Device for measuring the water content of a substrate, in particular the skin

The device comprises: a high frequency alternating voltage generator the output of which is connected to a sensor probe 5 which is to be applied to a substrate to be measured, an amplifier the input of which is connected to the sensor probe, and a detector arranged to furnish a D.C. voltage signal based on the output from amplifier. The generator, the amplifier and the detector are located inside a metal envelope enclosure. An electromagnetic radiation shield is disposed inside the envelope to separate the amplifier and the detector from the high frequency generator. In operation, voltage output from the detector is indicative of the electrical impedance of the substrate as sensed at the probe. In the preferred application, the substrate is skin and the measured electrical impedance is indicative of the water content of the skin.

FIELD OF THE INVENTION 
The invention relates to a device for measuring the water content of a 
substrate. The device is of the type including: an alternating voltage 
generator the output of which is connected to a sensor probe intended to 
be applied to the substrate, an amplifier the input of which is connected 
to the sensor probe, and a detection means arranged to furnish a D.C. 
voltage signal based on the output signal of the amplifier. The level of 
this D.C. voltage signal is indicative of the water content of the 
substrate. 
More particularly, the invention relates to a device for measuring the 
water content of human skin. 
BACKGROUND OF THE INVENTION 
Devices proposed thus far, although they do to some degree indicate water 
content, do not produce sufficiently reliable results. Two prior art 
devices measuring the water content in a single substrate may well 
indicate very different contents. 
SUMMARY OF THE INVENTION 
A first object of the invention is to reduce substantially such disparities 
in the measurement of different devices on the same substrate. In other 
words, a good correspondence between the water content of the substrate 
and the measurements obtained from the device is sought. 
Another object of the invention is to furnish a device with which 
measurement are performed in a simple manner, with a sensor that is easy 
to use, and which measure water content at a significant depth into the 
substrate. 
The invention for measuring the water content of a substrate has a 
generator, an amplifier, and a detection means, all of which are disposed 
inside a metal envelope. The generator is a high-frequency alternating 
voltage generator an electromagnetic radiation shield or screen is 
disposed in the envelope to separate the amplifier and the detection means 
from the generator. A sensor probe is located at one end of the envelope 
and is connected both to the output of the generator and to the input of 
the amplifier. 
The frequency of the generator is greater than 100 kHz, preferably on the 
order of 10.7 MHz. The amplitude and frequency of the signal output from 
the signal generator is unaffected by variations in the magnitude of our 
output by the generator. Preferably, this generator is connected at its 
output, via a resistor, to the sensor probe. The resistance of this 
resistor is selected to be sufficiently high to provide correct insulation 
of the voltage generator and sufficiently low to enable obtaining a 
sufficient signal at the input of the amplifier. This resistance is 
optimally on the order of 1 kiloohm. 
The amplifier is selected to have a high input impedance so as not to 
affect the voltage at the probe. This input impedance is optimally on the 
order of 100 kiloohms. 
The metal envelope in which the voltage generator, the amplifier, and the 
detection means are accommodated is preferably in the form of a cylinder. 
The electromagnetic radiation shield or screen disposed in this envelope 
comprises a metal plate. This plate is located in a diametral plane of 
this cylinder to divide the internal volume of the envelope into two 
half-cylinders. The high-frequency generator is located in one of these 
half-cylinders, while the amplifier and the detection means are located in 
the other half-cylinder. 
Besides the features described above, the invention comprises a certain 
number of other features, described in further detail below in terms of an 
exemplary, but in no way limiting, embodiment described in the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a device 1 for measuring the water content of a substrate, in 
particular of human skin. This device includes a generator 2 which outputs 
a high-frequency alternating voltage signal of a frequency greater than 
100 kHz. Preferably this frequency is 10.7 MHz. The output of generator 2 
is connected via a capacitor 3 and a resistor 4 to a sensor probe 5. Probe 
5 comprises a metal rod or stylus which is intended to be placed in 
contact with the substrate. The generator 2 is an alternating voltage 
generator whose output voltage amplitude and frequency are unaffected by 
the magnitude of current output by generator 2. 
The device 1 also includes an amplifier 6, the input of which is connected 
via a capacitor 7 to the sensor probe 5. The amplifier 6 has a high input 
impedance on the order of 100 kiloohms so that it does not affect the 
measurement on probe 5. 
Detection means 8, the input of which is connected to the output of the 
amplifier 6, is provided to furnish a D.C. voltage signal based on the 
signals coming from the amplifier. 
When the sensor probe 5 is put in contact with the substrate, in particular 
the skin, from an electrical point of view, the end of the probe 5 is 
connected to ground via an impedance 9. The value of this impedance 
depends on the water content of the substrate. The level of the D.C. 
voltage signal on the output of the detection means 8, therefore also 
depends on the value of the impedance 9 and hence on the water content of 
the substrate. 
Voltage generator 2, amplifier 6, and detection means 8 are accommodated 
inside a metal envelope 10. The metal envelope 10 takes the form of a 
cylinder closed at its two longitudinal ends by transverse walls 11, 12. 
