As with any technology, pH sensors are constantly advancing and improving. You select the pH sensor will depend on the pH range and measurement accuracy needed.
“pH” is a standard abbreviation for “potential hydrogen.” It is the negative log of the hydrogen ion activity in an aqueous solution. Essentially it measures how acidic or alkaline a liquid is on a scale of 0 to 14. A pH of 7 is neutral, below seven is acidic, and above 7 is alkaline or basic.
Here are some common terms to be familiar with when selecting a pH sensor:
- pH Range: The range describes the lower limit of the intended measurement range. For example, you can have a general-purpose 4-wire type sensor with an upper limit of 10. An industrial-grade 4-wire type sensor typically ranges from 0 to 14, while a lab-grade 4-wire type sensor can measure from -2 to 16.
- Resolution: The resolution refers to the slightest change in mV that a given device can measure once it has been calibrated in units of millivolts (mV) per pH unit. For example, a general-purpose pH sensor typically has a resolution of 0.02 pH units once calibrated in water with a reference value of 7. The resolution can be increased by obtaining a higher-grade sensor such as an industrial or lab-grade sensor.
- Accuracy refers to the precision and consistency of the sensor’s output over time and across environments. An industrial-grade type pH sensor typically offers better accuracy than a general-purpose or lower-cost sensor because it is designed for harsh and long-term use under various environmental conditions. Lab-grade sensors also provide high accuracy; however, their application is often limited to research laboratories due to cost and technical requirements.
- Electrical Output: The pH sensor’s electrical output represents the pH level in a given sample. There are two common types of outputs: current (in milliamps) and potential (in millivolts). You can convert one to another by using Ohm’s law, which states that voltage equals current times resistance or V=IR. For example, if you have an industrial-grade type sensor with 500 mV @ five mA for an applied voltage of +5 volts, then the resistance = (5/0.005), or 100,000 ohms (100 kilo-ohms or 100kΩ). This information will help you select the appropriate amplifier for your particular application.
- Temperature Coefficient: Temperature affects the electrical properties of most materials, including pH sensors. If not compensated for, temperature changes can affect the accuracy of a sensor’s output when measuring solutions with differing temperatures. The typical output of pH sensors is approximately -59 mV/�C.
- Resistance to Chemicals: Industrial-grade pH sensors are designed to withstand exposure to acids and bases for an extended period without degradation or interference from chemical reactivity. However, even industrial-grade type pH sensors should be rinsed and cleaned after use and stored in clean tap water during downtime, not to collect particulates that could clog the electrolyte cell and affect the sensor’s performance over time. Since general-purpose and lower-cost type pH sensors are usually not as chemically resistant, manufacturers typically do not recommend using harsh chemicals.
Here are the types of PH sensors.
Combination PH Sensors
Combination pH sensors are designed to measure the pH of gases and liquids. They contain electrodes for measuring hydrogen ion activity in liquid solutions and an oxygen electrode for measuring oxygen concentration in gas phase samples. The electrodes are enclosed in a single probe and work together to provide pH values of both gas and liquid samples. Combination type pH sensors usually require more maintenance than other sensors because they must be cleaned after each use, primarily if used to measure gases containing high concentrations of hydrogen ions or other acidic substances.
Differential PH Sensors
Differential type pH sensors consist of two separate electrodes housed in a single probe. One electrode measures hydrogen ion activity, and the other measures oxygen concentration, or vice versa. The probes are designed to be inserted into the sample chamber together to form an “electrical double layer” consisting of two parallel electrically charged wires separated by an electrolyte-containing gap. Because only the tips of both electrodes extend into the solution, differential type pH sensors can measure localized pH levels at various depths within a homogeneous liquid or solid medium, making them useful for monitoring processes that occur inside tanks or other containers where physical access is limited.
Consumer-Grade PH Sensors
Consumer-grade type pH sensors are designed to offer high-resolution readings in water for the home, lab, or field use. They are often lower quality than industrial-type pH sensors and are usually less accurate. This can be especially true if the sensor is not factory calibrated before shipping because calibrating the sensor requires an electronic meter with special calibration capabilities that consumer-grade meters don’t have.
Micro PH Sensor
A micro-electrode-type pH probe consists of a single electrode surrounded by a fluid-filled capillary tube. The orp probe connects to an external measuring device via a cable to provide measurements of very low levels of hydrogen ion concentration directly in the sample being measured, such as blood, saliva, cerebrospinal fluid, or other solutions containing small amounts of hydrogen ions relative to their normal level. Microelectrodes function independently from any surrounding solution and require no buffering capacity.
Industrial PH Sensor
Industrial-type pH sensors are designed for continuous or long-term chemical process monitoring of highly corrosive chemicals in a harsh operating environment. They can be manufactured from materials resistant to the corrosive medium, such as titanium or stainless steel, and contain thick protective external coatings that protect against abrasion and direct contact with the corrosive environment. The electrodes must withstand high pressures and extreme temperature variations, so they are usually enclosed in special housings designed to measure pressure and temperature. Some industrial type pH sensors include heaters on the probe tip to maintain a constant temperature between -40�C and 300�C (-40�F-572�F).
Lab-Grade PH Sensor
Lab-grade pH sensors are designed for general laboratory use. Generally, they include a temperature probe to measure the actual temperature of the sample measured and an optional reference electrode that can be used with either the standard hydrogen ion electrode or an additional oxygen electrode. The electrodes are protected with a thin glass membrane and connected to an external meter via flexible cables, which must withstand repeated flexing during normal lab use without deforming. Because they may need to detect very low levels of hydrogen ion concentration, lab-grade type sensors usually have a larger sensing tip than consumer types and require larger volumes of solution.
Spear Tip PH Sensor
Spear tip-type pH sensors contain two separated sensing tips housed within a protective metal sheath. A sensor is inserted into the body of the pH meter and retained by a rotating collar to form an electrical connection with the measuring probe. The collar also pierces two membrane seals to allow hydrogen ions or oxygen gas to reach each electrode tip. The spear tip design allows for insertion into hard-to-reach locations to measure specific portions of a liquid or gaseous solution.
Choosing the correct type of pH sensor for your application depends on several factors, including the pH range, operating environment, and desired features. With this guide, you should find the best match to fit your needs.