W. W. Harrison and A. W. Norvelle, Jr. University of Virginia
Charlottesville
II
A Simple Variable Speed Wavelength Drive
Emission measurements using a conventional spectrophotometer are greatly facilitated if a scanning wavelength drive accessory is avaiIabIe. However, such drive units are not standard equipment on most single-beam spectrophotometers. Drive accessories are usually available, but the somewhat substantial cost may convince the educator that manual methods will suffice. Furthermore, some instruments may not offer a drive accessory. The real advantage in using a wavelength scanning attachment results when a recording readout system is available. If this is not a,n int,egral function of the spectrophotometer, simple modifications as described in THIS JOURNAL' will provide a recording capability. A simple and inexpensive wavelength drive has been constructed for use in our Instrumental Analysis course. This unit, shown in Figure 1, has been specifically designed for use with a Beckman DU, although, with a different frame, the basic model could be used for ot,her spertrophotometers, such as the Beckman B or the Hitarhi-Perkin Elmer 139. The basis of the drive attachment is an SCR driven AC-DC reversing motor, gear coupled to t,he wavelength knob on the DU. A support frame cut from plate aluminum provides the neressary stability for the motor which is mounted on a baseplate connecting the two contoured support members. A ball joint stop positions the motor for proper gear mesh and also allows the motor to swing out to the side for manual wavelength adjustment. This also allows the motor to disengage in the event it
should be allowed to drive to the end of the wavelength scale. A silicon-controlled rectifier motor ~ o n t r o l , ~ clip mounted beside the motor, allows variation of input power to a Bodine speed reducer motor (ratio 900: 1) which is further reduced (5: 1) to a gear on the DU wavelength knob. A four-pole, three-position (FORWARD-OFF-REVERSE) switch connects the speed control to the motor. The motor used is not. suggested as necessarily being t,he optimum unit. but was available surplus to our department. A simple SCR. motor control circuit could be built into the frame of the wavelength drive, &her than purchasing a commercially available unit., if desired. However, the speed controls are inexpensive and find a sufficient number of other applications in a chemistry department as to possibly justify the approach t,aken here, where the unit. can be unplugged and used for othpr purposes.
' ~ \ ~ I L M S T : ~ D TH., V., BARNES,R . hl., .AND RODRIGUEZ,P. A,, J. CHEM.EDTTC., 41, 263 (1964). Jiffv XIotor Sneed Control tronics, Syxset,,L. I
598
(38.95).
/ Journal of Chemical Education
Lsfavet,te Radio Elec-
Figure 1 .
DU.
Voriab!e speed wavelength drive as odapted far o Beckman
This wavelength drive has been used by our students to record emission spectra of hollow cathode tubes and discharge lamps, as well as flame emission spectra. A DU has been modified for recording purposes by replacing the conventional blue and red phototubes with 1P28 and 1P22 photomultiplier tubes and measuring the output currents by means of a recording electrometer.' The photomultipliers are powered by a variable output battery power supply. The SCR control allows the rate of wavelength variation to be adjusted continuously from zero to a maximum speed which, on this unit, is considerably faster than any desired wave length scan rate. Drive rates of barely perceptible
wavelength movement proceed with adequate remaining torque to produce smooth and even scans. This control feature is particularly useful in scanning rapidly over a broad spectral region with a subsequent detailed examination at very slow speeds of the more complex or useful areas. If this were to he used with a linear dispersion spectrophotometer, known wavelength drive rates to correlate with chart speeds could be obtained by calibration of the SCR control potentiometer or by providing a step switch function. Additional specific details of construction will be supplied interested readers upon request to one of the authors (WWH.)
Volume 43, Number 1 1 , November 1966
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599
