There are a number of applications where it is either not possible to use air purging or desirable to minimize the volume of air used.
This could be to minimize compressed air energy costs, or to avoid a negative impact (combustion or thermodynamic).
The general principle is two-fold:
1. Avoid contaminants entering the sight tube and contaminating the lens or protective window.
2. Avoiding hot gas entering the sight tube and damaging the lens or protective window.
Using air purge - creates a positive pressure preventing gas or particles entering the sight tube.
Inlet diameter must be greater than exit diameter.
Not using air purge - this can be an option and really only utilized where a clean compressed air supply is not possible
This method serves to slow down the rate at which the sensor lens/window will become contaminated.
It is always recommended to use a protective window for the sensor. For Endurance, the front window as standard is a low-cost replaceable item. For T4.0 you will need to use the appropriate protective window accessory (can be ordered pre-installed with the corrective transmissivity parameter set during production.
Special Note:
Generally, a longer sight tube has less need for air purge or offers the possibility for lowest compressed air costs.
An Endurance sensor offers the change to use the longest tube due a high D:S specification and variable focus.
This can be important when you need to not have too large a spot size in the far field.
For lower D:S sensors like T4.0, the optimum focus is 200mm (CF2).