Buoyancy is an upward force exerted by a fluid that opposes the weight of an immersed object. If an umbilical cable is too light it will pull an ROV up, which makes piloting fiendishly difficult when manoeuvring through offshore structures. If the tether is too heavy it will push the ROV down making delicate work impossible. So how do we make sure we get it just right? Find out how we are able to test down to a single gram, up to 200 bar.