Smiths BCI created an information peace on pulse oximeter calibration. The oximeter calibration article can be found here.
"Pulse oximeters do not require a zero calibration because the design incorporates continuous automatic zero calibration. Gain calibration is not required because the measurement technique does not require gain accuracy."- Smiths BCI
Any type of product that has many manufacturers and has a significant range in price tend to have a range in quality. If an oximeter is reading incorrectly it is either broken or of poor quality.
A quality oximeter such as the Nonin Onyx Vantage 9590 has been clinically tested for accuracy and durability. Nonin also has military certification for their products. Nonin is the only company in the world that has received US military certification for their finger oximeters.
We do not sell inaccurate finger oximeters. We have created a pulse oximeter information page on inaccurate oximeters. The majority of the finger oximeter brands are from Chinese companies that focus on low prices instead of accuracy and durability.
If you require an oximeter to monitor the health of yourself and/or your patients we highly suggest you research the brand you are purchasing. There is a significant difference in quality and accuracy.