Cracked glass and fouled sensors cause incorrect pH measurements and slow the response time. However, unless the sensor fails completely and sends no signal, operators can find it difficult to detect problems. So, many plants frequently pull sensors for cleaning and calibration, at a large cost in labor and process downtime, to avoid problems. Enter the modern pH sensor with built-in diagnostics.
Some instrument vendors use a digital communications protocol such as HART on the instrument's 4–20-mA output to send diagnostic information to the control system, and employ software that analyzes the sensor readings to determine if a problem exists.
For instance, a transmitter like the Rosemount 5081-P simultaneously measures glass and reference impedance, raw sensor output, RTD resistance, pH and temperature. A glass sensor normally has an impedance of 50-150 MΩ; if cracked, impedance will drop to 0–5 Ω. A similar analysis of reference impedance can indicate sensor coating. Asset management software (Figure 5) can send alerts about glass breakage or coating buildup, and schedule appropriate maintenance.
Unnecessary maintenance can be a large expense. One pharmaceutical company found that pH sensors accounted for 46% of all field maintenance work at a batch fermentation facility. It installed sensors that send diagnostics via HART as well as asset management software, and saved over $110,000 per year in maintenance costs.
TALK TO THE EXPERTS
While this may be your first time trying to control pH in a particular service, sensor manufacturers likely have worked on numerous similar applications and, thus, can give you useful advice. Other reliable resources can be consultants, engineering firms and system integrators well versed in pH control and measurement. Table 2 lists some of the variables crucial for selecting a sensor that suits your needs.
The expert might even warn you off using a pH sensor. For example, if you're trying to control the concentration of a caustic solution, a conductivity sensor might be a better choice.
In other applications, especially where there's a lot of acid with a pH of less than 1 or base with a pH of more than 13, using measurements made by a conductivity, a near-infrared or a refractive index analyzer may make more sense. These analyzers may provide better accuracy because they're more linear at pH extremes.
The expert might recommend using a wireless pH transmitter far downstream as a final check on pH, e.g., before a stream enters the sewer system. In many cases, the U.S. Environmental Protection Agency requires this reading weekly; the wireless device and its associated wireless network may offer savings over sending a maintenance team out to take grab samples.
The control and measurement of pH often is difficult, and most chemical plants can't afford to employ a full-time expert. The solution is to use plant personnel's deep understanding of the process and the marketplace to select the right partner, and to work together to choose the correct sensor for the particular application.
DAVE JOSEPH is Irvine, Calif.-based senior industry manager for the Rosemount Analytical Division of Emerson Process Management. E-mail him at Dave.Joseph@emerson.com