Fluke 742A resistance standards repairs and performance checks




Intro

Every DMM and power measurement device are using electrical resistance standards to provide accurate and useful measurements. But if one need to verify precise instruments, highly stable and characterized fixed point resistance standards are required. Best examples of such artifacts are famous primary grade ESI SR104 or L&N 4210 units. These provide fixed resistance output, and essentially are custom made precision wirewound resistors mounted in nice rugged enclosure and provided with low-thermal binding posts for quality connections.

But in this article we will look at more common compact Fluke 742A resistance standards, designed for daily laboratory use. They might not be the best stability in the world and not the most stable devices, but way more practical and less demanding to typical handling. Fluke 742A performance still well suited for calibration of high-end long-scale multimeters, current sources, calibrators, thermometers and power measurement systems.

  • Good temperature stability, reducing need of airbath temperature control
  • Metrology grade with +18 °C to +28 °C temperature rating
  • Best 1 year specification is ± 4 ppm for 10 and 19 kΩ variants.
  • Each unit labeled with temperature α and β values.
  • Compact size 86 mm x 105 mm x 127 mm
  • 5-way low thermal binding posts, same as Fluke 57** calibrators or 8½-digit DMMs.

In this article collection of available information and benchmarks for the available Fluke 742A units will be performed. This would represent performance those particular units, as example what can be expected from such a device, given typical laboratory conditions.

Table below shows models that we measured directly and their test results summary. Models for 1 Ω and 10 kΩ are especially popular on secondary-market because they fit for artifact calibration needs of Fluke 5700A, 5720A and 5730A calibrators and Keysight 3458A multimeters.

Unit S/N Results Last known value Calibration Date, method
742A-1 6270023 Details 1 1.00004374 Ω ±0.43 ppm IET Labs external calibration, 27.OCT.2020
742A-1 6270023 Details 1 1.00004464 Ω ±0.33 ppm Internal xDevs.com DCC calibration, 20.MAR.2022
742A-1 7681001 Details 2 1.00000861 Ω ±0.4 ppm Internal xDevs.com calibration
742A-1 5825005 Details 1 1.00003507 Ω ±0.4 ppm Internal xDevs.com calibration
742A-1.9
742A-10 7170003 Details 10.0001170 Ω ±0.35 ppm Internal xDevs.com DCC calibration, 16.MAR.2022
742A-25
742A-100
742A-1k
742A-10k 5820007 Details 10000.0865 Ω ±0.38 ppm Internal xDevs.com HRB calibration, 9.APR.2022
742A-19k
742A-100k
742A-1M
742A-10M
742A-19M

Some additional custom values available from Fluke by special order. Fluke 742A standards are available for MSRP ~$3680 USD per piece.

There is also Fluke case designed to hold two 742A standards, Model 742A-7002. Case is available for $1315 USD.

Standard calibration fee for 742A with 17025 accreditation report from Fluke Everett calibration laboratory is $533 USD (2018). First report is included in price of the unit, if its ordered from Fluke new.


Image 1: Array of Fluke 742A secondary resistance standards

Fluke 742A-1, 1 Ω model, S/N 6270023 from Fluke PSL, 02-NOV-1999

Unit is manufactured around end of 1999, if we believe label on the back. Since it is low ohmic resistance model, there are kelvin 4-wire port on the front and grounding post.

Fluke 742A-1 6270023 Value Unit
Nominal value 1 Ω
Operation temperature range +18 … +28 °C
Maximum voltage 0.5 V
Maximum current 0.5 A


Image 2: Front of the 742A-1 unit


Image 3-4: Back of the 742A-1 and top side view

This unit usually lives in plastic hardcase, such as Apache 2800. This case can house two of such standards with nice amount of padding on each side. This is useful to protect resistance standard from shipping stress and reduce risk of damage.


Image 5-6: 742A in safety of hard plastic case

For temperature coefficient verification, next setup is used:

Temperature stability test of this unit

After initial validation and testing this 742A-1 was shipped to external calibration lab for formal calibration on their DCC bridge setup.

It came back with assigned value 1.00004374 Ω measured on October 27, 2020 with uncertainty ±0.46 ppm by IET Labs.

