Accurately testing SpO₂ performance at 70% presents a practical and ethical challenge in human-based testing. Controlled studies capable of reaching this level require specialized clinical environments, strict medical supervision, and carefully selected subjects. Even then, maintaining a stable and repeatable 70% SpO₂ level for systematic device evaluation is difficult. For this reason, controlled simulation of PPG optical signals provides a practical and repeatable method for validating low-saturation performance in a controlled setting.

Methods and steps for testing SpO₂ = 70% using a functional testing instrument:
1. Align the LED and PD of the pulse oximeter under test with the PD and LED of the functional testing instrument, respectively. This will synchronize the LED of the functional testing instrument with the LED of the pulse oximeter under test, simulating reflected light from the skin.

2. Set the heart rate values for the red and infrared PPG signals of the functional testing instrument. First, measure the blood oxygen level using a fixed heart rate value.

3. Adjust the AC and DC values of the red and infrared light, labeled R_ac, R_dc, IR_ac, and IR_dc respectively. This yields the perfusion indices (defined as AC/DC * 100%) for red and infrared light, PI(R) and PI(IR), respectively. The R value can also be calculated as: R = PI(R)/PI(IR) = (R_ac/R_dc)/(IR_ac/IR_dc).

4. For illustration purposes, assume a reference R-curve = 100 – 30R has already been defined (R-curves are device specific and requires a separate testing procedure[KG1.1]). To test SpO₂ = 70, R is set to 1, because SpO₂ = 100 – 30R, and when R = 1, SpO₂ = 70. To obtain R = 1, R_ac = IR_ac and R_dc = IR_dc.

5. Example setting of the functional testing instrument: Set heart rate = 70BPM, R_ac = IR_ac = 10mV, and R_dc = IR_dc = 300mV. With the red and infrared LEDs of the functional testing instrument activated, the pulse oximeter under test receives the corresponding optical signals. If the device displays SpO₂ = 70% under these defined conditions, this indicates that the device can correctly measure 70% saturation within this controlled test configuration.

The SpO₂ = 70% example represents only one test point. By adjusting the functional testing instrument parameters, a full range of SpO₂ values can be systematically simulated to verify accuracy, operating range, and stability. Because the red and infrared AC/DC components can be independently controlled, each condition can be reproduced consistently — unlike human-based testing, where physiological variability prevents repeatable testing.
 
Similarly, adjusting the AC signal frequency enables controlled simulation of different heart rates to test heart rate performance and evaluate its effect on SpO₂ stability under defined laboratory conditions.

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