Measurement of AC Resistance / Impedance of the Human Body

1. Aim

To estimate the human body’s AC impedance using SEELab3 with an AC source (WG) and the built-in high input impedance at A2.

2. Apparatus / Components Required

3. Theory & Principle

This is a measurement using a voltage divider model:

Let:

Current through the series path is: \(I=\frac{V_{A2}}{R_{in}}\)

Voltage across the body is: \(V_{body}=V_{A1}-V_{A2}\)

So the estimated impedance is: \(Z_{body}=\frac{V_{body}}{I}=(V_{A1}-V_{A2})\cdot\frac{R_{in}}{V_{A2}}\)

For comparison/learning (at moderate frequency like 1000 Hz), $Z_{body}$ can be reported in kilo-ohms.

4. Circuit Diagram / Setup

  1. Connect the AC source WG (sine wave) to one electrode and also to A1 for monitoring.
  2. Connect the other electrode to A2.
  3. Ensure SEELab3 GND reference is correctly wired (as per your hardware).
  4. Keep electrode contact area similar between trials.

5. Procedure

  1. Launch the SEELab3 app and open the AC resistance / impedance measurement screen (or oscilloscope/plot mode with $V_{rms}$ readouts).
  2. Set:
    • WG frequency = 1000 Hz
    • WG amplitude = start with a moderate value so RMS voltages stay within A1/A2 range.
  3. Enable analysis/readout for:
    • A1 (input RMS)
    • A2 (output RMS)
  4. Touch the electrodes with intact skin and hold contact stable for 3–5 seconds.
  5. Record:
    • $V_{A1,\text{rms}}$
    • $V_{A2,\text{rms}}$
  6. Calculate $Z_{body}$ using $R_{in}=1\text{ M}\Omega$: \(Z_{body}=(V_{A1}-V_{A2})\cdot\frac{10^6}{V_{A2}}\)
  7. Repeat for different contact conditions (dry, wet, increased contact area).

6. Observation Table

Reference: $R_{in} = 1.0\text{ M}\Omega$

Condition $V_{A1}$ RMS (V) $V_{A2}$ RMS (V) Estimated $Z_{body}$ (k$\Omega$) Remarks
Dry hands        
Dry hands with coin/contact increase        
Wet hands        

7. Precautions

  1. Use only the low voltage AC output from SEELab3 (WG). Never connect to AC mains.
  2. Do not touch with cut/bruised skin or if you have any wound.
  3. Keep contact stable to avoid large fluctuations.

8. Error Analysis

9. Troubleshooting

Symptom Possible Cause Corrective Action
RMS values look wrong or unstable Weak/no contact between electrodes and skin Reposition electrodes; maintain steady touch
$V_{A2}$ is very small Divider is not formed correctly Verify WG→electrode→body→A2 path
Computed $Z_{body}$ is unrealistically low/high Wrong electrode polarity or wiring Swap electrode connections and retake

10. Viva-Voce Questions

Q1. Why do we need the high input impedance at A2?

Ans: Human body resistance is typically very high (often in the M$\Omega$ / hundreds of k$\Omega$ range). A high $R_{in}$ ensures the divider current is measurable and the voltage drop can be detected.

Q2. Derive the formula for $Z_{body}$ from $V_{A1}$ and $V_{A2}$.

Ans: Current is $I=V_{A2}/R_{in}$. Body voltage is $V_{body}=V_{A1}-V_{A2}$. So $Z_{body}=V_{body}/I=(V_{A1}-V_{A2})\cdot R_{in}/V_{A2}$.

Q3. Why does wetting hands decrease the measured resistance?

Ans: Water (with dissolved salts) improves ionic conduction through skin, reducing resistance/impedance and increasing divider current.

Q4. Is $Z_{body}$ purely resistive at AC?

Ans: No. Skin and electrode interfaces add non-ideal effects (capacitive/complex impedance), but divider-based measurement still gives a useful estimate for comparison.