This resistor was recalibrated against freshly calibrated Ohm Labs 200 standard on 20.MAR.2022, with new assigned value 1.00004464 Ω with uncertainty ±0.33 ppm. Measurement was performed at 100 mA current using MIL 6010B DCC bridge. Both reference standard and test 742A-1 were fixed at +23.00 °C temperature in air bath chamber.

Fluke 742A-10, 10 Ω model, S/N 7170003, received damaged

Fluke 742A-10 7170003 Value Unit
Nominal value 10 Ω
Operation temperature range +18 … +28 °C
Maximum voltage 1 V
Maximum current 0.1 A

This resistor unit was acquired damaged and here we have rare chance to look inside during and see repair steps. All four low thermal binding posts were replaced with new, but bases were cleaned up and reused. No alterations to resistor element were performed.

10 Ohm standard useful to verify current sources on 10mA range and DMMs which often have lowest range 10 Ω.

Damage on this unit was severe enough that current LO black 5-way low-thermal binding post was bent and half of the plastic cap missing. There is no alternative here but to replace whole post.

Label with factory calibrated value is gone, but warranty cal-sticker is intact :).

Temperature coefficient is not the best in the world, but pretty good for low-resistance standard.

Another look on damages to post. Time to disassemble the standard to gain internal access for posts replacements.

Fluke chassis is bit tricky and required careful peeling off front face mylar and “Fluke” logo label. There are four anodized black allen socket (3/32” imperial size) bolts holding front panel to the extruded case.

After getting bolts loose whole resistor block just slides out. To prevent mylar sticking back to the front panel small pieces of label paper was inserted on corners. Heating up corners to +45…50 °C helps softening adhesive that holds mylar.

Now we can see pretty hermetic wirewound resistors mounted between two tinned copper plates with predrilled holes. This 10 Ω standard built using next resistors:

  • 5 pcs Fluke 809236 100.1 ±0.02% +1.2 ppm/°C TC resistors
  • 5 pcs Fluke 809235 100.1 ±0.02% with opposite -1.3 ppm/°C TC resistors
  • 1 pcs Fluke 9989 Ω ±0.02% small resistor to provide fine-tune value

By simple math 10 resistors with 100.1 Ω nominal connected in parallel provide primary resistance 10.01 Ω. Using half resistor with positive temperature coefficient and other half with negative greatly reduce overall temperature coefficient of the assembly. To further improve accuracy small 9989 Ω nominal trim resistor connected in parallel to main resistor group drops total resistance to 9.9999790 Ω. Wide and thick copper bars may account for some microohms to get very close to ideal desired 10.000000 Ω.

Binding posts are same ones used in Fluke 5725A and similar Fluke calibration instruments. Rods with rotating knob are available to purchase as a spare part and also same to ones used in Fluke 5700A series calibrators. We had some spare posts from 5700A repair projects, so will replace here all four posts with brand new parts.

Bases with copper insert cleaned up nicely and installed back in unit.

It was interesting to note some sort of adhesive/glue applied on all nuts, perhaps some kind of loctite. It made disassembly somewhat tricky and required use of the torque wrench to get nuts loose.

Wires are soldered close to middle point of both copper bus bars, one pair of wires each side current and sense terminals. Wires were terminated with small ring lug.

Replacing connectors was easy after this step and here are few photos of happy Fluke 742A-10 resistor with some friends.

To obtain preliminary value of this resistance standard after repair and new post installation next equipment was used:

  • Keysight 34420A, calibrated 13.SEP.2020 against xDevs standards and Fluke 5720A/03
  • Fluke 5720A as high-stability +10.00005 and -10.00005 mA DC current source
  • Fluke 742A-1 reference resistor, calibrated as 1.00004374 Ω on October 27, 2020 with uncertainty ±0.46 ppm by IET Labs.

Reference resistor was used to adjust Fluke 5720A output current to precisely 10.00000 mA in both polarity.

After doing few reversals of 10mA current initial value was obtained:

Better measurement was performed during comparison with calibrated Ohm Labs 200 1 Ω standard.

  • Ohm Labs 200 0.9999985 Ω , calibrated 1.MAR.2022 by Ohm Labs with U = ±0.3 ppm, using MIL 6242D bridge and their Ohm Labs 200 1 Ω standard.
  • Comparisons at xDevs performed by MIL 6010B DCC bridge at 10mA (F742A-10) and 100mA (1 Ω standard). Reversal time was set at 12 seconds.
  • Both standards were placed in two separate active air baths .

