IET Labs RS-925D Troubleshooting guide
www.ietlabs.com
IET LABS, INC.
Email: inf[email protected]
♦ ♦
RS-925D
Resistance Standard
User and Service Manual
Copyright © 2014 IET Labs, Inc.
Visit www.ietlabs.com for manual revision updates
RS-925D Sept. 2014
www.ietlabs.com
IET LABS, INC.
Email: inf[email protected]
♦ ♦
IET Model RS-925D
i
ii
Contents
Chapter 1 Introduction ..............................................................................1
1.1 Introduction........................................................................................................... 1
Chapter 2 Specifications ...........................................................................2
Specications ................................................................................................................ 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
4.2.2 Electrical Considerations ............................................................................ 5
4.2.3 Thermal emf Considerations....................................................................... 5
4.3 Dial Setting ........................................................................................................... 6
4.4 Rheostat Operation................................................................................................ 6
4.5 Power Considerations ........................................................................................... 6
4.6 Environmental Conditions .................................................................................... 7
4.7 Switch Conditioning ............................................................................................. 7
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 Trimming Considerations ........................................................................... 10
5.4.2 Trimming Procedure ................................................................................... 10
5.5 Replaceable Parts List........................................................................................... 12
iii
Figures and Tables
Figure 1-1: RS-925D Decade Resistance Substituter.................................1
Figure 2-1: Typical Operating Guide Affixed to Unit.....................................3
Figure 4-1: Rheostat Dial ............................................................................6
Figure 4-2: Rheostat Schematic..................................................................6
Figure 4-3: 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: RS-925D Replaceable Parts.....................................................12
iv
1
RS-925D
1Introduction
Chapter 1
INTRODUCTION
1.1 Introduction
The RS-925 Resistance Standard provides a very
broad-range high-performance resistance source.
The RS-925D Resistance Standard is a precision
resistance source with excellent characteristics of ac-
curacy, stability, temperature coecient, and power
coecient. All these features serve to make it a labo-
ratory resistance standard, exceeded in performance
only by stand-alone standard resistors.
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 coecient eects.
The unit has a xed minimum resistance of 10 mΩ.
This is implemented by mechanically limiting the
10 mΩdecade from going below the “1” position. In
this manner, no zero resistance subtractions have to
be made, and the accuracy given is for the absolute
reading.
The RS-925D Resistance Standard employs com-
pletely enclosed dust-tight very low contact resistance
switches. They feature solid silver alloy contacts and
quadruple-leaf silver alloy wipers which keep switch
contact resistance to under 1 mΩper decade, and more
importantly, keep switch contact resistance reproduc-
ible, insuring repeatable instrument performance.
High quality gold plated tellurium copper binding
posts minimize the thermal emf eects which would
articially reect 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.
TheRS-925D Resistance Standard is designed to al-
low very convenient maintenance of calibration over
time. Most decades are calibratable without changing
components or soldering resistors. The decades for
the 100 Ω through 100 kΩsteps are calibrated with
convenient trimmers. Trimming of the lower decades
is also possible.
With a resolution as low as 20 ΩΩand a maximum
available resistance of over 12.1 MΩ, the RS-925D-
11-0.001-RH may be employed for exacting precision
measurement applications requiring high accuracy
and stability. It can be used as components of dc and
low frequency ac bridges, for calibration, as transfer
standards, and as RTD simulators.
Figure 1-1: RS-925 Resistance Standard
2
RS-925D
2 Specifications
Chapter 2
SPECIFICATIONS
For convenience to the user, the pertinent specications are given in a typical OPERATING GUIDE, like the
one shown in Figure 2.1, axed to the case of the instrument.
SPECIFICATIONS
Resistance
per step
Total
decade
resistance
Max
current
Max
power Temperature
coecient
(±ppm/ºC)
Power
coecient
(±ppm/mW)
Accuracy* Stability
(±ppm/yr) Decade positions Resistor type
whichever applies rst
100 µΩ/division
20 μΩ resolution 10 mΩ 2 A NA 20 1 Continuous Rheostat
10 mΩ 100 mΩ 2 A NA 20 1
Ω)
Ω
10 positions "1"-"10"
(10 mΩ minimum reading)
100 mΩ 1 Ω 2 A NA 20 1
11 positions "0"-"10"
(12 positions "0"-"11" for
highest decade)
1 Ω 10 Ω 1 A 5 W 20 0.4
Wirewound
hermetically sealed
low-inductance
10 Ω 100 Ω 0.33 A 5 W 10 0.3
100 Ω 1 kΩ 0.1 A 5 W 3 0.1
1 kΩ 10 kΩ 33 mA 5 W 3 0.1 10 ppm
(<5 ppm typical)
10 kΩ 100 kΩ 10 mA 5 W 3 0.1
100 kΩ 1 MΩ 3 mA 2,000 V peak 3 0.1
Wiring and switch resistance NA 50 µΩ/ºC 0.2 µΩ/W NA
*At 23°C “true ohm” measurement, 30-70% RH, absolute reading, SI traceable
No zero subtraction required
Minimum resistance:
10 mΩ ± 0.5 mΩ; determined by the lowest settable
position, “1”, of the 10 mΩ/step decade
Resistance repeatability:
Better 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.
