When it comes to monitoring the weight of a lift axle, there are two basic configurations to be concerned with, integrated lift axles and independent lift axles. The only thing that separates these two configurations is how the air is supplied to the air bags.
Integrated lift axles are supplied by the same Height Control Valve as the fixed axles in the group. Meaning when the lift axle is down, the HCV now has to supply two extra air bags. When purchasing a gauge to monitor an axle group with an integrated lift axle, you will only need to buy one gauge to monitor the entire group. When checking the weight of your axle group using a Right Weigh gauge, the integrated lift axle must be in the same position that it was in when the gauge was last calibrated(except 201 series).
Independent lift axles are separate from the Height Control Valve (HCV) that supplies the fixed axles. Independent lift axles typically use a manual air regulator to adjust how much weight they will carry. When purchasing a gauge to monitor an axle group with an independent lift axle, you have two options:
Option one is to purchase a digital gauge with two air sensors (for two HCVs). This option will display exactly how much weight is on the fixed axle group and lift axle at all times.
Option two is to purchase one gauge to monitor only the fixed axles. With this option, you will calibrate and check your Right Weigh gauge only when the lift axle is in the up position. This will still allow you to ensure you are within the legal limit for the gross vehicle weight but does not allow you to know the exact weight for the axle group when the lift axle is down.