1014 Ω-class relay is a measurement-accuracy requirement rather than an upgrade.
HVFR-HI specifications
- Switching voltage to 1 kV (2 kV momentary)
- 1 A switching; pulse current 5 A / 10 A / 20 A
- Dielectric withstand 3 kV and above; insulation resistance 100 TΩ and above
- Switching within 1.2 ms; life above one billion operations
Why the part matches the front end
- Insulation. 100 TΩ (1014 Ω) holds the open-channel leakage below the pickup current and keeps scanned channels isolated from one another.
- Sealing. Glass-sealed contacts are isolated from dust, humidity and corrosive atmospheres around the test station.
- Weak-signal contact material chosen for minute currents rather than power duty.
- Voltage margin of 3 kVDC breakdown in the detector electronics.
- Life and channels: billions of operations for automated cycles, in 1- to 4-channel configurations that build a multi-point scanner from one package family.
Selection note
As in an electrometer, the input switch — not the amplifier input — is usually the component that caps achievable insulation in a tracer-gas detector. Specify the relay to the 1014 Ω class and then guard the board traces around it to the same standard; a relay that meets the figure cannot rescue a layout that leaks across its own pins.
About SHR AUTOSENSOR
SHR AUTOSENSOR TECH LIMITED (SHR AUTOSENSOR) manufactures high-voltage reed relays, mercury wetted relays and high-voltage DC contactors. Contact: [email protected] | +86 13761571029 | www.reed-relay.com