Voltcraft bl 20 trh digital thermo hygrometer voltcraft bl 20 trh p the voltcraft bl 20 trh thermo hygrometer 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 both the values simultaneously in the display p voltcraft bl 20 trh thermo hygrometer