Wheatstone Bridge


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Wheatstone Bridge


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Wheatstone Bridge (Direct Current Bridge) : 

Wheatstone Bridge (Direct Current Bridge) Direct current bridge is an instrument that used to measure the resistance or change in resistance and converts it to output current This bridge circuit are also used in control circuit, when one arm of the bridge contains a resistive element that is sensitive to the physical parameters (Temp, Pressure , Load) Wheatstone Bridge It consists of 2 parallel resistance branches, each branch contains two series elements (Resistance) DC volt is used as a power source Null detector (Galvanometer) is connected to detect balance Ch4

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Using the bridge to determine unknown resistance 1- Assume R4 is unknown ,, 2- Assume we can change one resistor in the bridge (say R1) till the balance condition At case of unbalance (reading= Va-Vb)

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Example (1) Determine the value of unknown resistor Rx in Fig. assuming the balance condition Solution At Balance Vo=0

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Sensitivity of the bridge When the bridge is in an unbalance, current flows through the galvanometer, causing deflection of the pointer. The sensitivity of the bridge = S Thevenin Theory for the bridge

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Thevenin Theory for the bridge

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Example Calculate the current through the galvanometer shown in Fig Solution

تهيئة إشارة القنطرة لتوصيلها على الحاسب : 

تهيئة إشارة القنطرة لتوصيلها على الحاسب + Vi Vo R1 R1 R2 R2 Difference Amp. Isolator (Buffer) + + + V1 V2

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Ch5 Blood Pressure and other Cardio Vascular Measurements Firstly the blood pressure is measured in arteries There are two kind of arteries pressure 1-Systolic pressure It is the pressure in arteries at case of heart contraction ≈ 120 mmHg 2- Diastolic pressure It is the pressure in arteries in the case of heart relaxation ≈ 80 mmHg Pressure = F = Force in Newton A is the area in m2 1 Pa = N/m2

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A small coin has a diameter of 1cm and a mass of 1.59 gram Find (a) Gravitational force (Weight) (b) The pressure caused by the coin Solution (a ) Force = Mass. Acceleration

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Pressure Measurements The air on the surface of the earth has a pressure value called atmosphere (1 atom) = 760 mmHg If the pressure is measured with respect to vacuum (0 atom) it is called absolute pressure ( zero pressure is reference) If the pressure is measured with respect to atmospheric pressure (1 atom) it is called gauge pressure (1 atom pressure (760mmHg) is reference ) Pressure in human circulatory system is measured with respect to atmospheric pressure Gauge pressure is usually given in mmHg above or below the atmospheric pressure Zero gauge pressure is 1 atom

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Blood pressure measurement There are many methods that can be used for blood pressure measurement 1- Direct measurements (Invasive) 2- Indirect measurements (Noninvasive) 1- Indirect Measurements This method is used for routine clinical measurements of blood pressure in human, a suitable technique without painful or hazard is required The instrument consists of an inflatable rubber bladder called cuff, rubber squeeze ball pump, assembly valve and manometer. The manometer might be a mercury column or dial gauge

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The cuff is wrapped around the patient upper arm; the stethoscope is placed over the artery. The cuff is inflated so that the pressure inside the cuff becomes greater than the expected systolic pressure. This pressure compresses the artery against the bone and shuts off the flow of the blood in the artery. 3-The pressure in the cuff then slowly released (using the valve) when the pressure of the cuff equal the systolic blood pressure the blood starts to flow and the operator can hear a crashing sound in the stethoscope. Then the systolic pressure can be watched on the dial gauge or in the mercury column. 4-The pressure of the cuff is lowered more and more and when the cuff pressure equals the diastolic pressure , the sound in the stethoscope is disappeared. Then the diastolic pressure can be watched

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The Ultra-sound blood Pressure Measurements The Ultra sound determination of blood pressure uses a Doppler sensor to detect the motion of blood vessel walls. The Fig. shows the placement of compression cuff over two small transmitting and receiving ultra sound crystals (8MHz) on the arm. The reflected signal (shifted in frequency) is detected with the receiving crystal. The difference in frequencies in the range from 40 t0 50 Hz, depends on the velocity of the wall motion and blood velocity.

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As the cuff pressure increased above the diastolic pressure (80mmHg) but below the systolic pressure, the vessel opens and close with each heart beat, the opening and closing of vessel are detected by ultra sound system. As pressured increased as shown in Fig. the time between the opening and closing decreases until they coincide. The reading at this point in the manometer or dial gauge is the systolic pressure. Conversely, when the pressure in the cuff is reduced, the time between opening and closing increases until the closing signal on pulse coincide with opening signal of the next one. The reading in this case is the diastolic pressure. The advantages of the Ultra sound method can be used with infants can be used in high noise environment

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Automatic Blood Pressure Measurements 1- The user adjusts the pressure of the system (by touching digital bottoms) this expected pressure above the expected systolic pressure 2- The pump triggers and gives a pressure to the cuff 3- The cuff pressure is measured using strain gauge system (calibrated) 4- The adjusted pressure from step 1 and the measured pressure by strain gauge and applied by the pump (step 2) are compared through a comparator 5- When the values of 2 pressures are equal, the comparator works in 2 ways (a) Give signal to stop the pump (b) Give signal to solenoid valve to start to release the cuff pressure gradually

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6- The pressure is always measured by the strain gauges and recorded all the time using a memory system 7- The microphone (which placed under the cuff) detect the first sound when the cuff pressure equal the systolic pressure. This will trigger the memory and then store s the systolic value 8- The output of microphone is connected with a comparator with a minimum level of sound can be recorded 9- When the pressure reaches to the diastolic pressure the microphone output sound reaches to the value of minimum level of sound, then the comparator works and gives a signal to the memory to record the value of diastolic pressure ,, also it gives a signal to repeat the process again (a) Close the solenoid (b) Trigger the pump to be ready to work again

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