USING SCANNERS TO AUTOMATE CALIBRATION

APPLICATIONS FOR SCANNERS IN CALIBRATION 1: Removing lead changing when calibrating Multi-product & Multi function Calibrators 2: Voltage Reference / Standard cell monitoring 3: Automating Standard Resistor Calibrations 4: Thermal transfer AC/DC switching 5: Measuring several PRT’s or thermocouples.

SCANNERS NEED SOFTWARE TO AUTOMATE MEASUREMENTS To take full advantage of a scanner the calibration system needs to be automated using PC to control the calibration process. This increases system complexity and cost.

ENGINEERS TIME Vs CALIBRATION TIME A significant consideration for the use of scanners is the saving in the engineers time as this can be greatly reduced. But this is not the only reason…………….

4 KEY ADVANTAGE CONSIDERATIONS 1: Time saving Possible to save both engineers time and make full use of the investment in the calibration system by calibrating unattended, for example performing calibrations overnight when equipment would otherwise be standing idle. Often ‘closed loop’ procedures still need an engineer to swap lead/connection in many cases negating the time saving of automation. Note the actual calibration time may not necessarily be reduced. This is because there is generally very little cost overhead in taking more measurements in a fully automated system, but an important improvement in calibration confidence.

4 KEY ADVANTAGE CONSIDERATIONS 2: Reducing Skill level The calibration process can be made simpler by avoiding lead changing, where errors, and even damage to equipment can happed due to incorrect connections, for example accidental misconnection to the wrong shunt when changing leads to measure high current.

4 KEY ADVANTAGE CONSIDERATIONS 3: Improving Uncertainties Automated systems can easily repeat measurements and reduce the ‘repeatability contribution in the uncertainty calculation.

4 - KEY ADVANTAGE CONSIDERATIONS 4: Improved calibration quality and repeatability By removing the human variation effect from the measurements The calibration becomes more repeatable, with more consistence results giving greater confidence in the quality of the calibration performed.

2 KEY DISADVANTAGE CONSIDERATIONS 1: Measurement degraded by switching A Scanner adds additional errors into the measurement by adding addition contacts in the signal path. The largest problem is thermally generated emf voltages. The design of the scanner and its operating environment can reduce these to less than 150nV This only effects low voltage DC measurements, even resistance measurements are not necessarily effected by emf if the ohms compensation function on the DMM is used.

2 KEY DISADVANTAGE CONSIDERATIONS 2: More Complex system The measurement system will need to be controlled from a PC. Therefore there will be software and procedures to write. There will also be more cables and connection to make. If all that is required is a ‘one off’ measurement a scanner is not the solution.

Calibration of Multi product Calibrators A scanner can be used to reduce lead changing, between the calibrator and the DMM - especially for current shunt selection. With some DMMs it is necessary to use external shunt to obtain the required uncertainties or range. The 4 terminal switching arrangement below requires different connection configuration than a normal scanner. Current Input Voltage output Current Shunt

Voltage Reference / Standard cell monitoring  A scanner can be used to measure differences between cells in either a solid state reference or a standard cell enclosure. Precision DMM PC control Solid State voltage standardScanner

STANDARD RESISTOR CALIBRATION  A scanner can be connected to a set of standard resistors in an oil bath and used to ratio compare against an unknown using a precision DMM. This technique can provide very low uncertainties. Standard resistors 4 wire connection Scanner Precision DMM PC control

DESIGN OF A PRECISION SCANNER The perfect scanner should switch signals without adding errors. The most important being thermally generated emf voltages. This is best achieved by minimizing the power dissipated in the scanner by the use of low power processors, latching relays and using external power supply. Great care must also be taken in the connection to use gold plated copper connections. Processor External Power Brick Relay Switching matrix Inputs PC Interface Output

CONCULSIONS The combination of a  Scanner  Digital Multi-meter  Software  Provides a cost effective measurement solution………