The FHZ76V energy-compensated Geiger-Müller tube is sensitive to γ (gamma) radiation and β (beta) radiation above 0.25MeV. It contains a Valvo 18550 tube (Barth 2012; Philips 1961, 1999), which is equivalent to the Centronic ZP1320, Mullard MX164 and LND-713 models (Di Luigi & Giarratana 2011 and Holm 2007). The sensitivity of the ZP1320 is given as 9 cps mR-1 s-1 for Cs-137 (Centronic 2009). For a natural radiation background of 0.025-0.045 mR h-1, this results in a counting rate of approx. 10-20 cpm. The resulting conversion 1 cpm = 15.625 nSv/h-1 was experimentally confirmed (see here) by comparing a FH40T geiger counter (which uses a FHZ76V tube) with a Berkeley Nucleonics Isotope Spectrometer SAM 940 in Tokyo in May 2011. The average natural radiation level was taken from Swedish Radiation Safety Authority's report Strålmiljön i Sverige (SSI-2007-02).
Further sources & documentation:
Juerg Wenger's webpages
Steffen Barth's webpages
DIY Geiger Counter webpages by John Giametti