I have a device for measuring blood pressure that can be powered by a AC/DC 6V adapter, inanition to batteries.  Since I don't have such an adapter amongst my many adapters, I bought a AC/DC 6V adapter from Trademe. However when the said adapter arrived yesterday the plug doesn't fit my blood pressure device - it is too big. I had naively assumed all 6V adapters would be the same. Clearly they are not. What do I need to ensure that I buy the right 6V adapter from my blood pressure device?  (I checked the manual for device and it just refers to an optional  "Adapter S").