In order to test the accuracy of a CO2 alarm, you should purchase a higher test gas concentration than the alarm requires.
For example: if you have a device that sounds an alarm at 10,000 ppm, you should buy test gas rated at 15,000ppm or 20,000 ppm.
The reason is that the gas concentration on a canister is not exact. 10,000ppm CO2 rated at 99% purity might only reach 9,900ppm, which would not be enough to trigger the alarm. You might think the sensor is defective.
Therefore, the only way to insure your test is successful is to use test gas that is over the threshold of the sensor.