IET Labs HARS-LX Troubleshooting guide

♦PRECISION INSTRUMENTS FOR TEST AND MEASUREMENT ♦
Email: inf[email protected]
TEL: [phone] • FAX: [phone]
www.ietlabs.com
IET LABS, INC.
HARS-LX
Laboratory Standard Decade
Resistance Substituter
User and Service Manual
Copyright © 2014 IET Labs, Inc.
Visit www.ietlabs.com for manual revision updates
HARS-LX-im November 2014

♦PRECISION INSTRUMENTS FOR TEST AND MEASUREMENT ♦
Email: inf[email protected]
TEL: [phone] • FAX: [phone]
www.ietlabs.com
IET LABS, INC.

♦PRECISION INSTRUMENTS FOR TEST AND MEASUREMENT ♦
Email: inf[email protected]
TEL: [phone] • FAX: [phone]
www.ietlabs.com
IET LABS, INC.
WARRANTY
We warrant that this product is free from defects in material and workmanship and, when properly used,
will perform in accordance with applicable IET speci cations. If within one year after original shipment,
it is found not to meet this standard, it will be repaired or, at the option of IET, replaced at no charge when
returned to IET. Changes in this product not approved by IET or application of voltages or currents greater
than those allowed by the speci cations shall void this warranty. IET shall not be liable for any indirect,
special, or consequential damages, even if notice has been given to the possibility of such damages.
THIS WARRANTY IS IN LIEU OF ALL OTHER WARRANTIES, EXPRESSED OR IMPLIED,
INCLUDING BUT NOT LIMITED TO, ANY IMPLIED WARRANTY OF MERCHANTABILITY OR
FITNESS FOR ANY PARTICULAR PURPOSE.
i

WARNING
OBSERVE ALL SAFETY RULES
WHEN WORKING WITH HIGH VOLTAGES OR LINE VOLTAGES.
Dangerous voltages may be present inside this instrument. Do not open the case
Refer servicing to qualifi ed personnel
HIGH VOLTAGES MAY BE PRESENT AT THE TERMINALS OF THIS INSTRUMENT
WHENEVER HAZARDOUS VOLTAGES (> 45 V) ARE USED, TAKE ALL MEASURES TO
AVOID ACCIDENTAL CONTACT WITH ANY LIVE COMPONENTS.
USE MAXIMUM INSULATION AND MINIMIZE THE USE OF BARE
CONDUCTORS WHEN USING THIS INSTRUMENT.
Use extreme caution when working with bare conductors or bus bars.
WHEN WORKING WITH HIGH VOLTAGES, POST WARNING SIGNS AND
KEEP UNREQUIRED PERSONNEL SAFELY AWAY.
CAUTION
DO NOT APPLY ANY VOLTAGES OR CURRENTS TO THE TERMINALS OF THIS
INSTRUMENT IN EXCESS OF THE MAXIMUM LIMITS INDICATED ON
THE FRONT PANEL OR THE OPERATING GUIDE LABEL.
ii

Contents
Chapter 1 Introduction ..............................................................................1
1.1 Introduction........................................................................................................... 1
Chapter 2 Specifications ...........................................................................2
Specications ................................................................................................................ 2
Chapter 3 Installation ................................................................................4
3.1 Initial Inspection ................................................................................................... 4
3.2 Installation............................................................................................................. 4
3.3 Repackaging for Shipment.................................................................................... 4
3.4 Storage .................................................................................................................. 4
Chapter 4 Operation ..................................................................................5
4.1 Initial Inspection and Setup .................................................................................. 5
4.2 Connection ............................................................................................................ 5
4.2.1 General Considerations............................................................................... 5
4.3 Electrical Considerations ...................................................................................... 5
4.3.1 Thermal emf Considerations....................................................................... 5
4.4 Dial Setting ........................................................................................................... 6
4.5 Power Considerations ........................................................................................... 6
4.6 Environmental Conditions .................................................................................... 6
4.7 Switch Conditioning ............................................................................................. 6
4.8 Meter Shunt Applications ..................................................................................... 7
4.9 Kelvin Bridge Applications................................................................................... 7
Chapter 5 Maintenance..............................................................................8
5.1 Maintainability and Reliability ............................................................................. 8
5.2 Preventive Maintenance........................................................................................ 8
5.3 Calibration............................................................................................................. 8
5.3.1 Calibration Interval..................................................................................... 8
5.3.2 General Considerations............................................................................... 9
5.3.3 Required Equipment ................................................................................... 9
5.3.4 Calibration Procedure ................................................................................. 9
5.4 Adjustments........................................................................................................... 10
5.4.1 Adjustment Considerations......................................................................... 10
5.4.2 Adjustment/Trimming Procedure ............................................................... 10
5.5 Replaceable Parts List........................................................................................... 12
iii

