Types of pH Sensors: Simulation and Design

August 15, 2019 Cadence PCB Solutions

Surface mildew from water on glass

 

Hard water scale problems in industrial heating, venting, and air conditioning systems dramatically increase operating costs. For example, a 1/8” thick layer of scale caused by hard water insulates heat transfer surfaces and causes a 20-25% loss of efficiency in boiler performance. Hard scale on boiler tube surfaces causes overheating and the failure of the boiler tubes. Scale within the chiller condenser loops inhibits water flow and results in an additional 30% increase in energy usage. Safety valves coated with scale can become inoperable and impact plant safety.

Maintenance teams can check for the presence of hard water through the use of test equipment that includes potential of hydrogen (pH) sensors. The pH level of water shows the level of acidity and measures on a logarithmic scale from one to 14. Because pH meters use a logarithmic scale, the meter gives a degree of acidity compared to another. 

A measurement of seven indicates a neutral balance between acidity and alkalinity. Any measurement below seven shows that the water has a higher level of acid that can cause the leaching of toxic metals—such as lead, zinc, and copper—from pipes and fixtures. When the measurement ranges from 8.5 to 14, the water has a higher percentage of alkaline and can cause scale build-up. 

A Quick View of pH Sensors 

All pH sensors rely on electrodes to measure the acidity of a wide range of liquids. The electrodes used for pH sensors may vary in terms of tip shapes, type of materials, junctions, and fills. While those variations define operating characteristics, every set of glass, plastic, or epoxy electrodes forms a circuit that connects to a meter for the purpose of providing a pH reading. Glass electrodes withstand high temperatures and highly corrosive materials. Epoxy electrodes resist impacts but cannot withstand inorganic solutions, solvents, or corrosive liquids.

We can divide the circuit according to sensing and referencing functions and according to the electrode junction type. Each electrode contains a sensing half-cell and a reference half-cell. Placing the half-cells together forms a complete pH circuit. The sensing half-cell contains a membrane that remains sensitive to the change in pH of a liquid. 

Some types of industrial water testing equipment often rely on combination pH sensors that operate through a sensing electrode and a reference electrode. While the sensing electrode detects any changes in the pH value, the reference electrode establishes that stable signal that the sensor can use as a baseline comparison. General purpose pH testing equipment usually use single-junction, gel-filled electrodes for testing liquids that do not contain materials that can interact with the reference electrode. Instead, single-junctions work well for testing clean water. Another part of the sensor—a high impedance pH meter—displays a millivolt signal in pH units. 

 

A digital pH sensor detects for hydroponics

A digital pH sensor detects for hydroponics

 

During the test of a sample, a single-junction electrode allows hydrogen ions to pass between the sensing and referencing half-cells. The movement of the hydrogen ions completes the circuit between the half-cells. Each electrode contains a trace amount of silver. Most types of electrodes connect to the pH meter through a BNC connection. 

Water scrubbers and other water, wastewater, and coating test equipment use differential pH sensors that offer enhanced stability, increased accuracy, and a longer service life. Unlike combination pH sensors that use single junction electrodes, differential pH sensors utilize double-junction electrodes with a salt bridge. While single-junction electrodes place the reference electrode directly into the reference solution, a double-junction electrode include an additional barrier called a salt bridge that prevents any chemical reaction between the sample material and the trace amount of silver found in the electrode. Double-junction electrodes work with samples that contain proteins, sulphites, organics, biological media, and heavy metals. 

The double-junction electrode places the reference wire and the reference solution inside salt bridge that includes an additional internal junction and protects against any interaction between the reference materials and the sample materials. Within the double-junction electrode, a salt bridge filled with an electrolyte connects the oxidation and reduction half-cells of the reference cell and maintains electrical neutrality. Eliminating the salt bridge would allow the half-cells to accumulate negative and positive charges.

In terms of operation, a differential pH sensor features a preamplifier, two electrodes for measuring the pH level and a third solution ground electrode. The two measurement electrodes—designated as process and reference--measure the pH level differently with respect to the solution ground electrode. This difference in measurement occurs with the reference electrode immersed in a known pH 7.0 buffered cell solution and in electrical contact with the measured liquid through the double-junction salt bridge. The process electrode has a direct electrical contact with the measured liquid. 

 

pH meter measuring while stuck into the soil next to a flower bush

Measuring soil acidity in a more reliable and sturdy ways helps in agriculture

 

pH Sensors and Circuit Design

As you develop your PCB design, you can consider the pH probe as a battery. Placing the probe in a liquid solution causes the sensing electrode to generate a voltage. The amount of voltage depends on the amount of hydrogen activity in the solution compared to the potential of the reference electrode. A low pH value represents a higher acidic level and translates into a positive potential at the sensor electrode. In contrast, a high pH value represents a high alkaline level and translates into a negative potential at the sensor electrode. The difference in potential between the sensor electrode and reference electrode becomes the measured potential.

Because electrode materials may consist of glass, epoxy, or plastic, the resistance of the material varies. A glass electrode has a much higher resistance than an epoxy electrode. The resistance occurs in series with the pH voltage source and can result in a significant voltage drop that reduces the voltage differential produced by the sensor electrode. Most pH circuits include a buffer amplifier that has a high input impedance and low input bias current to isolate the circuit from the high resistance and improve the accuracy of the data acquisition. To account properly for resistance, impedance, and achieve adequate voltage you might use SPICE modeling to simulate the parameters you’re looking for in your sensor design. 

Developing a meaningful display at the meter requires analog-to-digital conversion and noise-free resolution. Because pH meters are low-power applications, the pH meter circuit can operate with an analog-to-digital converter (ADC) that has a low sampling rate and lower power consumption. 

In many instances, industrial pH meter applications require RF wireless transmission. As a result, a PCB design that incorporates a pH meter may also require a transceiver and a microcontroller that controls the transceiver. In addition, the microcontroller serves as the interface between the protocol processing and the sensor. Because most pH meters rely on battery power, the circuit design must consider transceiver power consumption as well as the data rate. Some applications may require longer distance transmission and lower data rates that consume more power while others work with shorter distance transmission, higher data rates, and lower power consumption.

When considering if your pH sensor is up to the challenge presented for it, ensure that you’re using the best tools available for your design needs. Cadence’s suite of tools for simulation, layout, and analysis are beyond helpful in distinguishing your design and ensuring no problems arise. OrCAD’s PSpice Simulator is both proficient with managing low power consumption and modeling ADC conversion with minimal interference. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

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