I had a look for myself on Amazon and individual reviews. Nonin was terribly expensive. I read the TempIR - Pulse Oximeter review. The price was £29.95, a sort of in-between price. It's made in China, but has very peasant features, like you can orient the reading, either facing you, for facing downward, or facing the other way for a nurse to read. I compared with the Nonin that my nurse had and we had the same reading. hers cost £178/ she told me that this was a "normal" price for NHS purchase. I'm quite happy with the accuracy of mine!
The physiotherapist at the hospital told me not to be fixed on the result. a reading once a day is enough. I read it once after putting the bin outside, I was really short of breath. The reading went down to 66%. But I applied the following exercise that the physiotherapist had shown me:
Sit gently, put your hands just above your navel. Breathe into your navel(see your tummy go up with the in breath and down with the out breath) do this 10 times.
next, put your hands on the top part of your chest. Breathe in slowly. When you are near "the top" of the breathing in, Sniff forcefully that bit more air in and block the breathing for a couple of second. Relax deeply. do this 19 times.
This really works to get your oxygen level back. It also gives you a feeling of serenity and freedom with that feeling that you have enough air.