function conv( val, _from, _to ) {
	
	var val = prepare_in( val );
	var exc1 = _from[_from.selectedIndex].getAttribute( "exc" );
	var exc2 = _to[_to.selectedIndex].getAttribute( "exc" );
	
	var tmp = exc1 / exc2;
	tmp = (val * tmp) - ( -( _from.value - _to.value ) * ( 0.001 / exc2 ) );

	return prepare_out( tmp );
}