An electrometer is only as good as its weakest insulation
An electrometer is a voltmeter with input impedance high enough that the current into it can, for practical purposes, be treated as zero; input resistance is on the order of 1014 Ω. At that impedance a picoampere leakage path is a measurable error, which means every component on the signal path has to be chosen with its insulation in mind, and the switching element is very often the weakest link because an open contact sits directly across the measurement.
Electrometers are widely used in radiation detection: radiation ionizes gas in a chamber and produces a very small current, which is amplified and quantified to indicate radiation level. The switch routing that current faces two requirements at once — it must add almost no leakage, and it must not add capacitance or contact artifacts that bury the signal.
Why the usual switches do not hold up
- Semiconductor switches carry parasitic capacitance that swamps a femtoamp-level signal at the acquisition node.
- Open mechanical relays leak too much, and their contacts grow resistive surface films that need a certain voltage to “frit” through — a small signal may not cross the film at all.
A sealed reed contact avoids both: the glass capsule gives very high insulation resistance and a film-free contact surface. The HVFR-HI high-voltage, high-insulation series is built for this duty.
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 is specified here
- Insulation. At 100 TΩ the open contact adds negligible parallel conductance, so adjacent weak-signal channels do not leak into one another.
- Sealing. Contacts sealed in glass are isolated from dust, humidity and corrosive atmospheres in field detectors.
- Weak-signal contact material. The contact material is chosen for switching the minute currents of an acquisition front end rather than for power duty.
- Voltage margin. 3 kVDC breakdown leaves headroom in high-impedance, high-voltage measurement chains.
- Life and packaging. Billions of operations for continuous automated scanning, in 1- to 4-channel configurations that simplify multi-channel scanner design.
Selection note
In an electrometer or ionization channel the switch is usually the component that bounds the achievable insulation, not the amplifier input. Selecting for insulation resistance and a clean, film-free contact surface — and then guarding the surrounding layout to the same standard — matters more here than operate speed or current rating, which is the opposite weighting from a power-switching application.
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