Measurements of TCR with a two slow 3-day sweeps, one from +19 to +27 °C and second from +23 to +27 °C. Current result for α = +0.086 ppm/°C and β = -0.030 ppm/°C2. This is pretty close to factory sticker from 25 years ago.

Calibrated value at +23.0 °C on 16.MAR.2022 equals 10.0001170 Ω ±0.35 ppm, with test current 10.0 mA.

Both 742A-1 and 742A-10 kit boxed together in transport case:

Live temperature corrected standard resistance calculator

The temperature correction chart in the lid of the each 742A unit is helpful to correct the resistance of the standard resistor for different ambient temperature effects. We added interactive real-time calculator for this article to aid with the calculation. Just fill in blue boxes from your 742A label information and enjoy the calculated value of expected resistance in green box. Formula used for 1 Ω example shown below:

RSTD = 1 Ω + (( (α × ΔTEMP) + (β × ΔTEMP2) + RSTD_DEV) × 1 Ω / 1-6) Ω

This realtime calculator accepts resistor parameters from label or calibration certificate to provide temperature-corrected output resistance at arbitrary environment temperature. Just enter α, β, deviation from nominal and desired actual temperature:

Nominal resistance +23 °C Ω
Alpha α +23 °C ppm/°C
Beta β +23 °C ppm/(°C)2
RSTD deviation +23 °C ppm
Temperature to recalculate at °C

Standard Resistance output : Ω, ppm from ideal Ω

This resistance value may be used as given at +23°C, if the change in resistance for the temperature range to be encountered is acceptable. For example, temperature variations less than ±2 °C from a nominal +23 °C would result in a worst case resistance deviation less than -0.3 ppm. If this is an acceptable, then no temperature correction is required.

Here is also Python application to calculate resistance relations if you prefer to play with numbers more, based off ESI SR104 article .

Fluke 742A-1, 1 Ω model, S/N 7681001

Unit date of manufacture is unknown since no original factory sticker is present. Since it is low ohmic resistance model, there are kelvin 4-wire port on the front and grounding post.

Fluke 742A-1 7681001 Value Unit
Nominal value 1 Ω
Operation temperature range +18 … +28 °C
Maximum voltage 0.5 V
Maximum current 0.5 A


Image : Photos of the this 742A-1 unit

According to previous lab calibration sticker value of this standard was 1.000000 Ω measured with uncertainty ±4.9 ppm at +23.0 °C with 100mA test current. This calibration was done on 2003/10/24 if we take typical 1 year calibration period (sticker only has due date and cryptic date calendar label from August 2001).

This resistor was recalibrated against freshly calibrated Ohm Labs 200 standard on 1.APR.2022, with new assigned value 1.00000861 Ω with uncertainty ±0.4 ppm. Measurement was performed at 100 mA current using MIL 6010B DCC bridge. Both reference standard and test 742A-1 were fixed at +23.00 °C temperature in air bath chamber after initial warmup around 15 hours.

Fluke 742A-10k, 10000 Ω model, S/N 5820007, received damaged

Fluke 742A-10k 5820007 Value Unit
Nominal value 10000 Ω
Operation temperature range +18 … +28 °C
Maximum voltage 30 V
Maximum current 0.003 A

This resistance standard was acquired with heavy front damage on binding posts.

Unit seen better days with completely destroyed sense terminals due to excessive torque applied to posts. Hopefully damage is only external and resistance elements still in good shape inside.

Disassembly of 742A is bit tricky due to hidden bolts that hold front and rear panels to square body metal chassis.

This 10 kΩ unit built using four hermetic Fluke wirewound resistance elements, P/N 806224. Each of these have equal nominal resistance 39992 Ω trimmed to ±50 ppm precision.

Three elements are paralleled directly with copper shorting bars, while 4th connected with 31.759 Ω series mica wirewound resistor. With simple math we can conclude final resistance value should nominally equal 9999.9844 Ω which is pretty close to 10 kΩ nominal.

