The determination of resistance of living tissues of organism to the alternating current

Laboratory work

Topic: The determination of resistance of living tissues of organism to the alternating current (AC)

Description of device and principle of its work:

Tissues of organism have an ohmic resistance and reactance (capacitance). At the passing of AC through the living tissue we deal with the total resistance – impedance Z. When ACpasses through such circuit the impedance Z is:

          .                                                          (1)

Formula (1) are showing that the impedance depend on the AC frequency f. For low frequencies take place and R_t and Z_C. For high frequencies (when f®¥, Z_C®0 and Z= R_t) take place only R_t. It is illustrated on the Fig.1.

Fig.1 The dependence impedance Z on the frequency f.

               Setup for the definition of living tissues resistance to AC is shown on fig.2. AC of given frequency transmits from the generator of low-frequency signals to the electrodes trough the attenuator and measuring instruments (ammeter).

     

Fig.2. Setup for the definition of living tissues resistance to AC.

Workalgorithm:

1. Set all handles and buttons in positions according to table 1.

2. Switch on the generator and wait for 10 min. for the stabilization of operating duty.

3. Set the output voltage of generator to 4V (use the upper scale of generator’s voltmeter).

4. Set the attenuator’s handle in the position 10dB. If value of voltage 4V weaken to 10dB, therefore the electrode’s voltage will equal 1.26V.

5. Set the first frequency of AC (table 1).

6. Make finger-tips action to electrodes and take reading of AC.

7. Measure AC for all frequencies.

8. Calculate the impedance Z=V/I and fill table 2.

                                                                                        Table 1.

№	handle	position
1	диапазоны частот (frequency band)	2-2×103
2	Время индикации ( indication time)
	0.1S	Нажата (keystroke)
	1S	Отжата (pulled position)
3	время развертки (scanning time)	1
4	<dB	10
5	Быстродействие (speed of operation)	Выкл. (off)
6	Стоп (stop)	Отжата (pulled position)
7	Пуск (start)	Нажата (keystroke)
8	знак частотной отметки (sign of frequency marker)	Отжата (pulled position)
9	Предел (margin) ниж. (lower), верх (upper)	Отжаты (pulled positions)
10	Реверс (reversion) вниз (down) вверх (upward)	Отжаты (pulled position)
11	вых U (output U)	4
12	вход компрессии (compression entry)	вых U (output U)
13	Режимы (modes)	SIN
14	Усреднение (averaging)	1
15	развертка частоты (frequency scanning)	ручная (hand feed)

                                                                                                        Table 2

f, [Hz]	Bandswitch	V, [V]	I ´10-5 [А]	Z=V/I, [W]	lnf
50	2-2×103	1.26			1.69
500					2.69
1000					3.00
2000					3.30
5000	2-2×104				3.69
10000					4.00
20000					4.30
40000	2-2×105				4.60
60000					4.78
80000					4.90
120000					5.08
200000					5.30

9. Draw the frequency dependence of impedance. Use logarithmic scale lnf for the convenience (fig.1).

10. Find the R_t. with using of the graphic.

11. Calculate the capacity of living tissues (value Z take for frequency f = 1000Hz):

.

12. Write the conclusion.