Which statement is true regarding the routine maintenance of a phoropter?

Routine maintenance of a phoropter is essential for ensuring its accuracy, functionality, and longevity. Using a protective dust cover when not in use is a beneficial practice, as it helps to prevent dust, dirt, and other contaminants from accumulating on the device. This precaution can significantly reduce the need for frequent cleaning and maintenance, as a clean environment is essential for the reliable performance of the optometric equipment.

The other options do not reflect best practices for phoropter maintenance. Ammonia-based solutions can be too harsh and may damage the sensitive components of the phoropter. Regular recalibration is necessary, but daily recalibration is excessive; typically, it is done less frequently depending on usage. Additionally, storing the phoropter in high humidity is not advisable, as moisture can lead to damage or malfunction of the equipment.