With high electrode resistance comes the impact of capacitance
coupling and electrical noise (Figure 11). The lead wire exiting the
measuring electrode is prone to picking up spurious electrical
signals—an effect known as hand capacitance. Similar to
extending a radio antenna for pulling in distant stations, the
longer the electrode lead wire length, the more prone the
electrode is to picking up interference. In the case of hand
capacitance, merely waving a hand next to the cable will cause
the measuring instrument display to change erratically.
Shielding the electrode and using high quality cable in the
manufacturing process can minimize the effects of electrical
noise. Shielding is usually accomplished by incorporating a
metallic band within the glass measuring electrode body. This
band extends from the base of where the bulb is coupled to
the electrode body, up the shaft, and back to where the cable
exits the body. When the electrode is immersed in the solution,
the band effectively shields the measuring element from stray
interference through the solution. Combination electrodes are inherently shielded by the reference fill solution
that surrounds the entire measuring element.
When taking measurements, it is best to immerse a majority of the metal band (the measuring electrode shaft)
into the solution. However, the area where the electrode shaft joins a cap or cable connection should not be
immersed. As for shielding the glass bulb, the solution being measured acts as the shield.
Most instruments presently manufactured compensate for electrode resistance changes resulting from solution
temperature fluctuations. This compensation is almost always accomplished automatically using a temperature‐
sensitive device as part of the measuring circuit. In other cases, it is accomplished manually with an adjustment
to the feedback circuit of the instrument’s electronics. On analog instruments, this is achieved by setting a dial
to the correct temperature setting. For microprocessor‐based instruments, a temperature value corresponding
to the actual temperature of the solution being measured is manually entered.
With high electrode resistance comes the impact of capacitancecoupling and electrical noise (Figure 11). The lead wire exiting themeasuring electrode is prone to picking up spurious electricalsignals—an effect known as hand capacitance. Similar toextending a radio antenna for pulling in distant stations, thelonger the electrode lead wire length, the more prone theelectrode is to picking up interference. In the case of handcapacitance, merely waving a hand next to the cable will causethe measuring instrument display to change erratically.Shielding the electrode and using high quality cable in themanufacturing process can minimize the effects of electricalnoise. Shielding is usually accomplished by incorporating ametallic band within the glass measuring electrode body. This band extends from the base of where the bulb is coupled tothe electrode body, up the shaft, and back to where the cableexits the body. When the electrode is immersed in the solution,the band effectively shields the measuring element from strayinterference through the solution. Combination electrodes are inherently shielded by the reference fill solutionthat surrounds the entire measuring element.When taking measurements, it is best to immerse a majority of the metal band (the measuring electrode shaft)into the solution. However, the area where the electrode shaft joins a cap or cable connection should not beimmersed. As for shielding the glass bulb, the solution being measured acts as the shield.
Most instruments presently manufactured compensate for electrode resistance changes resulting from solution
temperature fluctuations. This compensation is almost always accomplished automatically using a temperature‐
sensitive device as part of the measuring circuit. In other cases, it is accomplished manually with an adjustment
to the feedback circuit of the instrument’s electronics. On analog instruments, this is achieved by setting a dial
to the correct temperature setting. For microprocessor‐based instruments, a temperature value corresponding
to the actual temperature of the solution being measured is manually entered.
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