Standard Horizon HX751 Owner's Manual page 19

Thermo Sensor
To measure a water temperature, soak the radio so the sensor is covered
by water for several minutes to see the actual water temperature.
It may take a few minutes for the temperature to register correctly.
The temperature sensor may not display the correct temperature
when the internal temperature of the radio is to hot; for example,
while transmitting a lot.
Microphone
The internal microphone is located here.
When transmitting, position your mouth about 1/2 to 1 inch (1.2 ~
2.5 cm) away from the small mic hole. Speak slowly and clearly into
the microphone.
Speaker
The internal speaker is located here.
Battery Pack Lock ( Bottom side )
Turn the Battery Pack Lock to the "OPEN" position for battery removal.
HX751
NOTE
NOTE
NOTE
Page 19