Selected elements are also not just random values, but actually matched for opposite TCR. There are +0.7 ppm/°C, +0.2 ppm/°C, -0.2 ppm/°C and trimmed -0.7 ppm/°C elements used here to give factory zero TCR output.

Overall repair was very similar to process shown above with Fluke 742A-10 box.

We will reuse original base inserts with copper bases and replace only binding post part with brand new parts.

Shortly after repair this standard was verified on APR.3.2022 for operation by direct measurement using Keysight 3458A DMM, which resulted for first value 10000.0700 Ω at nominal +23 °C with uncertainty ±3.6 ppm. There was 6 hour soak time at +23.0 °C and +30 °C points to verify thermal delay behavior of resistor under test.

Standard with new binding posts was cleaned up, free now of any rogue labels and now ready for our performance benchmarks.

This resistor was recalibrated first against calibrated transfer standard ESI SR104 on 9.APR.2022, with new assigned value 10000.08649 Ω with uncertainty ±0.38 ppm. Measurement was performed at 500 µA current using MIL 6000A resistance bridge with 12 seconds settling time per 15 readings and battery powered Fluke 732B 10V DC source. Detector in this measurement was again 8½-digit Keysight 3458A voltmeter.

Both reference standard and test 742A-10k were fixed at +23.00 °C temperature in air bath chamber after initial warmup around 24 hours.

More details about DIY air bath available in this dedicated article . Accurate temperature of Fluke 742A under test was obtained from Honeywell HEL-705 Pt1000 sensor placed directly in chassis 742As’ grounding post. Fluke 1529 Chub-E4 used to collect temperature data.

Temperature coefficient was also remeasured against same reference standard with MIL 6010B DCC bridge. For this test two air bath chambers were used, one at fixed +23.00 °C temperature with transfer Guildline 9330 1000.03470 Ω standard. Temperature in second air bath with DUT Fluke 742A was slowly changed from +17.7 °C to +30.3 °C over duration of 68 hours. Ramp up and ramp down temperatures were equal in duration, with ΔT speed 0.0062 °C/minute.

Resistance measurements were collected during temperature sweep, resulting in RAW data, shown in time-scale below. RAW-file available by clicking on plot.

After some basic math correlation of Fluke 742A-10k measured resistance to temperature was established. Temperature coefficient of this resistance standard now established at α = +0.006 ppm/°C ; β = -0.024 ppm/°C2 and zero TCR intersection point at +23.1 °C.

Manufacture date of this resistance standard is June 1993, which together with the calibration sticker from Canadian Hydro-Québec Research Institute with 2011 calibration allows us to give educated guesstimate of annual drift for this unit.

Daily DriftRSTD = Current value +8.04 ppm / Time between current calibration April 2022 and manufacture date June 1993 (10600 days) = +8.04 / 10600 = +0.00758 ppm/day

By running simple math we can estimate annual linear drift +0.00758 ppm/day * 365 days = +0.277 ppm/year. This is well inside ±8 ppm/year specified in Fluke instruction manual for Model 742A standards. Also running same math from ITEQ calibration sticker dated June 3, 2011 we can establish even smaller figure : +1.84 ppm / 3963 days = +0.00464 ppm/day = +0.169 ppm/year.

Model and serial Calibration date Assigned value Uncertainty Calculated drift Points in calculation
742A-10k 5820007 JUN.1993 10000.0060 Ω ± 1.00 ppm
742A-10k 5820007 3.JUN.2011 10000.0680 Ω ± 0.75 ppm
742A-10k 5820007 14.APR.2022 10000.0865 Ω ± 0.38 ppm
742A-10k 5820007 9.MAY.2022 10000.0872 Ω ± 0.35 ppm 115

We plan to use this 10 kΩ as a travel standard between xDevs members for resistance calibrations and various studies. It is small, easy to ship and have decent performance to meet most of practical metrology demands.

Fluke 742A-1, 1 Ω model, S/N 5825005

Fluke 742A-1 5825005 Value Unit
Nominal value 1 Ω
Operation temperature range +18 … +28 °C
Maximum voltage 0.5 V
Maximum current 0.5 A

This resistor was recalibrated first against freshly calibrated Ohm Labs 200 standard on 2.APR.2022, with new assigned value 1.00003507 Ω ([1] , [2] ) with uncertainty ±0.4 ppm. Measurement was performed at 100 mA current using MIL 6010B DCC bridge. Both reference standard and test 742A-1 were fixed at +23.00 °C temperature after initial warmup around 24hours.

