Metalscan 2000 from Arun Technology:
The Personal Spectrometer
by Dr. Trevor Smith, Technical Director, Arun Technology Ltd.
Low Cost Technology puts Alloy Analysis in the smallest of factories
The analysis of metal alloy content for certification purposes has always been regarded as a separate operation requiring major capital outlay and specially trained staff. For this reason, most smaller companies use external test houses to analyse samples for accurate chemistry.
Analysis is required to ensure that material has been manufactured to the correct alloy grade. Often national or international standards are called up where the content of each element in the alloy is specified within certain ranges. Sometimes, for special applications, the customer requires manufacture to his own specification.
There is increasing awareness of quality control and traceability, as more and more companies seek registration under ISO 9000. This has caused increased demand for certification of the elemental content of shipments. Smaller companies are faced either with larger test house costs and quarantine delay of shipments whilst these tests are performed or with an inability to bid for the work. Also, their raw material must always be ingot of the required specification.
New Spectrometer
A new desktop metals spectrometer called the Metalscan 2000 is now putting
analysis capability in the hands of the smallest of companies. The instrument is
based on the latest in microchip and optical technology. It uses a CCD detector
similar to the camera chip in a CAMCORDER and a special optical device called a
holographic flat field diffraction grating. The system, launched last year, was
developed by Arun Technology Ltd. at their factory in the
Conventional Spectrometers
Optical emission spectrometers use an electrical discharge with one of the electrodes as the sample under test. The light from the spark or arc contains specific colours which are associated with each of the elements present in the metal. The higher the concentration of a particular element, the brighter the light of its characteristic colours or wavelengths. Conventional metals spectrometers use large optical systems to disperse the light into the different wavelengths and large detectors to measure the amount of light associated with each element (Fig 1).
Fig 1 Geometry of Conventional
Spectrometer
A separate detector is needed for each element to be measured; these photo-multiplier tube detectors are large, expensive and fragile. The optical system images the light entering a narrow entrance slit onto exit slits by means of a concave diffraction grating. These optical components must be carefully aligned to within a few microns. Mechanical stability is essential and temperature variations resulting in movement of the optical components, due to thermal expansion, cause drift in the results as the optics goes out of alignment. In order to combat this effect, the whole spectrometer is usually mounted on a rigid cast iron base at least half a meter long and is therefore heavy and bulky.
CCD Detector
In the Metalscan 2000 a linear array charge-coupled-device (CCD) is employed to detect the spectrum. This consists of a long string of over 2000 small detector elements each about 12.5 m square. The spectrum is focused from the diffraction grating onto the surface of this silicon chip and electronic circuitry is used to clock out the results, much like one line from a TV picture. Because the spectrum is continuous, any movement with temperature can be tracked by automatic profiling software. Also, each alloy element has many different wavelengths at which it emits light, and the most suitable wavelength can be selected, under software control, to ensure that performance is optimal. The Metalscan 2000 is currently configured with over 800 lines and can be calibrated for several metal bases if required by a customer - including aluminium, brass and bronze, cast iron, steel, nickel alloys, zinc and magnesium.
Fig 2 Layout of the CCD
Spectrometer
Test time is short. Typically, argon gas is flushed through the sample chamber for 2 seconds prior to initiating the spark discharge. A high energy pre-spark is then used for 5 seconds to re-melt the surface of the sample under test and ensure that local variations of elements are made homogeneous, The detector signals are then integrated over 4 or 8 seconds.
The electrical discharge used in the Metalscan 2000 is very similar to that employed on more conventional instruments, although the weight and bulk of the electronics to drive it has been drastically reduced by Arun's team of designers. The source operates at up to 500Hz and can deliver about 155A peak during the preburn stage - suitable for re-melting the surface of the toughest of materials.
Operation of the instrument is made very simple. For routine use of the instrument, all operations can be performed using just a few clearly labeled buttons on the front panel. A keyboard is supplied so that the user can enter identification codes or melt numbers which can then be printed on the built-in printer, together with the analysis data.
For the best possible accuracy, users may set up their own calibrations and standardise these on type standards of independently analysed chemical composition. The system is very stable. In many applications the instrument will not require standardisation for days at a time.
The windows based software can be used in conjunction with a monitor and
mouse, for customers who use computer technology. All results displayed on the
screen are logged on a computer file. This file may be retrieved from the
instrument and transferred to a PC where the data can be read by many popular
spreadsheet or database applications. 
Aluminium Foundry Applications
The instrument is particularly popular with aluminium foundries, where the
accuracy and repeatability of results is very comparable with spectrometers
costing tens of thousands of dollars more. Table 1 shows results from two
aluminium grade samples taken by a customer in
Other Cost Savings
As a guide, a Metalscan 2000 costs approximately $30,000 for an installation covering one metal base. This cost can often be justified against external laboratory charges over only a couple of years. Typical foundry applications allow opportunities for cost savings in addition to those achieved by eliminating test house charges. Firstly, delays to shipments are all but eliminated, since certification can usually be performed whilst material is still being cast. Second, scrap and re-work can be drastically reduced since out-of-specification metal will not get cast if frequent checks are performed. Third, material costs can be reduced since lower priced ingot of questionable content can be purchased, after first analysing its content and the molten metal brought into specification later under control of the Metalscan 2000.