Finally got the HydroVolt temperature compensated Specific gravity meter.
The design is elegant in its simplicity.
The black bulb is squeezed, the acid is slowly drawn into the unit to completely fill the chamber. The temperature wheel and the SG wheel rotate based on the SG of the acid, and the temperature of the acid.
The SG pointer then points at the corrected value of SG.
The photo shows the device reading my acid reference solution which has a SG of 1.265.The temperature was ~ 40 F, yet the instrument gives the correct SG