Later on, Roche Diagnostics happened to be pondering the question ‘Wouldn’t it be useful to have a non-electrical version of the ion selective electrode?’ especially for use in critical care wards where the patients are monitored by a bank of electrical monitors. A device with a non-electrical front end is less likely to suffer cross-talk. Again, fluorescent PET sensors turned out to be a satisfactory answer since they have an optical front end \cite{optimedicalcom}. Now these fluorescent PET sensors are seeing use in ambulances so that blood sodium levels can be determined in seconds at the scene of an injury. What is good for a human is also good for other animals. So it is no surprise that fluorescent PET sensors are also seeing use in veterinary situations \cite{idexxcom}.