Figures and Tables
Figure 1-1: HARS-LX Resistance Substituter .............................................1
Figure 2-1: Typical Operating Guide Affixed to Unit.....................................3
Figure 4-1: Kelvin Bridge Connections........................................................7
Table 5-1: Trimming Potentiometers ............................................................11
Figure 5-1: Typical Trimmer Board...............................................................11
Table 5-2: Replaceable Parts List................................................................12
Figure 5-2: HARS-LX Replaceable Parts ....................................................12
iv

1
HARS-LX Series
1Introduction
Chapter 1
INTRODUCTION
1.1 Introduction
The HARS-LX Series Laboratory Standard Decade
Resistance Substituter provides a very broad-range
high-performance resistance source.
The HARS-LX Series substituter is a precision re-
sistance source with excellent characteristics of ac-
curacy, stability, temperature coefcient, and power
coefcient. All these features serve to make it a labo-
ratory resistance standard, exceeded in performance
only by stand-alone standard resistors. The special
design of the HARS-LX requires no zero resistance
subtraction from settings.
Hermetically sealed wirewound resistors are used
for 1 Ωsteps and over. These resistors are carefully
conditioned under power and temperature in order
to develop the best stability characteristics. Actual
experience has shown them to exhibit a storage sta-
bility of better than 5 ppm/year, improving as they
age. The low-resistance resistors are constructed with
resistance wire with a minimum of copper resistance
in series to limit temperature coefcient effects.
The HARS-LX Series employs completely enclosed
dust-tight very low contact resistance switches. They
feature solid silver alloy contacts and quadruple-leaf
silver alloy wipers which keep switch contact resis-
tance to under 1 mΩper decade, and more impor-
tantly, keep switch contact resistance reproducible,
insuring repeatable instrument performance.
High-quality gold-plated tellurium-copper binding
posts minimize the thermal emf effects which would
articially reect a change in dc resistance measure-
ments. All other conductors within the instrument,
as well as the solder employed, contain no metals or
junctions that contribute to thermal emf problems.
The HARS-LX is designed to allow very convenient
maintenance of calibration over time. Most decades
are adjustable without changing components or sol-
dering resistors. The decades for the 100 Ω through
1 MΩsteps are adjusted with convenient trimmers.
Trimming of the lower decades is also possible.
With a resolution as low as 1 mΩand a maximum
available resistance of over 12.2 MΩ, the HARS-LX
Series may be employed for exacting precision mea-
surement applications requiring high accuracy and
stability. They can be used as components of dc and
low frequency ac bridges, for calibration, as transfer
standards, and as RTD simulators.
Figure 1-1: HARS-LX Laboratory Standard
Decade Resistance Substituter 11 Dials with
Rack Mount
Figure 1-2: HARS-LX Laboratory Standard
Decade Resistance Substituter 2 Dials

2
HARS-LX Series
2 Specifications
Chapter 2
SPECIFICATIONS
For convenience to the user, the pertinent specications are given in a typical OPERATING GUIDE, like the
one shown in Figure 2.1, afxed to the case of the instrument.
SPECIFICATIONS
*At 23°C “true ohm” measurement, 30-70% RH, absolute reading, SI traceable; no zero subtraction required
Minimum resistance:
With this design, absolute accuracy, without
zero subtraction, is accomplished by having a
minimum settable resistance, which includes
all contact and wiring resistances. Absolute
accuracy applies for every setting. The minimum
resistance is determined by the lowest resolution
of the unit and by internal resistance. See Model
Information for the minimum settable resistance
for any model. Minimum settable resistance is
implemented by a mechanical stop in one of the
decades.
Resistance repeatability:
Less than 100 µΩ, short-term, average value.
Leakage resistance:
>10 GΩ
Environmental conditions:
Operating temperature: 0ºC to 55ºC
Storage temperature: -40ºC to 70ºC
Switch type:
Multiple solid silver contacts; dust-tight diallyl-
phthalate body.
To allow continuous rotation, a blank position is
added on most decades.
Resistor type:
Wirewound, hermetically sealed,
low-inductance
Terminals:
Four, 5-way, gold-plated, tellutium-copper binding
posts with low thermal emf and low resistance,
for four-terminal Kelvin measurements, plus one
binding post connected to case for shielding.
Rear outputs are available as an option.
An additional Kel-F washer is added under the
High and Low binding posts for decade boxes
that have a 10 MΩ/step decade.
Range and resolution:
See Model Information
Resistance
per step
Total
decade
resistance
Max current Max power Temperature
coefficient
(±ppm/ºC)
Power
coefficient
(±ppm/mW)
Accuracy* Stability
(±ppm/yr) Decade positions Resistor type
whichever applies first
100 μΩdivision
20 μΩresolution
(-RH Option)
10 mΩ2 A NA 20 1 Continuous Rheostat
1 mΩ10 mΩ2 A NA 20 1
±(20 ppm+0.5
mΩ)
20 ppm+0.5 mΩ
11 positions "0"-"10"
Resistance wire
10 mΩ100 mΩ2 A NA 20 1 10 positions "1"-"10"
(10 mΩminimum reading)
100 mΩ1 Ω2 A NA 20 1
11 positions "0"-"10"
(12 positions "0"-"11" for
highest decade, where
highest decade steps are
1 Ω, 10 Ω, 10 kΩ, 100
kΩ, 1 MΩ, and 10 MΩ.)
See Ordering Information
section below.
1 Ω10 Ω1 A 5 W 20 0.4
Wirewound
hermetically sealed
low-inductance
10 Ω100 Ω0.33 A 5 W 10 0.3
100 Ω1 kΩ0.1 A 5 W 3 0.1
1 kΩ10 kΩ33 mA 5 W 3 0.1
10 ppm
(<5 ppm typical)
10 kΩ100 kΩ10 mA 5 W 3 0.1
100 kΩ1 MΩ3 mA 2,000 V peak 3 0.1
1 MΩ10 MΩ1 mA 2,000 V peak 3 0.1
10 MΩ100 MΩ2,000 V peak 15 0.2 ±0.02% 50 ppm Metal oxide
Wiring and switch resistance NA 50 µΩ/ºC 0.2 µΩ/W NA