Same setup as in Fluke 742-10k case used to remeasure TCR as well. Two air bath chambers were used, one at fixed +23.00 °C temperature with reference Ohm Labs 200 0.9999985 Ω standard. Omega RTDCAP 3-wire 100 Ω temperature sensor was placed in Ohm Labs 200 thermometer well for accurate temperature data for correlation math.

Temperature in second air bath with DUT Fluke 742A was slowly changed from +17.5 °C to +30.3 °C over duration of 118 hours. Ramp up and ramp down temperatures were equal in duration, with ΔT speed 0.0056 °C/minute. Fluke 1529 Chub-E4 used to collect temperature data.

Resistance measurements were collected during temperature sweep, resulting in RAW data, shown in time-scale below. RAW-file available by clicking on plot.

After some basic math correlation of 742A-1 measured resistance to temperature was established. Temperature coefficient of this resistance standard now established at α = -0.109 ppm/°C ; β = -0.040 ppm/°C2 and zero TCR intersection point at +21.6 °C.

Estimated manufacture date of this resistor is June 1993, which together with the calibration sticker from Canadian Hydro-Québec Research Institute with 2011 calibration allows us to give educated guesstimate of annual drift for this particular Fluke 742A-1 standard.

Daily DriftRSTD = Current value +35.07 ppm / Time between current calibration April 2022 and manufacture date June 1993 (10600 days) = +35.07 / 10600 = +0.00331 ppm/day

By running simple math we can estimate annual linear drift +0.00331 ppm/day * 365 days = +1.20 ppm/year. This is well inside ±8 ppm/year specified in Fluke instruction manual for Model 742A standards. Also running same math from ITEQ calibration sticker dated June 3, 2011 we can establish similar figures : +12.37 ppm / 3963 days = +0.00312 ppm/day = +1.14 ppm/year. This gives some additional confidence that resistor survived these years well enough to keep that stability to present date.

Now this resistance standard is ready to keep precious 1 Ω and current ppms for xDevs.com lab, together with ovenized Fluke SL935.

Long-term drift history of Fluke 742A standards

Model and serial Age Last measured value Calculated drift Points in calculation
742A-1 5825005 29 years 1.00003507 Ω ±0.40 ppm +1.20 ppm/year (+1.14 since previous calibration in 2011) Jun 1993, Jun 2011, Apr 2022
742A-1 6270023 22 years 1.00004464 Ω ±0.33 ppm +1.40 ppm/year (+0.65 since previous calibration in 2020) Nov 1999, Oct 2020, Mar 2022
742A-10k 5820007 29 years 10000.0865 Ω ±0.38 ppm +0.28 ppm/year (+0.17 since previous calibration in 2011) Jun 1993, Jun 2011, Apr 2022

Measuring self-heating effect of Fluke 742A 1 Ω resistors and fixed-point TCR curves

Standard resistors can be calibrated at different currents. The main thesis of this section is that measurement current is heating the internal resistance element of the resistor and that heat, along with measurement time, is the primary cause for difference in calibration values at different currents. This is especially critical for high-power resistance standards and low-ohmic shunts, but also affecting even “normal” 1 Ω – 1000 Ω decade value resistance standards used to calibrate long-scale DMMs, calibrators, current and voltage sources.

During experiment, we establish TCR curves for two Fluke 742A-1 resistors, including theoretical 0 mW power curve.

Used Equipment:

DataProof scanner used to connect either resistor A or resistor B to RS terminals of the DCC bridge. Resistors are placed in TEC1 chamber at programmatically controlled temperatures (+19 °C to +27 °C). Resistor C is connected to RX terminals of the bridge. Resistor C resides in TEC2 chamber at constant +23.2 °C.

Theory

TCR of standard resistors are defined as quadratic function with α and β coefficients. We will call a “HAT” the flattest portion of the TCR curve, and T_HAT and R_HAT would be the temperature and resistance values corresponding to this flattest point.

Our 742A-10 resistor will be used at 1mW and 0.5 mW due to 10:1 RX to RS ratio. This minimizes its self-heating comparing to usual 10mW.

