It is much more accurate and easier to handle than psychrometers or even chilled-mirror hygrometers. Calibration every year is largely sufficient for most applications. Testo 645 is ideal for all museum applications like spot readings inside the museum and can serve as a reference for hygrothermographs and other climate measuring devices. Equipped with the stainless steel dust protection cap (see photo) it is ideal to check the conditioning of silica gel or other granular products: the probe can be stuck right into the bulk material. To measure the humidity inside stacks of paper you can use a sword shaped probe which uses the same sensor. Testo 645 uses a conventional 9V-battery.