3
HARS-LX Series
3Specifications
Mechanical Information:
Model Dimensions Nominal Weight
1 decade 7.7 cm W x 7.7 cm H x 8.4 cm D
(3" W x 3" H x 3.3" D)
0.45 kg
(1.0 lb)
3 decades 37.5 cm W x 8.9 cm H x 10.2 cm D
(14.8" W x 3.5" H x 4" D)
2.0 kg
(4.3 lb)
4-5 decades 43.9 cm W x 8.9 cm H x 10.2 cm D
(17.3" W x 3.5" H x 4.0" D)
2.2 kg
(4.8 lb)
6-10 decades 48.3 cm W x 17.8 cm H x 19.7 cm D
(19" W x 7" H x 7.8" D)
5.1 kg
(11 lb)
11 decades 48.3 cm W x 31 cm H x 19.7 cm D
(19" W x 12.3" H x 7.8" D)
6.6 kg
(14 lb)
Model Nominal total
resistance
No. of
decades Resolution
Minimum
settable
resistance**
HARS-LX-1-0.001 10 mΩ1 1 mΩ1 mΩ
HARS-LX-1-0.01 100 mΩ1 10 mΩ10 mΩ
HARS-LX-1-0.1 1 Ω1 100 mΩ100 mΩ
HARS-LX-1-1 11 Ω11 Ω1 Ω
HARS-LX-1-10 110 Ω1 10 Ω10 Ω
HARS-LX-1-100 1.0 kΩ1 100 Ω100 Ω
HARS-LX-1-1k 10 kΩ1 1 kΩ1 kΩ
HARS-LX-1-10k 110 kΩ1 10 kΩnone
HARS-LX-1-100k 1.1 MΩ1 100 kΩnone
HARS-LX-1-1M 11 MΩ1 1 MΩnone
HARS-LX-1-10M 110 MΩ1 10 MΩnone
HARS-LX-2-0.001 110 mΩ2 1 mΩ2 mΩ
HARS-LX-2-0.01 1.10 Ω2 10 mΩ10 mΩ
HARS-LX-2-0.1 12.0 Ω2 100 mΩ100 mΩ
HARS-LX-2-1 120 Ω2 1 Ω1 Ω
HARS-LX-2-10 1.10 kΩ2 10 Ω10 Ω
HARS-LX-2-100 11.0 kΩ2 100 Ω100 Ω
HARS-LX-2-1k 120 kΩ2 1 kΩ1 kΩ
HARS-LX-2-10k 1.20 MΩ2 10 kΩnone
HARS-LX-2-100k 12.0 MΩ2 100 kΩnone
HARS-LX-2-1M 120 MΩ2 1 MΩnone
HARS-LX-3-0.001 1.110 Ω3 1 mΩ*
HARS-LX-3-0.01 12.10 Ω3 10 mΩ10 mΩ
HARS-LX-3-0.1 121.0 Ω3 100 mΩ100 mΩ
HARS-LX-3-1 1.110 kΩ3 1 Ω1 Ω
HARS-LX-3-10 11.10 kΩ3 10 Ω10 Ω
HARS-LX-3-100 121.0 kΩ3 100 Ω100 Ω
HARS-LX-3-1k 1.210 MΩ3 1 kΩ1 kΩ
HARS-LX-3-10k 12.10 MΩ3 10 kΩnone
HARS-LX-3-100k 121.0 MΩ3 100 KΩnone
HARS-LX-4-0.001 12.110 Ω4 1 mΩ*
HARS-LX-4-0.01 121.10 Ω4 10 mΩ10 mΩ
HARS-LX-4-0.1 1.111 0 kΩ4 100 mΩ100 mΩ
HARS-LX-4-1 11.110 kΩ4 1 Ω1 Ω
HARS-LX-4-10 121.10 kΩ4 10 Ω10 Ω
HARS-LX-4-100 1.211 0 MΩ4 100 Ω100 Ω
HARS-LX-4-1k 12.110 MΩ4 1 kΩ1 kΩ
HARS-LX-4-10k 121.10 MΩ4 10 kΩnone
HARS-LX-5-0.001 121.110 Ω5 1 mΩ*
HARS-LX-5-0.01 1.111 10 kΩ5 10 mΩ10 mΩ
HARS-LX-5-0.1 11.111 0 kΩ5 100 mΩ100 mΩ
HARS-LX-5-1 121.110 kΩ5 1 Ω1 Ω
HARS-LX-5-10 1.211 10 MΩ5 10 Ω10 Ω
HARS-LX-5-100 12.111 0 MΩ5 100 Ω100 Ω
HARS-LX-5-1k 121.110 MΩ5 1 kΩ1 kΩ
HARS-LX-6-0.001 1.111 110 kΩ6 1 mΩ*
HARS-LX-6-0.01 11.111 10 kΩ6 10 mΩ10 mΩ
HARS-LX-6-0.1 121.111 0 kΩ6 100 mΩ100 mΩ
Model Nominal total
resistance
No. of
decades Resolution
Minimum
settable
resistance**
HARS-LX-6-1 1.211 110 MΩ6 1 Ω1 Ω
HARS-LX-6-10 12.111 10 MΩ6 10 Ω10 Ω
HARS-LX-6-100 121.111 0 MΩ6 100 Ω100 Ω
HARS-LX-7-0.001 11.111 110 kΩ71 mΩ*
HARS-LX-7-0.01 121.111 10 kΩ710 mΩ10 mΩ
HARS-LX-7-0.1 1.211 111 0 MΩ7 100 mΩ100 mΩ
HARS-LX-7-1 12.111 110 MΩ7 1 Ω1 Ω
HARS-LX-7-10 121.111 10 MΩ7 10 Ω10 Ω
HARS-LX-8-0.001 121.111 110 kΩ8 1 mΩ*
HARS-LX-8-0.01 1.211 111 10 MΩ8 10 mΩ10 mΩ
HARS-LX-8-0.1 12.111 111 0 MΩ8 100 mΩ100 mΩ
HARS-LX-8-1 121.111 110 MΩ8 1 Ω1 Ω
HARS-LX-9-0.001 1.211 111 110 MΩ9 1 mΩ*
HARS-LX-9-0.01 12.111 111 10 MΩ9 10 mΩ10 mΩ
HARS-LX-9-0.1 121.111 111 0 MΩ9 100 mΩ100 mΩ
HARS-LX-10-0.001 12.111 111 110 MΩ10 1 mΩ*
HARS-LX-10-0.01 121.111 111 10 MΩ10 10 mΩ10 mΩ
HARS-LX-11-0.001 121.111 111 110 MΩ11 1 mΩ*
Options:
-RH: 10 mΩ rheostat for lowest decades,
20 μΩ resolution
-RO: Rear output binding posts
-RM: Rack-mountable case for standard 19” rack
*Customer Selection
**With this design, absolute accuracy, without zero subtraction, is accomplished by having
a minimum settable resistance, which includes all contact and wiring resistances. Absolute
accuracy applies for every setting. The minimum resistance is determined by the lowest
resolution of the unit and by internal resistance. See Model Information for the minimum
settable resistance for any model. Minimum settable resistance is implemented by a
mechnical stop in one of the decades.
MODEL INFORMATION

