These inexpensive manometers measure positive, negative and differential pressures. Dwyer Flex-Tube U-Tube Manometers combine the inherent accuracy of the simple "U" Tube with the durability of tough, long-lasting plastic construction. Series 1221 Manometer, the simplest, lowest cost basic U-gage. Series 1222 Manometer, alll the features of the 1221 plus magnetic clips. Series 1223 Manometer, the finest U-gage for either portable or stationary use.