Dip meter

Once the coil has been incorporated into a tank circuit, a dip meter can be used to measure the resonance frequency. For a parallel resonant circuit, the dip meter can be used in the usual way, i.e., look for a "dip" indication as a function of frequency. For a series tank circuit, it is best to use the dip meter as an rf source acting like an artificial NMR signal and maximize the receiver output when the receiving system is fully hooked up. This will work also for the parallel tank. (See section V.C.9. on impedance matching.)... 

Aside from its canonical use as a device for finding the resonance frequency of a parallel tank circuit, the dip meter can be used as an rf source to mimic an NMR signal. Because of the high sensitivity of NMR receivers, the dip meter need only be close to the NMR apparatus. But remember the output level comment in the last paragraph. An old grid dip meter can be put, say, one meter away from the receiver system and tuned close to the phase detector reference frequency to produce an observable signal. A dip meter usually has to be closer. 

Crossed coil probes are easy to tune because the transmitter and the receiver functions are independent. Tune the receiver tank circuit for a maximum signal through the receiver from the dip meter. The transmitter coil can be optimized by maximizing the rf field amplitude at the coil as described in the section on transmitter tuning below. 

If you want to use the set-up as is and ignore the problem, tune the probe to maximize the received signal by looking, for example, at the detected signal from a dip meter. The loss in S/N of the received signal due to an improperly tuned probe is irrecoverable while that is not so for the transmitter pulse. In principle, an inadequate amplitude due to inefficient coupling between the transmitter and the coil can be overcome by simply having a more powerful transmitter. 

The receiver and preamp can be tuned with the dip meter signal at the same time the probe is tuned by adjusting the preamp/receiver for a maximum output. If the receiver has a built-in filter, it must be adjusted to the lowest cutoff (or threshold) frequency so that it will not distort the signal. Another common practice is to maximize the noise by the tuning. We find that the dip-meter signal is easy to generate and makes the tuning more accurate. 

For building probes, making tank circuits, and as an rf source which acts as a artificial NMR signal without any electrical connections to the circuit, a dip meter is nearly essential. In addition, it is useful in its intended role to passively measure the parallel resonant tank circuit s resonant frequency... 

The spring ensures a soHd closing action and is usually wound from stainless steel wire. The dip tube conducts the product from the container to the valve. It is usually extmded from polyethylene or polypropylene and has an inside diameter of over 2.54 mm, although it can be provided in capillary sizes having diameters down to 0.25 mm. These small tubes are used to reduce flow rate and may function in place of the Hquid metering orifice in the valve housing. 

We can determine an approximate value of the pH of an aqueous solution very quickly with a strip of universal indicator paper, which turns different colors at different pH values. More precise measurements are made with a pH meter (Fig. 10.11). This instrument consists of a voltmeter connected to two electrodes that dip into the solution. The difference in electrical potential between the electrodes is proportional to the hydronium ion activity (as will be explained in Section 12.10) so, once the scale on the meter has been calibrated, the pH can be read directly. 

A glass electrode, a thin-walled glass bulb containing an electrolyte, is much easier to use than a hydrogen electrode and has a potential that varies linearly with the pH of the solution outside the glass bulb (Fig. 12.11). Often there is a calomel electrode built into the probe that makes contact with the test solution through a miniature salt bridge. A pH meter therefore usually has only one probe, which forms a complete electrochemical cell once it is dipped into a solution. The meter is calibrated with a buffer of known pH, and the measured cell emf is then automatically converted into the pH of the solution, which is displayed. 

Figure 17-3 shows the range of pH and hydronium ion concentrations. The measurement of pH is a routine operation in most laboratories. Litmus paper, which turns red when dipped in acidic solution and blue when dipped in basic solution, gives a quick, qualitative indication of acidity. As Figure 17-4 shows, approximate measures of pH can be done using pH paper. Universal pH paper displays a range of colors in response to different pH values and is accurate to about 0.5 pH unit. For quantitative pH determinations, scientists use pH meters. 

A pH meter uses a probe to make electrical measurements between a reference solution containing acid at known concentration and the sample being tested. The pH meter is calibrated by dipping the probe in a solution of known pH and adjusting the meter to read the correct value. Then the probe is dipped into the sample solution, and the pH appears on a digital display. Example illustrates how to convert from pH measurements to ion concentrations. 

Glass pH electrodes are simple to use and maintain. They respond selectively to hydronium ion concentration and provide accurate measurements of pH values between about 0 and 10. They can be small enough to be implanted into blood vessels or even inserted into individual living cells. In precision work, these electrodes are calibrated before each use, because their characteristics change somewhat with time and exposure to solutions. The electrode is dipped into a buffer solution of known pH, and the meter is electronically adjusted until it reads the correct value. 

The beauty of this electrode is that the measured potential (measured against a reference electrode) is thus directly proportional to the pH of the solution into which it is dipped. A specially designed voltmeter, called a pH meter, is used. A pH meter displays the pH directly, rather than the value of E. 

Either one buffer solution, with a pH near that of the solution being tested, or two buffer solutions, with pH values that bracket that of the solution being tested, are usually used. When the pH electrode is dipped into a buffer solution during the standardization step, the pH reading on the meter is adjusted to the given pH using the standardize knob, or other control, on the meter. 

Soap-film flow meter A modified form of the soap film flow meter used by Krishnamurthi, Kumar, Datta, and Kuloor (K10) for collecting bubbles at atmospheric pressures is shown in Fig. 2. This device, which makes use of the movement of a soap film in a burette, can also be employed to calibrate low rate gas flow meters. Gas enters the apparatus from the top. A soap film is formed at the tip of the calibrated burette by raising a metallic loop dipped in soap solution. A water-seal is used to prevent the gas from escaping through... 

Electrokinetic Measurements. Electrophoretic mobilities were measured with a flat-cell apparatus manufactured by Rank Brothers, Cambridge, England. In addition, several mobility values were checked for accuracy with a Zeta Meter, New York. Mobilities were determined with a small volume of the suspension (approximately 25 cc) that had been prepared for the adsorption experiments. The pH of the solution was measured prior to determining the electrophoretic mobilities, which involved measuring the velocities of five to ten particles in each direction. An average value of the mobilities was recorded. Samples containing the flocculated particles were dipped into an ultrasonic bath for approximately one second prior to making the pH and mobility measurements. 

The. other mode by pulp meter was introduced by Messrs. Oowan from Oechelhaeusek,. the foreign inventor, and consists in making the pulp, whioh is kept, at an equal density, flow into a cistern, in which a box constantly dips, lifting an exact quantity of pulp. The capacity of this box con be increased, or diminished, ta correspond to. the different thicknesses, of paper required. The writer has seen this pulp.m.eter work fur twelve hquxs, and the workman in attendance nerd require fa touch it, while the paper produced would not vary ha weight one half pound per ream. 

To calibrate the electrode, dip it in a standard buffer whose pH is near 7 and allow the electrode to equilibrate with stirring for at least a minute. Following the manufacturer s instructions, press a key that might say calibrate or read on a microprocessor-controlled meter or adjust the reading of an analog meter to indicate the pH of the standard buffer. Then wash the electrode with water, blot it dry, and immerse it in a second standard whose pH is farther from 7 than the pH of the first standard. Enter the second buffer on the meter. Finally, dip the electrode in the unknown, stir the liquid, allow the reading to stabilize, and read the pH. 

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