Yes - Saturated (or unsaturated) salt solutions, and certain other chemicals, can be used to generate an environment of a particular relative humidity in an enclosed space. The value of relative humidity obtained depends on the particular chemical salt, the concentration of the solution, and the temperature of use, among other things.

Saturated salt solutions, with solid salt present, have the special property that a stable concentration is maintained - and hence a constant relative humidity - even if water migrates to or from the solution. Ready-made capsules are commercially available for a variety of relative humidity "fixed points". Such capsules are designed for insertion of relative humidity probes, with a seal providing a small, enclosed atmosphere of defined relative humidity. Certain of these, if in the form of solution with solid salt present, are re-useable, and can be calibrated to assign a traceable value to the reference humidity they provide.

Other humidity calibration systems employing unsaturated salt solutions exist; for example, in the form of single-use ampoules of solution. These are typically used to soak a pad in a housing designed for exposing a sensor to the humidity generated. These ampoules, too, may be supplied with a traceable calibration, on the basis of tests of samples from batches of ampoules.

As for any other calibration, traceability through a laboratory with UKAS or equivalent accreditation provides the best assurance of a reliable measurement.

Salt solutions can also be used in various ways to provide nominal (uncalibrated) values of humidity. Used in conjunction with a calibrated hygrometer, such nominal humidity environments can also be useful for calibration.

Just as it is for relative humidity in general, temperature stability is critical for calibration with salt solutions, particularly since the properties of the salt solutions change with temperature. Stabilisation after a change in temperature may take an hour, or many hours, depending on the size of the change. For freshly mixed solutions, full stabilisation may take days or weeks.

It is always vital to ensure that no salt contacts the sensing parts of the hygrometer, as this would cause errors in readings, and would destroy some sensors.

Forms of "salt check" offered for sale as a "kit" to be mixed into solution by the user, is not usually suitable for providing any kind of traceable calibration or reliable check by itself, except when used in conjunction with a calibrated hygrometer. Salts used in dry form, in particular, may provide only very approximate values of humidity, and may degrade quickly. Unsaturated solutions also generate atmospheres of relative humidity, but since their concentration gradually changes with repeated use, so does the generated humidity.

Further information and tables are given in the publication "A Guide to the measurement of Humidity".