Before use the particle counter take off the Dust cap

Particle counter which can help us check the air quality around us.
Also can use demo air purifier. but what to do before you use the laser particle counter?
There has a dust cap at the top of the right. It is to protect the sensor.
When we do not use the machine , some dust will easy go inside the machine.
So we use the dust cap to avoid dust go inside the device, let it can work long and result correct.