Calibration simplified
The Fluke 8508A reference multimeter won Test & Measurement World's Test Product of the Year Award.
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 4/1/2003
Perhaps this versatility is the reason why Test & Measurement World readers voted the Fluke 8508A reference multimeter as the 2003 Test Product of the Year. From the 12 products chosen as Best in Test Award winners by our technical editors (see December 2002 ; www.tmworld.com/bit ), our readers preferred the 8508A.
The 8508A calibrates voltage, current, and resistance sources as well as temperature probes. "It serves the market for low-frequency metrology and temperature metrology," says product manager Peter Dack.
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Members of the project team responsible for bringing the Fluke 8508A to market. |
While researching the precision DMM market, Dack found that metrologists wanted more information about a measurement than just the result. So, Fluke added a bit-map display to the meter. "When measuring resistance, our users of older high-end DMMs wanted to view the excitation current as well," Dack noted. "Users also wanted raw resistance measurements in addition to temperature readouts when calibrating RTD probes."
The meter's engineering project manager, Mike Ashcroft, says the Fluke engineers had to overcome several technical hurdles, such as finding a better way to measure resistance. The 8508A uses a ratiometric method to derive resistances from reference resistors. "To get the improved performance," Ashcroft notes, "we turned away from hermetically sealed foil resistor networks and used thin-film resistor networks. We still need foil resistors as reference resistors, but using the thin-film networks lets us compensate for temperature coefficients in the foil resistors."
The resistor networks, combined with both front and rear terminals, let engineers perform ratiometric measurements on not only resistance but also on any other parameter. Paul Livingstone, a design engineer at Krohn-Hite (Brockton, MA), has used an 8508A. He comments, "It's nice to have front and rear terminals to make ratiometric measurements. With the two sets of terminals, I can connect any voltage, current, or resistance reference source to one set of terminals and measure an unknown value with the other set." Livingstone describes the 8508A as "a very versatile instrument" that he can use to calibrate his company's DC voltage and current sources.
To develop the meter, Fluke engineers needed to improve the accuracy and linearity of the company's proprietary multislope analog-to-digital converters (ADCs). Warren Wong, Fluke's engineering manager, said that engineers increased accuracy by reducing noise: The 8508A's ADC's internal noise is half that of the ADCs used in the company's older high-end DMMs. Fluke's engineers also increased the ADC's linearity between two and three times, using a Josephson junction array to verify the improvements.
Wong added, "We improved performance through careful component selection. Having engineers both in Fluke's Everett, WA, headquarters and in our Norwich, UK, facility (the former Wavetek Datron facility), we shared information and found the lowest thermal-EMF relays as well as op amps with lower noise and offset voltages than either engineering group had found on its own."
