Field meter

For an accurate measurement using a field voltmeter, the meter must be caUbrated (or zeroed) before a test. The charge on a grounded conductor is measured. A grounded conductor should have a charge equal to zero, which means the voltage would be zero. Therefore, the field meter should read zero (Fig. 2a). 

Static Voltmeter These instruments are calibrated to indicate the potential (V) on an ungrounded conductor and usually have more than one calibrated meter/surface spacing. They can be used, for example, to indicate the potential on ungrounded persons or equipment. A meter that indicates in volts or kilovolts is not an electric field meter. 

Electric Field Meter These meters are calibrated to indicate the polarity and magnitude of the electric field (V/m) at the sensor. They should have only one calibrated meter/surface spacing and should be... 

To determine the level of electrification on an insulating surface, an elec tric field meter should always be used. There is a direct relationship between the charge density on the surface of an insulator and the elec tric field intensity at the surface. Measurements should be made at locations where the insulating surface is several inches away from other insulating or conduc tive surfaces. The area of the measured surface should be large, compared to the field of view of the meter. In locations where a flammable vapor-air mixture has an MIE greater than 0. 2 mj, field intensities of 500 kV/m or more should be considered unsafe. 

Field Milb and Vibrating Sensor Field Meters... 

The applications of field meters are discussed in [ 136,217]. Typical uses are... 

Measurement of space potential in a large container using the field meter head as a potential probe (3-5.3.4). 

The surface potential on a plane nonconductor, such as a charged web or belt, can be measured using normal field meter techniques. The readings are very useful for comparative purposes but are not... 

FIGURE 7.8 Low-frequency electromagnetic field meter used to measure magnetic and electric fields. 

Protect sample containers and field meters from contamination with dirt and dust by placing them into large shallow plastic pans or keep them covered in coolers or boxes. 

There is no uniform operating procedure for field meters, field portable detectors, and field screening kits because different manufacturers make them in different formats. To use them correctly, we must strictly follow the manufacturer s instructions. When selecting a particular model, we need to evaluate its ruggedness, portability, selectivity, sensitivity, and reliability. To produce usable data, we must have a good understanding of the measurement mechanism, its applicability and limitations, and be concerned with the issues of field instrument calibration and maintenance. In this chapter, we will review some basic general chemistry definitions applicable to field measurements and focus on the common types of field analysis. 

If we do not own field meters, we will rent them for a sampling event. The rental company should calibrate the meters and make sure that they are in good repair. 

Practical Tips General rules for field meter use... 

Choose the field meter type and capabilities based on the parameter to be measured and the type of water to be sampled. 

Always follow the manufacturer s instructions for the operation and maintenance of the field meters. 

The accuracy of the ORP measurements depends on the temperature at which a measurement is taken. For solutions with reactions involving hydrogen and hydroxyl ions, the accuracy also depends on the pH of the water. In natural waters, many redox reactions occur simultaneously each reaction has its own temperature correction depending on the number of electrons transferred. Because of this complexity, some of the field meters are not designed to perform automatic temperature compensation. The temperature correction for such meters may be done with a so-called ZoBell s solution. It is a solution of 3 x 10 3 mole (M) potassium ferrocyanide and 2 x 10 2 M potassium ferricyanide in a 0.1 M potassium chloride solution. The Eh variations of the ZoBell s solution with temperature are tabulated for reference, and the sample Eh is corrected as follows ... 

The ZoBell s solution and the tabulated reference values are usually part of the field meter kits. It may also be purchased separately from several field instrument manufacturers. 

Field meters typically measure ORP and pH simultaneously with a probe that shares a common reference electrode for both measurements, but has two different sensing electrodes. The field ORP meters have a typical range of 0-1500 mV and an accuracy of 0.5 percent. 

The relationship between the electromotive force and pH is defined as a straight line. To establish the slope and the intercept of a meter system at a given temperature, the meter must be calibrated with standard solutions prior to use. A typical field meter kit includes three calibration standards, which are buffer solutions with known pH values (usually pH 4.01, 7.00, and 10.01 at 25°C). By immersing the probe into the buffer solution with pH 7, we establish the intercept (also called offset or zero) of the probe. If the reading is different from 7.0 at this point, we must adjust it with a control knob labelled Offset or Zero. The buffer solutions with the pH values of 4.01 and 10.01 allow us to verify and adjust the slope (span) of the calibration line. 

Conductivity is measured in the field with a portable probe. It is typically part of the temperature and pH meter, enabling us to measure all three parameters simultaneously. The meter software will perform the temperature correction to 25°C. The probe calibration is verified with a standard prepared as a solution of potassium chloride of a known conductivity. For better accuracy, field meters may have up to four different ranges of conductivity measurements, for example, 0-20pmho/cm ... 

Most of the field meters measure dissolved oxygen in the range from 0 to 20 mg/1 and have an accuracy of +0.03 mg/1. For better accuracy in the low concentration range, DO meters may require an additional calibration point for low range measurements (less than 2 mg/1). We prepare a zero DO calibration standard by adding an excess of sodium sulfite to water in order to salt out dissolved oxygen and use this solution for the preparation of the water-saturated air standard. 

Field meters use the nephelometric method for turbidity measurements and read turbidity in Nephelometric Turbidity Units (NTUs). A nephelometer measures the intensity of light scattered by a sample and compares it to the intensity light scattered by a reference standard. Sample turbidity is proportional to the intensity of the scattered light. The reference standard is a formazin suspension in water, which has a known turbidity value. 

If renting a field meter, make sure that it has been calibrated by the rental company and have the calibration standards on hand in the field. 

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