We apologise for this. A batch of our test strips were not functioning as they were supposed to be. This was identified quickly, and as far as we are aware, free of charge replacements were sent to all affected users. In case you did not receive your free of charge replacement please let us know so we can take appropriate action. Please use our chat helpline on poolsan.uk.com so that one of or experts can help you in case you have any questions or need any help using our products.