Some people infected with Covid-19 get themselves a pulse oximeter to check the oxygen level (saturation) of their blood, since sinking levels can be an early sign they’re developing pneumonia without obvious symptoms. A clip-like device that’s usually placed on a fingertip, it uses light beams to estimate the oxygen saturation and pulse rate. Medical opinions vary on the usefulness of this practice. Pulmonologist Dr Jens Geiseler is sceptical of pulse oximeters for home use, which can be purchased in pharmacies, for example. He says they’re often too inaccurate, especially at low saturation l…