Liquid analyzers have been used for years in industrial applications for pH, conductivity, oxygen, chlorine and other measurements. In recent years, it has become common practice to have line-powered multiparameter analyzers make two or more measurements; using signal cards for each desired measurement type enables a single device to meet the needs of the application. Yet, many users don’t realize that adding or changing signal cards gives these analyzers the flexibility to handle new or evolving applications — solving problems beyond their liquid measurement functions quite cost effectively and possibly reducing costs in other areas of the plant as a result.
For instance, inputting non-analytical measurements into a liquid analyzer can improve the accuracy of the main measurement. All the most common liquid analytical measurements require a temperature measurement to correct a measured value to a standard temperature. In some cases, the main sensor may lack a temperature element or may have a very slow temperature response, sometimes as long as 30 minutes. An accurate reading won’t occur until the temperature reaches equilibrium. Inputting a fast-responding temperature measurement can improve the accuracy of the main measurement — as well as its efficiency. All the plant operator has to do is add a simple temperature input.
Two common oxygen measurements, % oxygen in gas and % saturation of oxygen, depend upon the total pressure. Changes in the total pressure will cause errors in these measurements if the total pressure isn’t measured and corrections applied. Inputting a pressure measurement enables the analyzer to correct these oxygen measurements for total pressure changes. Again, the addition of a non-analytical measurement — pressure — to the multiparameter analyzer saves time and money and improves accuracy.
Non-analytical measurements also can be very useful and provide cost savings by:
• Preventing damage to the analytical sensor. Measurement of sample temperature or pressure can allow diverting sample flow if the temperature or pressure exceeds the sensor’s limits.
• Detecting loss of sample flow conditions. Measurement of flow in monitoring and data logging applications can identify when an analytical sensor is out of contact with the sample process, a situation that can lead to false readings and, thus, false alarms.
For multiparameter analyzers used for water treatment, it’s often useful to have a flow totalizing capability. This allows observing and logging the rate and total volume of water treated. A flow input signal card can accommodate a low-cost pulse flow sensor as well as any full-featured flow transmitter.
Getting readings for pressure, temperature and flow only requires a simple connection of a two-wire transmitter, without the need for an additional power supply. This saves money and reduces complexity while improving the accuracy of the analytical measurement or integrating another needed non-analytical measurement into the application.
A signal card is used for inputting 4–20-mA signals for temperature, pressure and pulse flow. Because multiparameter analyzers are line-powered (85–230 VAC, 47.5–65.0 Hz, 20 W; or 20–30 VDC, 20 W), they not only can receive a transmitter signal but also, if needed, can power the transmitter that provides the signal. All that’s required to input a transmitter signal is to connect the transmitter to the powered input of the signal board in the analyzer.
The analyzer software recognizes the presence of the analog input signal card; the only configuration required is to scale the analog input and assign a unit. Once the input is scaled, it becomes an integral measurement of the analyzer, no different than pH or any other analytical measurement. An input measurement can be applied to any of the analyzer features such as HART transmissions, data logging, and discrete and analog outputs. The increased functionality associated with the external measurement incurs only a small increase in cost.
Multivariable analyzers can do more than provide measurements. For instance, they can handle a number of traditional water treatment functions and controls, including on/off control, on/off control with delay (to allow time for mixing), interval timer (typically for sensor cleaning), and a date and time signal (sprinkler timer) for routine chemical additions. There’s also event-based relay activation, which can be based on totalized flow or the activation of another relay.
In addition, control functions include time proportional control (TPC, duty cycle), using a standard proportional-integral-derivative (PID) algorithm and one or more of the analyzer’s relays. PID control also is available using one or more of the analyzer’s analog outputs. As is the case with TPC, this relies on a standard PID algorithm; the control variable can be any analytical or input measurement.
The analyzer’s discrete and analog control capabilities also make it easily adaptable as a single station controller for applications such as skids, utilities, waste treatment and outfalls that require basic PID control but aren’t hooked up to the plant’s distributed control system. Control applications have included PID for pH control, TPC with ORP for chlorine destruction, and differential pressure input for level control. In one case, a multivariable analyzer was used to provide temporary water level control during a plant turnaround. Because the site already had the instrument installed, all it had to do was simply add a 4–20-mA signal card and configure it for level control. The instrument was quickly adapted at minimal cost. In general, the only questions that have arisen in applying the control capability of the analyzer have been whether to opt for direct or reverse control action, a common control question.