I usually use the tax_query
parameter:
$args = array(
'post_type' => 'my_cpt_name',
'post_status' => 'publish',
'posts_per_page' => -1,
'tax_query' => array( array(
'taxonomy' => 'uom_system',
'field' => 'slug',
'terms' => 'metric-system'
) )
);
$results = get_posts( $args );
See http://codex.wordpress.org/Class_Reference/WP_Query#Taxonomy_Parameters