4
HARS-LX Series
4 Specifications
Figure 2-1: Typical Operating Guide Affixed to Unit (See label on your specific unit for actual specifcations)
HARS-LX-5-100 OPERATING GUIDE
CONSULT INSTRUCTION MANUAL FOR PROPER INSTRUMENT OPERATION
HARS-LX-5-100 lbl/5/6/2015
WARNING
Observe all safety rules when working with high voltages or line voltages. Connect the (GND) terminal to earth ground in order to maintain the case at a safe voltage. Whenever hazardous voltages (>45 V) are used, take all mea- sures to avoid accidental contact with any live components: a) Use maximum insulation and minimize the use of bare conductors. b) Remove power when adjusting switches. c) Post warning signs and keep personnel safely away.
IET LABS, INC. www.ietlabs.com
CAGE CODE: 62015
• Long Island, NY • Email: [e-mail] • Tel: [phone] • 800-899-8438 • Fax: [phone]
Accuracy:
±(20 ppm+0.5 mΩ)
*
At 23°C “true ohm” measurement, 30-70% RH
, absolute reading
No zero subtraction required
Minimum resistance:
100 Ω ± 0.5 mΩ; determined by the lowest settable position, “1”, of the 100 Ω/step decade
Resistance repeatability:
Better than 100 µΩ, short-term, average value
Environmental Conditions:
Operating Temperature: 0°C to 55°C Storage Temperature: -40°C to 70°C
Switch Type:
Multiple solid silver contacts; dust-tight diallyl-phthalate body. To allow continuous rotation, a blank position is added on most decades.
For Best Performance:
Whenever the unit has been idle, turn each switch 7-10 times both ways before using. This switch "break-in" procedure is standard metrology practice required for best accuracy to remove any silver oxide lm on the contact surfaces, typically <1 mΩ.
Terminals:
Four, 5-way, gold-plated, tellurium-copper binding posts with low thermal emf and low resistance, for four-terminal Kelvin measurements, plus one binding post connected to case for shielding. Rear outputs are available as an option.
MODEL: HARS-LX-5-100 SN: D5-1422571
Resistance
per step
Total decade
resistance
Max current Max power Temperature
coefficient
(±ppm/ºC)
Power
coefficient
(±ppm/mW)
Stability
(±ppm/yr) Decade positions
whichever applies first
100 Ω1 kΩ0.1 A 5 W 30.1 20 ppm+0.5 mΩ10 positions “1”-”10”
(100 Ωminimum reading)
1 kΩ10 kΩ33 mA 5 W 30.1
10 ppm
(<5 ppm typical)
11 positions “0”-”10”
10 kΩ100 kΩ10 mA 5 W 30.1 100 kΩ1 MΩ3 mA 5 W 30.1 1 MΩ11 MΩ1 mA 1,500 V peak 30.1 12 positions “0”-”11”
Wiring and switch resistance NA 50 µΩ/ºC 0.2 µΩ/W NA

