Shark attacks on humans are incredibly rare, but they do still happen from time to time. As with any danger that could potential end your life, companies have developed what they see as solutions to the problem and in this case it’s in the form of electronic shark deterrents that claim to keep the aquatic predators at bay.

Whether or not these kinds of devices actually do anything is the subject of a new study published in PLOS One, and unfortunately it seems that there’s some pretty huge differences depending on which brand you choose.

The researchers tested a commercially available wearable called the Electronic Shark Defense System, or ESDS. Its creators claim that it produces an electrical field that drives sharks away, and it’s designed to be worn around the ankle of a surfer or diver.

To determine just how effective the device is, the researchers tested it against a large population of white sharks in a shark hotspot in South Africa. While the device was active, the team logged nearly 400 “encounters” with over 40 individual sharks. That’s a whole lot, and as the scientists explain in their paper, the frequency of shark encounters was the same whether the device was on or off.

The small bit of good news is that the frequency of shark biting seemed to be less when the ESDS was active, but other aggressive behavior like bumping actually increased, suggesting that it’s not a great option for protection from sharks.

“Given the very short effective range of the ESDSTM and its unreliable deterrent effect, combined with the fact that shark-bite incidents are very rare, it is unlikely that the device would significantly reduce the risk of a negative interaction with a white shark,” Dr. Ryan Kempster, lead researcher, said in a statement.

The team had previously tested another brand of electronic shark deterrent called Shark Shield, and while both it and the ESDS claim to work in a similar manner, the Shark Shield actually proved to be worth its price. The Shark Shield kept sharks at an average distance of around 1.3 meters, the researchers say.