True Lift
True Lift reflects a considerably more accurate lift capacity number. While the numbers may seem smaller, they are reflective of usable lift available to the diver while wearing the unit in the water. Manufactures that are not using the "new method" of determining lift will likely have "over-inflated" ratings. Sometimes as much as 30% higher than the exact same product rated under the new standard.

Under the "old method" a BC is put flat (open) in water, fully inflated. Weights are then suspended to, or put on the vest, until it sinks. The amount of weight determines the lift capacity rating. Unfortunately, this method introduces a few errors into the measurement:

Under the old method the BC is measured open, while in actual diving conditions it is bent around the diver’s torso. This change in conditions can make a difference of up to 20%.

In many cases under the old method weights are added in full 1 or 2 pound increments. It may only require a few more grams to sink the BC at the limits of its buoyancy. However since the increments are higher, the lift capacity numbers may inaccurately reflect this.

As mentioned above, under the old method the amount of buoyancy is equal to the amount of weight required to sink the BC. It does not take into account the lift of the water onto the weights (Archimedes Principal). This decreases the actual weight in water by about 10%.

Considering the above points, a BC rated with 44lbs of lift under the old method might decrease to an actual buoyancy of 29 lbs! This breaks down to reduction of 20% (9 lbs) because it was measured flat as opposed to fitted around a diver; a reduction of 2 lbs due to the measuring increments used; and a reduction of 10% (4 lbs) when the lift of the water on the weights is considered.

In contrast, under the current method the BC vest is assembled on to a tank with the straps closed as if being worn by a diver. The tank has additional ballast in order to keep the whole unit continually submerged. The unit is suspended to a weight gauge and fully immersed. The weight in water of the system is measured with the BC fully inflated and deflated. The difference between the two is the actual buoyancy of the BC vest, which is the buoyancy (True Lift) a diver can really use. This method of measuring lift capacity is also independent from the weight-in-air of the vest.

True Lift numbers are especially important for cold-water divers and smaller persons. Cold-water divers require higher lift capacities because they use more environmental protection and consequently more ballast (lead weights). Smaller persons use smaller BC's that simply put, generally have less lift because there is less material. On a side note, rear inflation buoyancy compensators (as opposed to jacket style) almost always have the same True Lift across the sizes. If you are replacing or upgrading a buoyancy compensator it is also valuable to keep this in mind. Under the new standard a BC with comparable lift capacity may be rated at 30% less than the one you currently own.

We hope this information sheds some light on why current lift capacity (True Lift) numbers may seem smaller. When choosing a Buoyancy Compensators it is important to know that a manufacturer is rating their lift capacities by the current method.