# signal conditionning

Thread Starter

#### alitronic

Joined Jun 13, 2020
60
Hello everyone, I have a question about the operation of the circuit.
I know that the Wheatston bridge is a sensor but how pressure measurment is obtained.
Thank you for your answers.

#### Delta Prime

Joined Nov 15, 2019
1,311
Is it just me or is everything out of focus this morning. Oh my goodness I believe it's me bye-bye good morning everyone!

#### AlbertHall

Joined Jun 4, 2014
12,396
This might be a bit easier to see?

#### Reloadron

Joined Jan 15, 2015
7,580
In a balanced state the bridge output is zero volts. If we change the resistance in one leg the balance is upset resulting in a voltage out. So I can have for example three 350 Ohm resistors and a 350 Ohm strain gauge. The strain gauge resistance is 350 Ohms making a balanced bridge but as I apply a strain (pressure force) on the strain gauge the bridge becomes unbalanced with an output proportional to the amount of strain. I suggest you read this or similar to understand Wheatstone Bridge Theory of Operation.

Ron

Thread Starter

#### alitronic

Joined Jun 13, 2020
60
Good morning, can you explain me how the following circuit work (how the pressure measurment is made by the circuit)
Thanks

#### MisterBill2

Joined Jan 23, 2018
19,459
Tje bridge circuit, as already described, has the four resistors being devices known as strain gages, which means that their resistance increases as the conductors are stretched and decreases as they are compressed. These resistors are bonded to a diaphram in a housing so that as some fluid pressure increases the diaphram flexes, which causes two if the resistors to stretch and the diagonally opposite pair to be compressed . This unbalances the bridge, causing a difference in the two voltages at the side corners. Normally that voltage would be fed to a circuit called an instrument amplifier. But the circuit shown is not an instrument amplifier, it is instead a differential input amplifier. The over-al performance is similar but the operation is different from an actual instrument amplifier.
The voltages at the two sides of the bridge are (in theory) equal when the pressure is zero and they change a small amount as the bridge becomes unbalanced. The amplifier only amplifies this small difference voltage, providing an analog voltage in proportion to the pressure deforming the diaphram.

Thread Starter

#### alitronic

Joined Jun 13, 2020
60
Thread Starter

#### alitronic

Joined Jun 13, 2020
60
Tje bridge circuit, as already described, has the four resistors being devices known as strain gages, which means that their resistance increases as the conductors are stretched and decreases as they are compressed. These resistors are bonded to a diaphram in a housing so that as some fluid pressure increases the diaphram flexes, which causes two if the resistors to stretch and the diagonally opposite pair to be compressed . This unbalances the bridge, causing a difference in the two voltages at the side corners. Normally that voltage would be fed to a circuit called an instrument amplifier. But the circuit shown is not an instrument amplifier, it is instead a differential input amplifier. The over-al performance is similar but the operation is different from an actual instrument amplifier.
The voltages at the two sides of the bridge are (in theory) equal when the pressure is zero and they change a small amount as the bridge becomes unbalanced. The amplifier only amplifies this small difference voltage, providing an analog voltage in proportion to the pressure deforming the diaphram.
Thank you for replying, but I have a question about the following circuit where the same sensor is used but the measured quantity is flow

Thanks again.

#### AlbertHall

Joined Jun 4, 2014
12,396
That bridge arrangement could be used for any quantity which can alter a resistance - weight, temperature, RF power, pressure, etc. The changing resistance is used in one, or possibly two) arms of the bridge.

#### MisterBill2

Joined Jan 23, 2018
19,459
Measuring flow with a pressure sensor is often done. The simple method is measuring the pressure drop, (Differential pressure) across a calibrated orifice. This can be quite accurate but generally has a lower span that is usable. In addition the orifice method depends on the fluid having a consistent viscosity. For gas flow measurements there is a special class of orofices called "critical venturi flow meters" that have special properties making them more accurate than other types. So that is how flow measurement is done with a strain gage pressure sensor.

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