#  Measuring Resistance with AC 

 



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#### The Requirement of Alternating Current in Data Acquisition

 ![ohmmeter](/sites/g/files/omnuum4601/files/2024-11/ohmmeter.png)

 

Salt water (aqueous sodium chloride NaCl) is an ionic fluid that consists of positive (sodium, Na+) and negative (chloride, Cl-) ions. These ions flow freely in the solution and are attracted to voltage potentials introduced to the solution, for instance, if you’d place a negatively charged plate in a saltwater solution the positively charged sodium ion (Na+) will start to migrate towards the plate. This poses a problem when measuring resistance in the traditional sense because an ohmmeter applies a small known voltage and measures the current in order to calculate the resistance. What would follow when measuring the resistance of a ionic fluid is the small voltage applied charges the two leads of the ohmmeter, one positively and one negatively, thus leading to ions pooling towards their attractor. The solution itself then becomes capacitive, storing a potential, and discharges into the ohmmeter causing a erroneous reading. Thus, applying any direct current (DC) voltage to the sensor will cause it to store charge and render the resistance measurement impossible.

 ![dc-problem](/sites/g/files/omnuum4601/files/2024-11/dc-problem.png)

 

This phenomenon can be verified by simply attempting to measure a saltwater solution with a multimeter for duration of 30 seconds and switching to voltage measurement - there will be a potential of a few millivolts that decays, additionally, swapping the leads with result in a negative reading of a few millivolts.

 ![ac-measure](/sites/g/files/omnuum4601/files/2024-11/ac-measure.png)

 

 ![voltage-divider](/sites/g/files/omnuum4601/files/2024-11/voltage-divider.png)

 

To circumvent this issue, we measure the voltage across a known resistor in series with the sensor while applying alternating current as the voltage source - this is known as a voltage divider. The alternating current ensures that there is no charge stored in the sensor since the ions do not get a chance to pool towards either lead/electrode. The same principle is found in water conductivity meters. An example circuit is shown above. When the sensor changes its resistance, the voltage drop across the sensor and the known resistance changes and thus the resistance of the sensor can be calculated from \\(V\_M=V\_S \\times \\frac{R\_{KNOWN}}{R\_{SALT} + R\_{KNOWN}}\\) , which gives \\(R\_{SALT}=R\_{KNOWN}\\times\\frac{V\_S-V\_M}{V\_M}\\).

Most data acquisition devices with analog input and output, such as the NI USB-6351 should have the capability to drive and measure small negative voltages and thus can provide and measure the sinusoidal AC signals required for the sensor. However, given the correct circuitry, an operational amplifier can be used to generate an AC signal and the rectified sensor voltage response can be measured using a standard microcontroller with an analog-to-digital converter (ADC), thus reducing the cost of the overall project greatly at the inconvenience of analog circuitry.

### **Bibliography**

Preechayasomboon et. al. [Multi-Modal Sensing and Actuation in Biomechanical Hydraulic and Pneumatic Systems.](/publications/multi-modal-sensing-and-actuation-biomechanical-hydraulic-and-pneumatic?admin_panel=1)

[Export Citation](/bibcite/export?content_filter%5B0%5D=1333742&format=bibtex)

### **Contributors**

Pornthep Preechayasomboon

Gaurav Mukherjee

Eric Rombokas