Instrumentation Sensors Book

Change in pH = 0.69 − 2.495 = −1.805

12.5.2pH Measuring Devices

The pH is normally measured by chemical indicators or by pH meters. The final color of chemical indicators depends on the hydrogen ion concentration, and their accuracy is only from 0.1 to 0.2 pH units. For indication of acid, alkali, or neutral water, litmus paper is used, which turns pink if acidic, turns blue if alkaline, and remains white if neutral.

A pH sensor normally consists of a sensing electrode and a reference electrode immersed in the test solution, which forms an electrolytic cell, as shown in Figure 12.9. One electrode contains a saturated potassium chloride (alkaline) solution to act as a reference. The electrode is electrically connected to the test solution via the liquid junction. The other electrode contains a buffer, which sets the electrode in contact with the liquid sample. The electrodes are connected to a differential amplifier, which amplifies the voltage difference between the electrodes, giving an output voltage that is proportional to the pH of the solution. A temperature sensor in the liquid is used by the signal conditioning electronics to correct the output signal for changes in pH caused by changes in temperature.

12.5.3pH Application Considerations

The pH of neutral water varies with temperature. Neutral water has a pH of approximately 7.5 at 32°F, and approximately 6 at 212°F. pH systems are normally automatically temperature compensated. pH test equipment must be kept clean and free from contamination. Calibration of test equipment is done with commercially available buffer solutions with known pH values. Cleaning between each reading is

essential to prevent contamination. For continuous monitoring of pH in a production environment, a conductivity method is normally used.

12.6Smoke and Chemical Sensors

The detection of smoke, radiation, and chemicals is of great importance in industrial processing, not only as it relates to the safety of humans, and to the control of atmospheric and ground environment pollution, but also is used in process control applications to detect the presence, absence, or levels of impurities in processing chemicals.

Smoke detectors and heat sensors (e.g., automatic sprinklers) are now commonplace in industry for the protection of people and equipment, and for the monitoring and detection of hazardous chemicals. Low-cost smoke detectors using infrared sensing or ionization chambers are commercially available. Many industrial processes use a variety of gases in processing, such as inert gases (e.g., nitrogen), to prevent contamination from oxygen in the air. Conversely, gases or chemicals can be introduced to give a desired reaction. It is necessary to be able to monitor, measure, and control a wide variety of gases and chemicals. A wide variety of gas and chemical sensors are available, and the Taguchi type of sensor is one of the more common.

12.6.1Smoke and Chemical Measuring Devices

Infrared sensors detect changes in the signal received from an LED due to the presence of smoke, or some other object, in the light path.

Ionization chambers are devices that detect the leakage current between two plates that have a voltage between them. The leakage occurs when carbon particles from smoke are present and provide a conductive path between the plates.

Taguchi-type sensors are used for the detection of hydrocarbon gases, such as carbon monoxide, carbon dioxide, methane, and propane. The Taguchi sensor has an element coated with an oxide of tin, which combines with the hydrocarbon to give a change in electrical resistance that can be detected. To prevent depletion of the tin oxide, the element is periodically heated and the chemical reaction is reversed, in order to reduce the coating back to tin oxide.. The tin oxide can be made sensitive to different hydrocarbons by using different oxides of tin and different deposition techniques.

12.6.2Smoke and Chemical Application Consideration

Many hazardous, corrosive, toxic, and environmentally unfriendly chemicals are used in the processing industry. These chemicals require careful monitoring during use, transportation, and handling. Analysis labs and control rooms must meet safety codes. Further information can be obtained from the ISA series RP 60 practices. All processing plants and labs must have an alarm system, which can shut down certain operations if a problem occurs. These systems are regularly tested, and are often duplicated to provide built-in fail-safe features, such as redundancy as protection against sensor failure. Table 12.4 gives some of the chemicals used in industry and the type of sensor used for measurement.

12.7Summary

A number of different types of sensors were introduced in this chapter. These are not the main sensors used in process control, but are very important in many industries. This chapter introduced humidity, the definition of water vapor and its relation to a saturated gas using both volume and pressure definitions, and its relation to dew point. Humidity measuring devices, such as psychrometers, hydrometers, and dew point measuring devices, were described, as well as methods for measuring moisture content in materials.

Density, specific weight, and specific gravity were defined for both liquids and gases. Some of the various methods and instruments for measuring these quantities are described.