5
HARS-LX Series
5Installation
Figure 2-1: Typical Operating Guide Affixed to Unit (See label on your specific unit for actual specifcations)
Chapter 3
Installation
3.1 Initial Inspection
IET instruments receive a careful mechanical and
electrical inspection before shipment. Upon receipt,
verify that the contents are intact and as ordered.
The instrument should then be given a visual and
operational inspection.
If any shipping damage is found, contact the carrier
and IET Labs. If any operational problems are en-
countered, contact IET Labs and refer to the warranty
at the beginning of this manual.
Save all original packing material for convenience
in case shipping of the instrument should become
necessary.
3.2 Installation
For a rack mounted model, installation in a 19 inch
rack may be made using the slots in the rack mount-
ing ears. A mounting location that does not expose
the unit to excessive heat is recommended. For bench
models there is no required installation.
Since this is a high accuracy instrument, it is rec-
ommended that a space be provided that would not
expose it to mechanical abuse and keep it maintained
at laboratory standard temperatures near 23ºC.
3.3 Repackaging for Shipment
If the instrument is to be returned to IET Labs, contact
the Service Department at the number or address,
shown on the front cover of this manual, to obtain a
“Returned Material Authorization” (RMA) number
and any special shipping instructions or assistance.
Proceed as follows:
1. Attach a tag to the instrument identifying the
owner and indicate the service or repair to be
accomplished. Include the model number,
the full serial number of the instrument, the
RMA number, and shipping address.
2. Wrap the instrument in heavy paper or
plastic.
3. Protect the front panel and any other protru-
sions with cardboard or foam padding.
4. Place instrument in original container or
equally substantial heavy carton.
5. Use packing material around all sides of
instrument.
6. Seal with strong tape or bands.
7. Mark shipping container “DELICATE
INSTRUMENT,” “FRAGILE,” etc.
3.4 Storage
If this instrument is to be stored for any lengthy period
of time, it should be sealed in plastic and stored in a
dry location. It should not be subjected to temperature
extremes beyond the specications. Extended expo-
sure to such temperatures can result in an irreversible
change in resistance, and require recalibration.