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.
Mechanical Information:
Model Dimensions Weight
8 decades
(11 lb)
3
RS-925D
3Specifications
RS925D OPERATING GUIDE
MODEL: RS925D SN: D5-2307704
WARNING
IET LABS, INC. www.ietlabs.com
Accuracy
±(20 ppm+0.5 mΩ)
At 23°C "true ohm" measurement, 30-70% RH,
absolute reading, case grounded, SI traceable
No zero subtraction required
Minimum resistance:
10 mΩ ± 0.5 mΩ; determined by the lowest settable
position, “1”, of the 10 mΩ/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.
Resistance
per step
Total
decade
resistance
Max
current
Max
power Temperature
coefficient
(±ppm/ºC)
Power
coefficient
(±ppm/mW)
Stability
(±ppm/yr) Decade positions Resistor type
whichever applies first
100 µΩdivision
20 µΩresolution 10 mΩ 2 A NA 20 1
20 ppm+0.5 mΩ
Continuous Rheostat
1 mΩ 10 mΩ 2 A NA 20 1 11 positions "0"-"10"
Resistance wire
10 mΩ 100 mΩ 2 A NA 20 1 10 positions "1"-"10"
(10 mΩ minimum reading
100 mΩ 1 Ω 2 A NA 20 1
11 positions "0"-"10"
1 Ω 10 Ω 1 A 5 W 20 0.4
Wirewound
hermetically sealed
low-inductance
10 Ω 100 Ω 0.33 A 5 W 10 0.3
100 Ω 1 kΩ 0.1 A 5 W 3 0.1
1 kΩ 10 kΩ 33 mA 5 W 3 0.1 10 ppm
(<5 ppm typical)
10 kΩ 100 kΩ 10 mA 5 W 3 0.1
100 kΩ 1 MΩ 3 mA 2,000 V peak 3 0.1 12 positions "0"-"11"
Wiring and switch resistance NA 50 µΩ/ºC 0.2 µΩ/W NA
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 fi 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.
RS925D OPERATING GUIDE
MODEL: RS925D SN: D5-1645634
WARNING
IET LABS, INC. www.ietlabs.com
Accuracy
±(20 ppm+0.5 mΩ)
At 23°C "true ohm" measurement, 30-70% RH,
absolute reading, case grounded, SI traceable
No zero subtraction required
Minimum resistance:
10 mΩ ± 0.5 mΩ; determined by the lowest settable
position, “1”, of the 10 mΩ/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.
Resistance
per step
Total
decade
resistance
Max
current
Max
power Temperature
coefficient
(±ppm/ºC)
Power
coefficient
(±ppm/mW)
Stability
(±ppm/yr) Decade positions Resistor type
whichever applies first
100 µΩdivision
20 µΩresolution 10 mΩ 2 A NA 20 1
20 ppm+0.5 mΩ
Continuous Rheostat
1 mΩ 10 mΩ 2 A NA 20 1 11 positions "0"-"10"
Resistance wire
10 mΩ 100 mΩ 2 A NA 20 1 10 positions "1"-"10"
(10 mΩ minimum reading
100 mΩ 1 Ω 2 A NA 20 1
11 positions "0"-"10"
1 Ω 10 Ω 1 A 5 W 20 0.4
Wirewound
hermetically sealed
low-inductance
10 Ω 100 Ω 0.33 A 5 W 10 0.3
100 Ω 1 kΩ 0.1 A 5 W 3 0.1
1 kΩ 10 kΩ 33 mA 5 W 3 0.1 10 ppm
(<5 ppm typical)
10 kΩ 100 kΩ 10 mA 5 W 3 0.1
100 kΩ 1 MΩ 3 mA 2,000 V peak 3 0.1 12 positions "0"-"11"
Wiring and switch resistance NA 50 µΩ/ºC 0.2 µΩ/W NA
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 fi 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.
Figure 2-1: Typical Operating Guide Affixed to Unit
4
RS-925D
4 Installation
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 specications. Extended expo-
sure to such temperatures can result in an irreversible
change in resistance, and require recalibration.
5
RS-925D
5Operation
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
specications.
4.2 Connection
4.2.1 General Considerations
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.
4.2.2 Electrical Considerations
In order to make proper use of the full performance
capabilities of the RS-925D 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 signicant for the task be-
ing performed.