Viscosity was introduced, along with the formulas used in its measurement, the various types of viscometers used, and its effect on motion within a fluid.

An introduction to sound intensity and pressure waves has been provided, as well as the use of sonic and ultrasonic waves for distance measurement. Sound reference levels were discussed with the formulas used to measure sound levels.

The need for measuring pH is given, and its relation to acidity and alkalinity is discussed. The types of instruments used in its measurement were given.

Smoke and chemical sensors were introduced, and the various types of sensors used in their detection and measurement listed.

Definitions

Dew point is the temperature of a saturated mixture of water vapor in air or in a gas.

Dry-bulb temperature is the temperature of a mixture of water vapor and air (gas), as measured by a dry thermometer element.

Humidity is a measure of the relative amount of water vapor present in the air or in a gas.

Psychrometric chart is a combined graph showing the relation between dry-bulb temperatures, wet-bulb temperatures, relative humidity, water vapor pressure, weight of water vapor per weight of dry air, and enthalpy (Btus per pound of dry air).

Relative humidity () is the percentage of water vapor by weight that is present in a given volume of air or gas, compared to the weight of water vapor that is present in the same volume of air or gas saturated with water vapor, at the same temperature and pressure.

Specific humidity, humidity ratio, or absolute humidity is the mass of water vapor in a mixture, divided by the mass of dry air or gas in the mixture. Wet-bulb temperature is the temperature of the air (gas), as measured by a moist thermometer element.

References

[1]Roveti, D. K., “Choosing a Humidity Sensor; A Review of Three Technologies,” Sensors Magazine, Vol. 18, No. 7, July 2001.

[2]Lauffer, C., “Trace Moisture Measurement with Aluminum Oxide Sensors,” Sensors Magazine, Vol. 20, No. 5, May 2003.

[3]Wiederhold, P. R., “The Principles of Chilled Mirror Hygrometry,” Sensors Magazine, Vol. 17, No. 7, July 2000.

[4]Sparks, D., et al., “A Density/Specific Gravity Meter Based on Silicon Microtube Technology,” Proceedings Sensors Expo, September 2002.

[5]Zang, Y., S. Tadigadara, and N. Najafi, “A Micromachined Coriolis-force Based Mass Flowmeter for Direct Mass Flow and Fluid Density Measurements,” Proceedings Tranducers, 2001.

[6]Gillum, D., “Industrial Pressure, Level, and Density Measurement,” ISA, 1995.

[7]Sparks, D., and N. Najafi, “A New Densitometer,” Sensors Magazine, Vol. 21, No. 2, February 2004.

[8]CRC Handbook of Chemistry and Physics, 62nd ed., Table F-12, CRC Press Inc., 1981–1982.

[9]Walsh, K., “Simplified Electrochemical Diagnostics and Asset Management,” Sensors Magazine, Vol. 17, No. 5, May 2000.

Pressure adjustment

Spring

Vent

Valve

Gas under pressure

Figure 13.5 Automatic pressure safety valve.

valve is closed until the pressure on the lower face of the valve reaches a predetermined level set by the spring. When this level is reached, the valve moves up, allowing the excess pressure to escape through the vent.

13.2.3Level Regulators

Level regulators are in common use in industry to maintain a constant fluid pressure, or a constant fluid supply to a process. Level regulators can be a simple float and valve arrangement, as shown in Figure 13.6(a), or an arrangement using capacitive sensors, as given in Chapter 6, to control a remote pump. The arrangement shown in Figure 13.6(a) is a simple, cost-effective method used to control water levels in many applications; two common uses of this device are in swimming pools and toilet cisterns. When the fluid level drops, the float moves downward, opening the inlet valve and allowing fluid to flow into the tank. As the tank fills, the float rises, causing the inlet valve to close, maintaining a constant level and preventing the tank from overflowing.

The float controls the position of the weight in Figure 13.6(b). The position of the weight is monitored by position sensors A and B. When the weight is in position A (container empty), the sensor can be used to turn on a pump to fill the tank, and when sensor B (container full) senses the weight, it can be used to turn the pump off. The weight can be made of a magnetic material, and the level sensors would then be Hall effect or MRE devices.

Figure 13.6 (a) Automatic fluid level controller, and (b) means of detecting full level or empty level in a fluid reservoir.