6
HARS-LX Series
6 Operation
Chapter 4
OPERATION
4.1 Initial Inspection and Setup
This instrument was carefully inspected before ship-
ment. It should be in proper electrical and mechanical
order upon receipt.
An OPERATING GUIDE is attached to the case
of the instrument to provide ready reference to
specications.
4.2 Connection
Four insulated low-thermal-emf binding posts labeled
CURRENT HI, CURRENT LO, SENSE HI, and
SENSE LO provide two current and two potential
terminals, respectively, for 4-terminal measurement.
2-terminal measurements may be made by shorting
CURRENT HI to SENSE HI, and CURRENT LO
to SENSE LO, preferably with shorting links or other
substantial means. The four terminal connection is
important of course for low resistances.
A fth metal binding post labeled GND (Ground) is
connected to the case and may be used as a guard or
shield terminal. It may also be connected using the
shorting link to either terminal to implement a 2-ter-
minal as opposed to a 3-terminal measurement.
4.3 Electrical Considerations
In order to make proper use of the full performance
capabilities of the HARS-LX unit, especially if low
resistance or low resistance increments are important,
care must be taken in connecting to the terminals of
the decade box.
In particular, to keep contact resistance to a mini-
mum, 4-terminal connections should be employed,
and the most substantial and secure connection to the
binding posts should be made. They accept banana
plugs, telephone tips, spade lugs, alligator clips, and
bare wire. The largest or heaviest mating connection
should be made, and, where applicable, the binding
posts should be securely tightened.
These considerations may be relaxed whenever single
milliohms not considered signicant for the task be-
ing performed.
4.3.1 Thermal emf Considerations
The highest quality low emf components are used in
the HARS-LX series. In particular, the terminals are
made of gold plated tellurium copper, which exhibits
low emf and low resistance. There nevertheless may
be some minute thermal emf generated at the user’s
test leads where they contact the HARS-LX binding
posts. This will depend on the test lead material.
Whenever this is critical, brass and iron materials
should be avoided.
This emf will not manifest itself if an ac measurement
instrument is employed. It will also be eliminated if
a meter with so called “True Ohm” capability is used.
Otherwise it may appear as a false component of the
dc resistance measurement, and can be the order of
milliohms.

7
HARS-LX Series
7Operation
4.4 Dial Setting
The resistance setting may be read directly from the
dial settings. For additional exibility and range,
each decade provides a “10” position setting. This
“10” position on any one decade equals the “1” posi-
tion on the next higher decade if any. It adds about
11% to the nominal total decade resistance. The
most signicant decade also has an “11” position
to extend the resistance range to over 1.2 MΩ. The
l0 mΩdecade, however, does not go below the
“1“ position in order to maintain a precise and
constant minimum resistance of 10 mΩ, so that no
subtraction of zero resistance is required.
To determine the resistance obtained when any one
or more “10” or “11” settings are used, simply add 1
to the next higher decade. For example, a setting of
“10-11-10-10-10-10” becomes:
11 1 1 0 0 0 0
10 1 0 0 0 0
10 1 0 0 0
10 1 0 0
10 1 0
10 1
1
10
Total 1 2 1 1 1 1
In order to obtain a zero in the l Ωposition, set the
l Ωdecade to the “10” position, i.e. l0 Ω, and take the
l0 Ωsetting, in the next decade, into consideration.
To get 2 Ω, for example, the switches should be set
to show “1-10”.
Since the highest decade has an additional “11” po-
sition, resistance values of over 1.2 times nominal
maximum value can be obtained.
4.5 Power Considerations
To maintain the maximum possible accuracy and pre-
cision, power applied to the HARS-LX should be kept
as low as possible, preferably below 0.1 W. For best
protection of the instrument, it is advisable to limit
the input power to 1 W. This may be implemented
with a series resistor or fuse.
4.6 Environmental Conditions
For optimal accuracy, the decade box should be used
in an environment of 23ºC. It should be allowed to
stabilize at that temperature for at least four hours
after any signicant temperature variation.
Humidity should be maintained at laboratory condi-
tions of 30% to 70% RH.
4.7 Switch Conditioning
The switch wipers employed in this unit are self
cleaning. They have solid silver alloy contacts.
After being left idle, the wipers and contacts must
be conditioned or “broken in” again to remove the
lm of silver oxide that develops over time. This is
standard metrology practice when high accuracy is
required. This effect is of the order of less than 1 mΩ,
So it may be ignored whenever measurements of that
magnitude are not important.
To perform this “breaking in,” simply rotate each
switch seven to ten times in each direction.

8
HARS-LX Series
8 Operation
4.8 Meter Shunt Applications
To measure the current in a 4-terminal resistor:
1. Measure the voltage drop between the two
terminals of the resistor not connected to the
current source.
2. Determine the current by the ratio of the
measured voltage to the known resistance.
Using the 4-terminal technique helps avoid errors
caused by the voltage drops in the current-carrying
leads and contacts. Errors caused by lead and contact
resistances in the voltage measuring circuit are negli-
gible if the current in this circuit is small.
4.9 Kelvin Bridge Applications
Use 4-terminal resistance standard for all Kelvin
bridge measurements. When connected as shown in
Figure 4-1, errors caused by lead and contact resis-
tances can be made negligible because they appear as
part of the generator or yoke resistance, or in series
with high resistance bridge arms.
Figure 4-1: Kelvin Bridge Connections
HARS-LX
For maximum protection and accuracy, limit power
input to HARS-LX to 1 Watt. This may be accom-
plished by placing a resistor in series with the bridge
generator or battery.
The value of this resistance can be calculated from
the following formula:
where:
R = value of the power-limiting resistor
E = open circuit voltage of the generator
The protective resistor should have a power rating
of 4 W or more. Input power should be limited to
1/10 W or less for most accurate measurements.
R=
E2
4

