Results of the experimental investigation of large volume DC glow discharge with coaxial grid electrodes in low-pressure helium are presented. Measurements of plasma parameters were carried out with a single-probe technique using the Impedans Langmuir Probe system. The distribution of the main plasma parameters including electron energy distribution functions was obtained. It is shown that the discharge volume is represented by negative-glow plasma with low electron temperatures of the order 0.3 eV. The second derivatives of probe I-V characteristics with respect to the potential evinced the presence of spectra of fast electrons born in reactions of Penning ionization of impurity particles with helium metastable atoms. We propose and implement a simple method for monitoring the presence of impurities and metastable atoms of the main gas and for estimation their densities.