La Crosse Technology WS-9121 - Manual (Page 11)

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7. Insert the batteries to the third transmitter as soon as the "channel
2" icon and outdoor data are displayed on the temperature station.
Then within 2 minutes, the channel 3 outdoor data from the third
transmitter will be displayed and the channel icon will shift back to
"1" once the third transmitter is successfully received. If this is not
happen, user shall restart the setting up from step 1.
Note: User shall insert the batteries into the third transmitter within 45
seconds of reception of the second transmitter.
8.
In order to ensure sufficient 915 MHz transmission however, this
should under good conditions be a distance no more than 100