Mass. Gen. Laws ch. 149, § 109

The preceding section shall not apply to textile factories equipped with such a number and type of standardized self-registering hygrometers, or psychrometers, or such a hygrometric system as the department approves, or using the sling hygrometer frequently to determine the actual moisture and temperature of the weaving or spinning department; provided, that the manner of using all such instruments or such system is approved by the industrial health inspector in whose district the factory is situated, and that the records of the readings from said instruments or system are not destroyed without the knowledge and consent of such inspector.