At the 2005 Measurement Science Conference, a consultant described instrument cards as “mindless beasts,” because they are measurement instruments whose uncertainties can’t be quantified. While that may be true to some extent, PXI measurement cards perform important production measurement functions every day. Manufacturers may calibrate PXI cards with NIST-traceable standards during production, but what happens to the calibration once the cards are placed in service?

PXI cards present the same calibration problems as their PC-plug-in and VXI bretheren: Cards calibrated in a cal lab may not be in the chassis where they are used every day, and are thus calibrated in an environment that does not represent reality. Differences in temperature, air flow, and EMI can affect measurement uncertainty.

Perhaps calibration wasn’t so important when the only PXI measurement cards were 12-bit data-acquisition cards. But today, you can get PXI digital multimeter (DMM) cards with as much as 7.5 digits of resolution, rivaling many bench meters. As resolution improves, calibration becomes more important.

If you use PXI systems with analog measurement cards such as DMMs, data-acquisition cards, oscilloscopes, or signal sources, do you ever calibrate them? What do you do to calibrate PXI cards?

• We remove the cards from the chassis and send them to our cal lab.
• We send the whole chassis to our lab for calibration.
• We send just the cards to an outside cal lab.
• We send the whole chassis to an outside cal lab.
• We calibrate the cards in place by bringing the calibration equipment to the test station.
• We return the analog PXI cards to the manufacturer for calibration.
• We don’t calibrate card-based instruments.

Which do you do? What calibration standards do you use to calibrate the instruments? How long is your system out of commission during calibration? How often do you calibrate PXI cards?

Tell us your story by sending an e-mail to mrowe@tmworld.com, and we’ll consider including it in a future article about system calibration.