2. Skin impedance Flashcards
Which current is this?
Direct current
Which current is this?
Alternating current
Which current is this?
pulses
What does Ohm’s law state in case of direct current?
- In case of direct current U = I·R, that is, voltage (potential difference) is proportional to the current intensity.
- The constant of proportionality R is called resistance.
What does Ohm’s law state in case of alternating current?
- In case of alternating current, Urms = Irms·Z .
- The root-mean-square (rms, i.e., effective) voltage is proportional to the effective (rms) current.
- The constant of proportionality Z is called impedance.
What information does Impedance (resistance in an AC circuit) measurements provide?
Information about the skin which are valuable in understanding the biological effects of the electric current and its hazards, contact protection and diagnostics.
Determination of the resistance (R) or of the impedance (Z) of the skin is attributed to the measurement of the ___ and ___ according to the circuit diagram
- voltage
- current
In Determination of the resistance (R) or of the impedance (Z) of the skin,
Which devices we need to use?
- an arbitrary waveform generator as a source of DC or AC
- a digital multimeter as an ammeter
- Two metal electrodes - auxillary and measuring electrodes - are applied for the connection with the body.
In the equivalent circuit diagram of the measuring circuit (Fig. 3) two skin layers of different area (A1, A2) but the same thickness (l ), and a muscle tissue are considered to be connected in ___
series
Describe Schematics of the measurement of skin resistance. (Thickness of the skin is enlarged in the figure).
- resistance and capacitance of the skin under the measuring electrode is R1 and C1 respectively,
- resistance of the muscle is Rm,
- resistance and capacitance of the skin under the auxiliary electrode is R2 and C2 respectively.
Is the skin a conductor or insulator? What is the consequence? Which type of circuit will it be?
Skin is considered a conductor and insulator at the same time
→ the piece of skin under the electrodes of area A and thickness l is represented with a resistor R and capacitor C connected in a parallel circuit (Fig. 4).
How many circuit elements does the electric model of skin have?
5
take a look at the situation of the direct current measurement (central part of Fig. 5)
→ Which elements do not play a role? Why?
In this case capacitances C1 and C2 do not play a role, as current flows through the resistances only.
take a look at the situation of the direct current measurement (central part of Fig. 5)
→ Comparing the surface area of measuring electrode and axillary electrode?
→ What is the relationship between the resistance and area?
- The surface area of the measuring electrode is much smaller than that of the auxiliary electrode, A1 << A2,
- The resistance is inversely proportional to the area
(where p is the specific resistance (resistivity) of the skin.)
take a look at the situation of the direct current measurement (central part of Fig. 5)
→ What is the equivalent resistance? How can we reach this value?
Resistance of muscle, Rm,which is rich in electrolytes is small compared to the resistances R1 and R2.
This way both Rm and R2 are negligibly small compared to R1
=> the equivalent resistance of the resistances connected in series circuit is
take a look at the situation of the direct current measurement (central part of Fig. 5)
→ What is the only significant resistance in the circuit?
the resistance of the skin under the measuring electrode R1.
Let us consider the circumstances of the alternating current measurement (right side of Fig. 5), by assuming that the frequency is “large enough”.
Capacitive reactance of the capacitor is ___
where f is the frequency of the alternating current
=> the greater the frequency, the smaller the capacitive reactance.
Let consider the circumstances of the alternating current measurement (right side of Fig. 5)
→ Capacitive reactances of the capacitors (XC1, XC2) are much __ compared to the resistances connected in parallel
→ What is the consequence?
smaller
→ at high enough frequencies the electric current flows mostly through the capacitors. → Resistances R1 and R2 can thus be neglected.
Let consider the circumstances of the alternating current measurement (right side of Fig. 5)
As the surface area of the measuring electrode is much smaller than that of the auxiliary electrode
→ What is the relationship between the skin capacitance and area under electrodes
As the surface area of the measuring electrode is much smaller than that of the auxiliary electrode, A1 << A2
→ capacitance of the skin is proportional to the area under the electrodes
(where e is the dielectric constant of the skin)
Skin impedance
Describe resistance of muscle
Resistance of muscle, Rm,which is rich in electrolytes is small compared to the resistances R1 and R2
Let consider the circumstances of the alternating current measurement (right side of Fig. 5)
→ What is the the equivalent impedance of the reactances?
the equivalent impedance of the reactances connected in series circuit is:
Let consider the circumstances of the alternating current measurement (right side of Fig. 5)
→ The equivalent capacitance of the measuring system at a sufficiently high frequency of the AC current is determined mostly by ___
the capacitance C1 of the skin under the measuring electrode.
Skin impedance
What information does direct current (DC) measurement provide?
the resistance R of the skin under the measuring electrode
Skin impedance
What information does the alternating current (AC) measurement provide?
capacitive reactance XC of the skin under the measuring electrode
Formula for the resistance of a conductor
R = p(l /A)
where p is the specific resistance of the material, l is the length, and A is the cross sectional area of the conductor. Its unit is the ohm (Ω).
Formula of capacitance (elements and units)
Its capacitance is C = e( A / l)
- e is the dielectric constant
- A is the surface area of the plate
- l is the distance between the plates.
- Its unit is the farad (F).
What is the equivalent resistance of the resistors connected in a series circuit?
the sum of individual resistances.
Re = R1 + R2 + …
What is the total impedance of capacitive reactances connected in series?
the sum of their individual capacitive reactances. Ze = XC1 + XC2 +…
What is SPECIFIC RESISTANCE (RESISTIVITY) OF THE SKIN?
resistance of the unit area of the skin:
p * = R·A.
The unit of specific resistance is Ωm2, or kΩcm2.
What is specific capacitance of the skin?
capacitance of the unit area of skin: y *= C /A.
The unit of specific capacitance is F/m2, or F/cm2.
