e-SI-Amp partner VTT (Finland), PTB (Germany) and e-SI-Amp stakeholder Tampere University of Technology (TUT, Finland) have been improving the measurement techniques and traceability of small electric currents in the Single Charge Aerosol Reference (SCAR) located at TUT.

Aerosol particles may play an important role in global climate change. Fine and ultrafine particles have also found to cause severe effects on human health. Regulation and control of aerosols requires robust techniques for monitoring and quantifying particle concentrations.

One way of measuring aerosol concentration in a gas sample is to charge up the particles and detect the resulting electrical current when they are accumulated in a ‘Faraday cup’ electrode. If the particle charging is accurately controlled, the electric current represents precise information on the particle concentration and can be used as an accurate reference to calibrate the sensitivity of other types of instrument.

Tampere University of Technology have developed a novel reference standard – the Single Charge Aerosol Reference (SCAR) to calibrate aerosol measurement devices. Its traceability is based on verifying aerosol charging and flow division with traceable current readout in the femtoampere (10^-15A) range. Working at such small current levels is difficult and at low aerosol concentrations the uncertainty of the SCAR is dominated by the noise and calibration uncertainty of the current measurement.

As part of the e-SI-Amp project, VTT used the Ultrastable Low Noise Amplifier (ULCA), developed by Physikalisch-Technische Bundesanstalt (PTB), to improve the calibration uncertainty of the current meter used in SCAR. The improved readout enabled the current noise of the SCAR itself to be characterized and suppressed, for example, by improving the calibration process and the design of the Faraday cup.