What is a key characteristic of direct-reading instruments?

Direct-reading instruments are designed to provide immediate numerical results or readings when exposed to a particular environment or substance, such as hazardous gases or radiation. This characteristic means that users can quickly assess conditions in real-time, which is crucial in situations like confined space rescues where time is often of the essence. The instant feedback from these instruments helps responders make informed decisions promptly and ensure safety.

Other options highlight aspects that are not associated with direct-reading instruments. While some instruments may require recalibration for accuracy, this does not define direct-reading capability. Similarly, manual calculations pertain to other types of analytical instruments that require processing data after measurement, which is contrary to the purpose of direct-reading tools. Finally, while direct-reading instruments can be employed in radiation detection, they are not exclusively used for this purpose, as they also play vital roles in monitoring toxic gases and various environmental hazards.