Those handheld skin analyzers are based on devices used to quantify skin hydration in many cosmetic studies.
The most commonly used one is the Corneometer made by Courage-Khazaka. While there have been a few studies looking at differences in measurements between the Corneometer and other devices, like the Skicon made by I.B.S., very few have looked at how well they work at actually measuring the hydration of the skin.
These devices measure either the conductance or capacitance of the skin. Conductance is the skin’s ability to conduct or pass an electrical charge, whereas capacitance is the measurement of the skin’s ability to hold a charge. The idea is that an increase in the capacitance or conductance of the skin indicates an increase in the amount of water within the skin. The Corneometer is based on skin capacitance, and the Skicon is based on skin conductivity.
The problem is that many things can increase the capacitance or conductance of the skin, like moisturizers. When you’re measuring your skin with one of these devices, you’re not just measuring the water in the skin…but also everything that’s dissolved in it.
An unpublished paper compared the Corneometer and Skicon against solutions of different substances commonly found in skin care products and found that they can have a large effect on the results. Increasing concentrations of salts like sodium chloride or calcium chloride increased the capacitance and conductance measurements of the devices. Different types of solvents like glycerin or ethanol also affected the readings, and the Corneometer tended to be more sensitive to dissolved substances than the Skicon.
So these devices should be left in the range of fun and interesting, but probably aren’t a great indication of actual hydration levels in your skin. Even if you apply it on the same area of skin, using the same type of water (tap water’s conductance and capacitance changes day to day), things like the room’s humidity, or if you’ve sweat recently will affect the readings. As well, using the devices on cleaned skin doesn’t capture the effect created by the moisturizer.
Ideally one would measure the moisturizer, the skin with the moisturizer on, and then “cancel” out the effect from the moisturizer itself – but I think that is beyond what the consumer devices are capable of, especially the ones that rate skin hydration with smiley faces :)!