Low Thermal Scanner Applications

 

CONTENTS

Introduction
Introduction
Low Thermal
   Scanner

Voltage Measurements
*  Maintaining The
   Laboratory Volt
*  VoltRef Software
*  Simplified Voltage
   Scaling System
*  10-Volt Josephson
   Junction Array

Resistance Measurements
*  OhmRef: Software
*  Ring Method for
   Standard Resistors
*  Guarded High-Ohm
   Resistance System

Temperature Measurements
*  Thermocouple
   Calibration System
*  Simplified System
   for Thermocouple
   Comparisons
*  PRT Comparisons

 


Data Proof
2562 Lafayette Street Santa Clara, CA 95050
phone 408 919-1799
fax 408 907-3710
iwww.dataproof.com

INTRODUCTION
Working together with leading laboratories around the world, we have identified a variety of automated systems that optimize results for low level DC measurements. It is our goal to see the best methods implemented in as many laboratories as possible. This paper presents various methods for measuring voltage, resistance and temperature with uncertainties below 1 ppm. In each case the goal of automation is not simply to save labor, but also to improve accuracy as well.

Substantial progress has been made in automating test systems over the past quarter century. However, they have not been useful for maintaining reference standards due to the lack of suitable switches. Most computer controllable switches generate thermal offsets of about a microvolt. A scanner with very low thermal offsets is now available that makes possible many exceptional improvements in automating precision measurements. The scanner used to achieve these results has thermal offsets typically below 20 nanovolts. It has a high leakage resistance of only 1012 ohms, or as high as 1014 ohms for the guarded version of the scanner.

The scanner is presently being used together with a commercial software package, to maintain the volt in hundreds of standards laboratories around the world including the national laboratories of most industrialized nations. Although the scanner was designed for making voltage comparisons, more recently it is being precision measurements as well. Over the last few years, much innovative work has been done to automate

precision temperature and resistance measurements.  Several methods are achieving errors as low as 0.1 ppm. This paper reviews some recent innovations so that others can use these techniques to improve the uncertainties in their laboratories.


Data Proof is committed to seeing the best available methods spread throughout the worlds measurement community.


LOW THERMAL SCANNER
The device that makes this scanner possible is a sensitive latching relay originally manufactured for the telephone industry. The major problem with using conventional relays is the thermal voltage caused by the heat generated by the current in the relay coil. With the latching relay a short pulse of only 10 milliseconds is all that is required to toggle the contacts from one side to the other. Thus the heat generated is negligible.

These latching relays make gold-to-gold contact directly on specially plated printed circuit boards. This arrangement minimizes the number of thermal-producing solder connections required. The relay makes connection by shorting together adjacent gold pads. Because the contacts are in pairs and are very close together on the board, any emfs generated by the relay contacts are canceled out.

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The two drawings on this page show the relay arrangement. The gold-plated contacts attached to the relay armature make contact with hard gold pads on the printed circuit boards. A permanent ceramic magnet holds the armature in place against one of the two pole pieces. If the coil on the side opposite is energized, the armature will toggle so that it is held against the other pole piece. Once the armature has toggled, the current in the coil can be removed. This requires only a few milliseconds.

The relays used have 3-pole, 2-throw bifurcated contacts. The contacts short out parallel traces on the printed circuit board. When the relay is in the closed position two of the contacts connect the high and low input lines to one of the two output lines. The third contact is used to switch the guard shield on the guarded version of the scanner.

ISOTHERMAL BOX: Switching assemblies with eight relays to a PC board are housed in a heavy machined aluminum box. This isothermal enclosure helps to maintain a uniform temperature at each of the relay contacts. The PC board edge connectors carry only the relay coil circuits. All the input and output lines are soldered directly to the board to prevent thermal voltage offsets caused by connectors. The relays attach to the boards with spring clips so that they can easily be removed for replacement if necessary.

DUAL SCANNER DESIGN: The switch is a dual scanner; that is, each input channel can be switched independently to one of two output lines. This makes possible comparisons between two inputs. Also, true four-terminal measurements are possible because both the positive and negative side of each channel is switched. This makes the scanner ideal for a very broad number of applications.

STANDARD CELL PROTECTION: These scanners have been carefully designed to protect standard cells from being damaged by scanner or operator error. A contact on each relay is tied to a series protection circuit. All relays must be open before power can be applied to close any relay. This protection circuit is available at the rear panel so that multiple scanners can be cascaded and all cells in a large system will have this protection feature.

CONVENIENT OPERATION: The relay circuits are activated either by front panel push buttons or by means of an IEEE-488 bus. A simple three-character bus command sets the interface to remote, opens any previously closed relay, and then closes the specified relay. Because of the dual output arrangement and low thermal design the scanner is very versatile. It can be used for many applications to automate low level measurements while avoiding errors caused by the switching.

Relay Diagram.gif (65282 bytes)Ralay & PC Board.gif (66270 bytes)

Diagram shows relay and
rocker arm assembly

Diagram showing how relay
mates with PC board

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