A photoelectric device that is applied externally to a site on the body surface of a paediatric or adult patient for transcutaneous measurement the percent oxygen saturation of haemoglobin in a volume of tissue (StO2) to assess the amount of oxygen delivered to that tissue, typically during circulatory or perfusion examinations of skeletal muscle or when there is a suspicion of compromised circulation. It is connected to a tissue saturation oximeter via a fibreoptic cable. This is a single-use device that has previously been used on a patient and has been subjected to additional processing and manufacturing for an additional single-use patient application.