What is used to indicate that a desiccant lay-up is within requirements?

Humidity indicator cards are specifically designed to show whether the moisture levels in a desiccant lay-up are within acceptable limits. These cards change color based on the level of humidity, providing a visual and straightforward method to determine if the environment is suitable for the preservation of materials or equipment. This is particularly important in maintaining the integrity of sensitive components that can be adversely affected by moisture.

Other options, while related to measuring humidity, do not serve the same purpose as humidity indicator cards. A humidity gauge measures current humidity levels but does not provide a direct indication of whether the desiccant is still effective. A hygrometer also measures humidity but may not offer a clear visual cue like the humidity indicator cards. A dew point meter measures the temperature at which air becomes saturated with moisture but is not designed to indicate the effectiveness of desiccants. Hence, humidity indicator cards are the most suitable choice for assessing whether a desiccant lay-up meets the necessary requirements.