9
HARS-LX Series
9Operation
4.10 Rheostat Operation (-RH Option)
For high-resolution applications, an optional rheostat
may be used. The rheostat provides up to 10 mΩ of
resistance in 0.1 mΩ steps. To get the rheostat read-
ing in milliohms, multiply the dial setting by 0.1.
See Figure 4-1.
Figure 4-1: Rheostat Dial
In order to eliminate contact resistance and thermal
emf, the HARS-LX integrates the rheostat as shown
in Figure 4-2. This way, the wiper is in the low
potential circuit, which is the high impedance lead.
As a result, voltage and contact resistance effects
are removed by being effectively added to the input
impedance of the measuring instrument.
Figure 4-2: Rheostat Schematic
HI SENSE
HI CURRENT
LO SENSE
LO CURRENT
DISCRETE
DECADES
RHEOSTAT

10
HARS-LX Series
10 Maintenance
Chapter 5
MAINTENANCE
5.1 Maintainability and Reliability
It is possible to maintain Model HARS-LX inde-
nitely. It is reliable due to its closed design and sealed
switches and resistors. It is possible to adjust the unit,
if necessary, because it has adjustable decades for 1
Ω decades and above and resistance wire for lower
decades which can be trimmed. The unit is resistant
to electromagnetic interference (EMI) because of its
metal enclosure.
5.2 Preventive Maintenance
Keep the unit in a clean environment. This will help
prevent possible contamination.
The Model HARS-LX is packaged in a closed case
and uses completely sealed switches. This limits the
entry of contaminants and dust to the inside of the
switches. If it is maintained in a clean or air-condi-
tioned environment, cleaning will seldom be required.
Should cleaning be needed:
1. Remove the 4 housing screws from the side
of the instrument, and remove the housing.
2. Remove any dust or debris using optical
grade dry compressed air or a clean brush.
3. Replace the housing and reattach the 4 hous-
ing screws.
The front panel should be periodically cleaned to
eliminate any leakage paths from near or around the
binding posts. To clean the front panel:
Wipe the front panel clean using alcohol and a
lint-free cloth.
5.3 Calibration
The Model HARS-LX may be employed as a stand-
alone instrument or as an integral component of a
system. If used as part of a system, it should be
calibrated as part of the overall system to provide an
optimum system calibration.
If the HARS-LX is employed as a stand alone device,
the following should be observed:
• Calibration Interval
• General Considerations
• Required Equipment
• Calibration Procedure
5.3.1 Calibration Interval
The recommended HARS-LX Series calibration
interval is twelve (12) months.
If the instrument is used to transfer resistance values
only, recalibration is not required, assuming that there
has been no drastic change in the deviations of any
individual resistors.

11
HARS-LX Series
11Maintenance
5.3.2 General Considerations
Before starting the calibration procedure, you need
to consider the following:
• Calibration environment should be 23°C and
less than 50% relative humidity.
• Test instruments should be sufciently more
accurate than the HARS-LX unit, and/or the
uncertainty of the measurement instrumenta-
tion has to be considered in the calibration
Test Uncertainty Ratio (TUR).
• The testing equipment and the HARS-LX
unit should stabilize at laboratory conditions
for at least 24 hours.
• Kelvin type 4-wire test leads should be
used to obtain accurate low resistance
measurements.
• Steps should be taken to minimize thermal
emf effects, such as using a meter with “True
Ohm” capacity.
• Accepted metrology practices should be
followed.
5.3.3 Required Equipment
Many combinations of standards, transfer standards,
meters, and bridges may be used to calibrate this in-
strument. The following are some possible choices:
• Resistance Standards or Transfer Standards
for 1 Ω, 10 Ω, 100 Ω, 1 kΩ, 10 kΩ, 100 kΩ,
1 MΩ, and 10 MΩper step, calibrated to
±10 ppm. IET options include the follow-
ing models:
• HATS-LR
• HATS-Y
• SRL Series
The 1 Ω, and 10 Ωtransfer standards are
optional, and are only used to take advan-
tage, if desired, of the adjustability of these
two decades
• Precision resistance measurement bridge
or multimeter, with a transfer accuracy
of ±l ppm. Options include:
• Guildline Model 9975
• Measurements International Model
6000A
• ESI model 242, 242A, 242C, or
242D
• A high-precision, high-stability digi-
tal multimeter (e.g. Fluke 8508A)
along with a set of resistance stan-
dards for ratio mode.
5.3.4 Calibration Procedure
To calibrate the HARS-LX unit, proceed as follows:
1. Set up the calibration equipment in the re-
sistance measurement mode and exercise the
switches 10 times in each direction.
2. Allow the switches to cool for 15 minutes.
3. Conrm the minimum resistance of the unit.
Allow a condence band for the uncer-
tainty of the measuring instrument and
setup.
4. Determine the allowable upper and lower
limits for each resistance setting of each
decade based on the specied accuracy and
the condence band.
For the HARS-LX series, the limits for
any resistance “R” are:
[R±(20 x 10-6R + 0.0005Ω)].
5. Conrm that the resistances fall within these
limits.
6. If any resistances fall outside these limits,
the associated switch assembly may require
service or replacement.

