pH Transmitters Information
pH transmitters, commonly called analyzers, provide electrical outputs that are proportional to potential of hydrogen (pH) inputs. The pH scale is used to express the acidity or alkalinity of a solution by measuring the concentration of hydrogen ions in the solution. pH transmitters are part of a larger system which monitors and analyzes the pH of a solution. The transmitters function under the principle that an acidic solution has a higher concentration of hydrogen ions than an alkaline solution. The device includes a pH sensor, which uses electrodes or wires to test the solution's pH, and a controller which processes the raw signal from the pH sensor and delivers it to the transmitter. In this way, a pH transmitter is able to convert the pH of a solution into an electrical signal. A pH transmitter differs from a pH meter because it has a communication interface which can transmit data to a control system or computer.
The device employs a stabilization feature which prevents the transmitter from accepting a buffer pH reading that has not reached a prescribed level of stabilization, in terms of pH change per time. This feature helps eliminate the common problem of not allowing enough time for the pH to approach the buffer value that leads to a lower observed slope and higher than necessary slope correction during calibration. pH transmitters have to be regularly "buffered" with solutions of known pH to establish the calibration. They are usually made of glass electrodes and are very delicate.
pH Transmitter Calibration. Video credit: BTC Instrumentation / CC BY-SA 4.0
|Design Tip: When installing a pH transmitter, remember that it will need to be accessed regularly for maintenance, buffering, etc.|
There are several basic types of pH transmitters. These include,
- Loop pH powered transmitter: A two-wire device typically powered by a 24 VDC powered transmitter or a 120/240 VAC powered analyzer. These pH transmitters often include a built-in preamplifier which conditions the signal coming from the pH sensor and provides a 4-20 mA output signal to the transmitter.
- Ion sensitive field effect transistor (ISFET): A type of transistor that responds to changes in the hydrogen ion concentration in a solution. When the hydrogen ion concentration or pH changes, less electrical current flows through the transistor. Consequently, ISFETs are well suited for use in pH sensors and pH transmitters. ISFET silicon chips are more durable and reliable than glass pH electrodes, which may break in harsh environments. ISFET-based pH electrodes also offer more stable pH measurement, with a response rate typically ten times faster than glass electrodes. ISFET-based pH transmitters are popular in pharmaceutical applications and the food processing industry. Their durability allows them to be easily sterilized without the risk of breaking during handling.
Schematic ion-sensitive field effect transistor (ISFET) diagram. Image Credit: TIMA.
In addition to type, industrial buyers should consider operating requirements, performance specifications and key features when selecting a pH transmitter
- Power requirements vary depending on the type and model of the transmitter. Many devices are stationary, but battery operated portable devices are available.
- Enclosure of the transmitter is important to consider. The application of the device will determine the size, mount options, weatherproof and resistance to corrosion needed to ensure proper function in the field.
- Input/Output options can include single or dual sensor versions. Dual sensors can be programmed to account for several parameters including pH, dissolved oxygen, and temperature measurements.
- Compatibility of the device with multiple systems and in multiple industries is important to consider when selecting a pH transmitter. Most systems come with computer software for easy use and customization to user needs.
- Display type is generally a high-contrast digital LCD screen and can come with options including plain language prompts and help screens.
- Analog systems are traditional one-directional communication protocols. They transmit one variable per loop and the device must be calibrated on the device itself. The current is proportional and scalable to the sensor signal.
- Digital communication protocols allow for simultaneous transmission of additional measurements. The signals are bi-directional allowing for remote monitoring and off-site configuration to troubleshoot and reduce false alarms.
Wireless pH Transmitter. Image Credit: Emerson
- Security of the device and system should be considered. Many devices offer various levels of password protection and alarming systems.
- Range defines the level of saturation the device can measure. This range can be between 0 and 99.9% saturated with hydrogen ions. Range can also identify the pH scale accepted, generally this is 0.00 to14.00 pH
pH Scale. Image Credit: Chemistry StackExchange
- Resolution is the detail in which the device can give a reading. Standard resolution for transmitters is
- pH: 0.01 pH
- mV: 0.1mV; 1MV
- Temperature: 0.1 °C
- Accuracy describes the pH, mV, and temperature tolerance of the transmitter.
- Provide assistance during buffer calibration and diagnostics for the transmitter and its electrodes.
- Provide solution pH temperature compensation in addition to the standard Nernst temperature compensation. Temperature compensation is important to correct the pH measurement for changes in solution pH.
- All solutions above 7 pH will exhibit at least some decrease in pH with increasing temperature, due to changes in the dissociation constant of water, which can be compounded by changes in the dissociation constants of weak base.
- Solutions below
- Neutralization systems
- Heavy metals recovery
- Plating control
- Scrubber control
- Pool and spa control
- Environmental study
- Water treatment
- Water quality monitoring
- Waste treatment
Seal, A. M. Practical Process Control. London: Arnold, 1998. Print.