4.2.3 Thermal emf Considerations
The highest quality low emf components are used in
the RS-925D 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 RS-925D banana
jacks. 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.
6
RS-925D
6 Operation
4.3 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” position
on the next higher decade if any. It adds about 11
% to the nominal total decade resistance. The most
signicant decade also has an “11” position to extend
the resistance range to over 1.2 MΩ. The l0 mΩde-
cade, 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
“10-11-10-10-10-10-1-10” becomes:
11 1 1 0 0 0 0. 0 0 0
10 1 0 0 0 0. 0 0 0
10 1 0 0 0. 0 0 0
10 1 0 0. 0 0 0
10 1 0. 0 0 0
10 1. 0 0 0
1 . 0 1 0
10 . 0 1 0
Total 1 2 1 1 1 1. 0 2 0
In order to obtain a zero in the l0 mΩposition, set
the l0 mΩdecade to the “10” position, i.e. l00 mΩ,
and take the l00 mΩsetting, in the next decade, into
consideration. To get 1.000 Ω, for example, the
switches should be set to show “0.-9-10-0”.
Since the highest decade has an additional “11” po-
sition, resistance values of over 1.2 times nomianl
maximum value can be obtained.
4.4 Rheostat Operation
For high-resolution applications, an optional rheostat
may be used. It provides up to 10 mΩ of resistance in
0.1 mΩ steps. To get the rheostat reading 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 RS-925D 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 eects are removed by
being eectively added to the input impedance of the
measuring instrument.
Figure 4-2: Rheostat Schematic
4.5 Power Considerations
To maintain the maximum possible accuracy and pre-
cision, power applied to the RS-925D 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.
HI SENSE
HI CURRENT
LO SENSE
LO CURRENT
DISCRETE
DECADES
RHEOSTAT
7
RS-925D
7Operation
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 signicant 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 eect 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.
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 mea-
sured 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 4-3, 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 4-3: Kelvin Bridge Connections
For maximum protection and accuracy, limit power
input to RS-925D to 1Watt. 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.
RS-925D
R=
E2
4
8
RS-925D
8 Maintenance
Chapter 5
MAINTENANCE
5.1 Maintainability and Reliability
It is possible to maintain Model RS-925D indenitely.
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 6 trimmable decades. 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 RS-925D 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-conditioned envi-
ronment, 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 RS-925D 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 RS-925D 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 RS-925D Series calibration inter-
val 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.
9
RS-925D
9Maintenance
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 suciently more
accurate than the RS-925D 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 RS-925D 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 eects, 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 advantage,
if desired, of the trimmability 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 RS-925D 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. Conrm the minimum resistance of the unit.
Allow a condence 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 specied accuracy and
the condence band.
For the RS-925D series, the limits for
any resistance “R” are:
[R±(20 x 10-6R + 0.0005Ω)].
5. Conrm that the resistances fall within these
limits.
6. If any resistances fall outside these limits,
the associated switch assembly may require
service or replacement.
10
RS-925D
10 Maintenance
5.4 Adjustments
If one or more resistances fall outside the limits, the
associated resistor should be trimmed. This applies
for 1 Ωsteps and over. There is a trimming network
provided for each resistance in these ranges. These
may be accessed by removing the housing and access-
ing the particular decade PC board.
If the minimum 10 mΩ needs to be adjusted, consult
IET Labs. For other resistors, 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 trimmed, that all
the resistances of that decade be tested and trimmed
as required.
To adjust any of the resistances, the following should
be observed:
• Trimming considerations
• Trimming procedure
5.4.1 Trimming Considerations
Before trimming any resistances, observe the
following:
The equipment measuring the unit should conform to
the guidelines provided in Section 5.3.
CAUTION
The RS-925D front panel should be
connected to the test instrument’s
guard point.
5.4.2 Trimming Procedure
To trim any resistances, proceed as follows:
1. Stabilize RS-925D at laboratory temperature
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 RS-925D to a resistance
meter.
4. Rotate all knobs approximately 10 times.
Breaking in the switches helps elimi-
nate any residual contact corrosion
resistance.
5. Set all dials to 0, except for the 10 mΩ
decade.
10 mΩ decade has a minimum stop at
position 1.
6. Measure the 10 mΩ setting
If an adjustment is needed, consult IET
technical support.
7. Measure 1 mΩ, 10 mΩ, and 100 mΩ steps.
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
remeasuring.
11
RS-925D
11Maintenance
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
7T6 T7
8T7 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
Pots
12
RS-925D
12 Maintenance
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
4RS-925D-4300-KNB Knob Assembly
5RS-925D-RH Rheostat Assembly
Not Shown RS-925D-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: RS-925D Replaceable Parts
3
2
1
4
5
Table of contents
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