12
HARS-LX Series
12 Maintenance
5.4 Adjustments
If one or more resistances fall outside the limits, the
associated resistor should be adjusted. There is a
trimming network provided for each resistance in
these ranges. These may be accessed by removing the
housing and accessing the particular decade PC board.
Adjust the resistors from lowest to highest. The order
of decades does not matter.
Whereas it is possible to adjust any one resistance
step, note that the nth step of a decade is the sum of
resistances 1 through n, so that errors are cumula-
tive. It is therefore recommended that whenever any
resistance of a particular decade is adjusted, that all
the resistances of that decade be tested and adjusted
as required.
To adjust any of the resistances, the following should
be observed:
• Adjustment considerations
• Adjustment procedure
5.4.1 Adjustment Considerations
Before adjusting or trimming any resistances, observe
the following:
The equipment measuring the unit should conform to
the guidelines provided in Section 5.3.
CAUTION
The HARS-LX front panel GND
binding post should be connected to
the test instrument’s ground point.
5.4.2 Adjustment/Trimming
Procedure
To trim or adjust any resistances, proceed as follows:
1. Stabilize HARS-LX at laboratory tempera-
ture of 23 °C for at least 8 hours.
2. Remove the 4 housing screws from the sides
of the instrument and remove the housing.
3. Using the binding posts in the 4-terminal
connection, connect HARS-LX to a resis-
tance meter.
4. Rotate all knobs approximately 10 times and
be careful not to rotate the minimum setting
decade beyond the stops.
Breaking in the switches helps elimi-
nate any residual contact corrosion
resistance.
5. Set all dials to 0, except for the minimum
setting decade.
The minimum setting decade has a mini-
mum stop at position 1.
6. Measure the minimum resistance setting
If an adjustment is needed, consult IET
technical support.
7. Measure 1 mΩ, 10 mΩ, and 100 mΩ steps as
applicable to the HARS-LX being trimmed.
If adjustment is needed, proceed as follows:
• If the resistance is high, reduce it by add-
ing a small bit of solder to an exposed
portion of the wire
• If the resistance is low, increase it by
gently ling the wire in one spot.
8. Measure the higher decades. If an adjust-
ments is needed, proceed as follows:
Starting with the lowest setting, adjust the
associated trim pots.
See Table 5-1 and Figure 5-1 for
reference
9. Replace the housing and reinstall the 4 hous-
ing screws.
CAUTION
Make these steps carefully and slowly,
allowing the wire to cool down before
measuring again.

13
HARS-LX Series
13Maintenance
Switch Position Potentiometer Designation
All decades but 1 k Ω
Potentiometer Designation
1 kΩ decade
1 T0 T1
2 T1 T2
3 T2 T3
4 T3 T4
5 T4 T5
6 T5 T6
7 T6 T7
8 T7 T8
9 T8 T9
10 T9 T10
11 T10 (1 M steps only)
Table 5-1: Trimming Potentiometers
Figure 5-1: Typical Trimmer Board
Trim
Potenti-
ometers

14
HARS-LX Series
14 Maintenance
1
2
3
5
5.5 Replaceable Parts List
Model Ref IET Pt No Description
1BP-1000-RD Binding Post, Red
2BP-1000-BK Binding Post, Black
3BP-1000-GN Binding Post, Green
4HARS-LX-4300-KNB Knob Assembly
5RS-925D-RH Rheostat Assembly
Not Shown HARS-LX-3100 Foot
Not Shown HARS-4000-LX-.001 1 mΩ/step Decade Switch Assembly
Not Shown HARS-4000-LX-0.01 10 mΩ/step Decade Switch Assembly
Not Shown HARS-4000-LX-0.1 100 mΩ/step Decade Switch Assembly
Not Shown HARS-4000-LX-1 1 Ω/step Decade Switch Assembly
Not Shown HARS-4000-LX-10 10 Ω/step Decade Switch Assembly
Not Shown HARS-4000-LX-100 100 Ω/step Decade Switch Assembly
Not Shown HARS-4000-LX-1k 1 kΩ/step Decade Switch Assembly
Not Shown HARS-4000-LX-10k 10 kΩ/step Decade Switch Assembly
Not Shown HARS-4000-LX-100k 100 kΩ/step Decade Switch Assembly
Not Shown HARS-4000-LX-1M 1 MΩ/step Decade Switch Assembly
Not Shown HARS-4000-LX-10M 10 MΩ/step Decade Switch Assembly
Table 5-2: Replaceable Parts List
Figure 5-2: HARS-LX Replaceable Parts
4