These walls are also metal. An electromagnetic radiation screen 13 
disposed in the envelope 10 separates amplifier 6 and detection means 8 
from the high-frequency generator 2. This screen 13 comprises a metal 
plate, disposed and fixed along a diametral plane of the metal envelope 
10, which is thus divided into two half-cylinders. The generator 2 is 
located in one of these half-cylinders, while the amplifier 6 and 
detection means 8 are located in the other half-cylinder. The envelope 10 
and the screen 13 can be made of a lightweight alloy, in particular an 
aluminum-based alloy. 
Sensor probe 5 is formed by a rod located at one end of the envelope 10. 
This rod lengthens along its axis orthogonal to the wall 12. The wall of 
the envelope is provided with a cylindrical extension 14 projecting toward 
the outside and surrounding rod 5. An insulating sleeve 15 of plastic 
material, in particular polytetrafluoroethylene, is provided to surround 
the rod 5 inside the extension 14. The inside end of the rod 5 is 
connected on one side of the screen 13 to the resistor 4 and on the other 
side of the screen 13 to a capacitor terminal 7, via respective short 
electrical conductors 16 and 17. Due to the particular configuration of 
the divider screen 13, and due to the reduced length of the conductors, it 
is not necessary to use coaxial cables for these electrical connections. 
The resistor 4 is selected such that the best response to the input of the 
amplifier 6 is obtained. The resistance of the resistor 4 is high enough 
to satisfactorily insulate the generator 2. The resistance is also low 
enough to permit obtaining a sufficient useful signal at the input of the 
amplifier 6. This resistance of the resistor 4 is optimally on the order 
of 1 kiloohm. 
The transverse wall 11, opposite that equipped with the rod 5, is 
penetrated in its central region by a cable 18. This cable has a conductor 
19 to supply the circuitry, particularly generator 2, with electrical 
power. This cable also has a conductor 20 for transmitting the output 
signal outputted from the detection means 8 to a unit 21. Unit 21 includes 
an energy source for supplying both the circuitry in the envelope 10 and 
display means 22 with power. Display means 22 preferably provides a 
digital readout of the signal level at the output of the detection means 
8. The numerical value displayed on the display means comprises the result 
of the measurement per se. 
A means (not shown) is provided to enable applying the probe 5 against the 
substrate at a constant pressure, with automatic triggering of the 
measurement once a desired application pressure has been obtained. This 
means may include a strain gauge placed on the extension 14. This means 
may also a sliding mount for attaching extension 14 and probe 5 to 
envelope 10. An elastic means, the compression of which determines the 
application force desired, would be included between the device and the 
substrate. 
To interface this device with a computer, an oscillator circuit controlled 
by the amplitude of the signal at the output of the detection means 8 is 
provided in the envelope 10. The output of this oscillator circuit is 
connected by a cable to the computer. The period of the signals furnished 
by the oscillator circuit contains the measurement information transmitted 
to the computer. The time interval between two leading edges of the signal 
furnished by the oscillator circuit comprises an image of the voltage of 
the output of the detection means 8. 
The function of the device described above is as follows. Once the device 
has been actuated, the unit 21 assures the supply of energy to the 
circuits of the envelope 10. The user then grasps the envelope 10 with one 
hand and presses the probe 5 against the skin at which the measurement is 
to be made. The measurement of the water content occurs in a direction 
substantially orthogonal to the surface of the skin. The user exerts 
sufficient force on the envelope 10 to obtain the desired pressure of 
application of the probe 5 to the skin. Reading of the measurement is 
performed at the display means 22 of the unit 21. 
Tests performed with a variable-frequency generator by plunging the sensor 
probe 5 into a brine has shown that a frequency of 10.7 MHz leads to 
highly dynamic measurement. This elevated frequency makes it possible to 
perform a deep measurement of the water content of a substrate. 
The physical, metallic separation between the generator 2 and the amplifier 
6, on the one hand, and between the generator 2 and the detection means 8 
on the other, provides electromagnetic insulation between these 
components. Accordingly, operation of the amplifier and the detection 
means is unaffected by radiation from the generator 2. 
By adjusting the gain of the generator 2 and amplifier 6, the same response 
curve can be obtained for different apparatuses. Measurements performed 
with different apparatuses in brines have made it possible to obtain very 
close results, with only about 1% deviation. The high input impedance of 
the amplifier 6 makes it possible to avoid inaccurate measurements due to 
loading of the generator 2. 
The sensor probe 5, which may be a simple rod or stylus, is of reduced bulk 
and is particularly simple to use. The arrangement of circuitry inside 
envelope 10, with separation by the screen 13, makes it possible to 
dispense with a shielded coaxial cable to connect the sensor probe 5 to 
the generator 2 and to the amplifier 6. 
In a variant of the present invention, the power and display devices 21 and 
22, respectively, are contained in the measurement head, thereby making 
the system portable.