W. ROBERT BARNARD
teaching
The Ohio Stote University Columbw, Ohio WITH
JOHN C. KELLEY The Ohio Stote University Columbur, Ohio
ROBERT GIDDEN' The Ohio State University Columbus, Ohio
WILLIAM H. EBERHARDT Georgia Institute of Technology Atlantm. Georgia
A lecture Room Digital Multimeter
This paper describes an instrument capable of displaying numerical values of pH, temperature, pressure, and voltage over wide ranges. From it, even in a large lecture hall, students may take down unusually legible quantitative data from lecture experiments. The instrument has been designed to be simple and quick to operate while giving adequate response for the purpose. The heart of the equipment is a digital voltmeter giving four significant figures. To this have been added two main accessory units. One, a plug-in for the digital voltmeter which converts the parameters being measured linearly to a voltage driving the digital voltmeter, and the ot,her, a slave display unit which duplicates the reading of the digital voltmeter on a scale large enough to be legible a t distances up to 100 it. Various probes or t,ransducers are used in the measurement of pH, temperature, and pressure and these have been chosen to be small and easily manipulated so that they can be used in as versatile a manner as possible. The transducer for pressure measurements, however, is large enough so that variations in the level of a mercury manometer are visible to a class. Figure 1 shows the instrument demonstrating several capabilities. The plug-in unit will accept more than one input so that alternate measurements of more than one variable are possible without changes in wiring or set-up of the display system. For example, this might be used to display alternate readings of temperature and pressure in a demonstration involving the gas laws. A few uses to which this equipment has already been put will be suggested; although, of course, the usefulness is limited only by the ingenuity of the demonstrator. The voltage measuring capability of the digital voltmeter itself has been used to demonstrate electrode potentials relative to a standard hydrogen electrode of simple metal-metal ion equilibria, and redox, and precipitation equilibria. The system may be used as a readout in a potentiometric titration, readings being A project of the Teaching Aids Committee of the Advisory Council on College Chemistry supported in part by the National Science Foundation. The idea for this device was suggested at bhe ACa conference on Lecture Experiments held at Harvey Mudd College in December 1963. The Leys School, Cambridge, England. 1 Present ddress:
206 / Journal of Chemical Educufion
taken a t particular steps in the titration. The pH capability has been used to demonstrate the buffer capacity of a solution of a weak acid and its salt. The data thus obt,ained could be used b?, the students to estimate the pK, of the acid. The sensitive temperature ranges have been used to demonstrate heat of crystallization and depression of freezing point, whiie the wide range temperature iacility has been used to demonstrate the variation in temperature between various points in a gas-air flame. The capability of the instrument to take a measurement a t an instant determined by pressinga hand-held button and then to hold this reading indefinitely is particularly useful in this last case. Both the temperature probes and the pH probe may be mounted on a flexible arm which facilitates transferring the probes from one vessel to another. The pH probe presently in use with the instrument is a combination electrode so that only one tubular electrode assembly has to be inserted into the measurement vessel. With this assembly conical flasks may easily be used if desired.
he lower photograph shows the digital multimeter with the Figure 1. porent unit, the slave unit, the special plug-in unit, several prober with the pH probe mounted an o flexible lever arm ond immerged in o solution. The upper photographr ,how the face of the slave unit during measurementr of pressure ond celcivr temperature.
Figure 2. A rchmatic diagram of the power supply for the auxiliary display ""it.
10 V M A X
IOV
CONNECTOR
+
4
Figure 3A. A block dimgmm of the rpeciol plug-in unit showing the voriovr probe connections.
THERMISTOR
A4 B4 AMP
PH I O Y I N . MIX.
7 AMPLIFIER
BATTERY
Figere 3L same unit.
A rchcrnotic drawing of the
GLASS PROBES
0 -s
E$ Z2M
AMPLIFIER
1-35"
Hq BAT.
rDW L~~
Volume 45, Number 3, March 1968
/
207
Figure 4. A diagram of the lecture table manometer used in pressure measurements. The open end manometer tube i s constructed of 3 / ~ i n . 0.d.. L/win. wall polyrarbonote tubing. Inserted in the tubing is a '/rin. polycorbonate rod. Spacers hove been cemented to the rod to keep it centered in the tubing. Tha remaining area uses only sever01 hundred milliliters of Hg to give the illusion d the large tube being completely filled. T o l d height of the tube is 42 in. A 30-in. Rvorercent tube mounted behind the tronduccnt backing for the manometer maker the 3/1-in. diameter column easily virible.
The Voltmeter and Slave Units
The digital voltmeter used is the Hewlett Packard model 3440A. This instrument has plug-in capabilities for a slave unit and for a remote-control system. It also has a control by which the sample rate can he varied from five samples per second to one sample each five seconds. A push button remote control switch can be used to hold a reading on the light panel or to takes new sampling. The auxiliary display (or slave) unit was made by housing six nixie tubes with 4-in. high characters and a power supply (Fig. 2) in a cabinet originally made for a Hewlett Packard model 3440 digital voltmeter. A decoder board was prepared for each nixie tube and housed in this cabinet. With the unit thus constructed, the reading on the digital voltmeter can he transmitted to the nixie tubes (Burroughs model 7034) to provide the numeral display. Two additional nixie tubes (Burroughs model 8971) were used to display symbols such as "C, "I
