Voltcraft set hygrometer bl 20 trh plus moisture meter fm 200 voltcraft bl 20 trh fm 200 p this voltcraft set consists of the hygrometer bl 20 trh and moisture meter fm 200 p p the u voltcraft bl 20 trh thermo hygrometer u has been designed for measuring ambient air temperature in deg c or deg f and ambient air humidity the minimum and maximum readings taken during the measurement process can also be displayed the bl 20 trh is a neat compact hand held meter is able to show bot