Additionally, it’s T_HAT at point +24.5 °C so it has the flattest section of its TCR between +23 °C and +25 °C and therefore even with some small amount of internal heating its resistance should be very stable.

Nominally Fluke 742A-1 resistors are calibrated at 10 mW power (100 mA). We will conduct all experiments with 10mW power and also half power 5mW (70.7 mA) to derive the effects of power heating.

Due to it’s internal construction Fluke 742A-1 resistors have significant thermal isolation between their enclosure and internal resistive elements. Due to this design, it takes a very long time for the temperature difference to sink in or self-heating to equalize and stabilize making precise experiments with Fluke 742A relatively time consuming. At the same time this thermal isolation also helps to keep resistance stable, even with small fluctuations in ambient temperature during normal use.

After settling TEC1 air-bath to set temperature and letting resistors to soak for about 10 hours, we will measure resistors interchangeably Res A for 10 hours, Res B for 10 hours, Res A for 10 hours and again Res B for 10 hours. Since we are constantly measuring against RX, the resistor C will always be at constant current i.e. constant internal temperature and also constant external temperature of TEC2 (+23.2 °C).

In each cycle we will observe how resistors A and B go from being cold i.e. internal temperature being equal to chamber TEC1 temperature, to being measured and heated by the measurement current. While one resistor is being measured the other resistor is cooling down back to chamber temperature. Repeatability of measurements using transfer 10 Ω standard was confirmed by multiple measurements with same settings.

This process allows us to see how many hours it takes for a measurement to reach its terminal value, as well as values within ±0.02 ppm and ±0.05 ppm of the final value.

There experiments are done at +19 °C, +21 °C, +23 °C, +25 °C and +27 °C temperatures. While just 3 temperature points are sufficient to produce TCR α and β coefficients, recording 4 or 5 points helps with additional data cross checking and confidence.

After running all measurements at 100 mA test current we will repeat them at 70.7 mA. For Resistor A recorded values in Ohms are as follows:

TEMP, °C Current 100 mA Current 70.7 mA
+27 °C 1.00004298 Ω 1.00004308 Ω
+25 °C 1.00004387 Ω
+23 °C 1.00004454 Ω 1.00004459 Ω
+21 °C 1.00004492 Ω
+19 °C 1.00004507 Ω 1.00004508 Ω

This data available in graphical and Excel-file format:

For resistance standard B similar data sequence was collected:

TEMP, °C Current 100 mA Current 70.7 mA
+27 °C 1.00000764 Ω 1.00000770 Ω
+23 °C 1.00000870 Ω 1.00000877 Ω
+21 °C 1.00000912 Ω
+19 °C 1.00000958 Ω 1.00000964 Ω

This data available in graphical and Excel-file format:

Earlier initial calibration obtained from MIL 6010B bridge also shown with cyan diamond at fixed +23.0 °C resistor body temperature.

Simple formula calculation from Fluke 742A manual produces new calculated α and β:

Resistance standard Test current, A α coefficient, ppm/°C β coefficient, ppm/°C2
Unit A, S/N 6270023 0.1000 -0.261 -0.032
Unit A, S/N 6270023 0.0707 -0.250 -0.032
Unit A, optimized values 0.1000 -0.261 -0.032
Unit B, S/N 7681001 0.1000 -0.243 -0.006
Unit B, S/N 7681001 0.0707 -0.242 -0.006
Unit B, optimized values 0.1000 -0.240 -0.006

Please note that measured β is identical to manufacturers provided β of -0.032ppm, while both resistance value at +23 °C and α changed over years since resistor was manufactured.

Given α and β R_HAT or the highest RS resistance point regardless of temperature was calculated to be 1.00004508 Ω at 100 mA and 1.00004509 Ω at 70.7 mA or only 0.009 ppm different. (formulas for calculation are in Excel).

This corresponds to difference in resistor C resistance at 1 mW and 0.5 mW and could be considered negligible as it is comparable to the used DCC bridge noise and repeatability. Therefore, we consider resistance C to be constant for this experimentation.

T_HAT was calculated to be +18.93 °C at 100 mA and +19.10 °C at 70.7 mA. Considering that β coefficient are the same for 100 mA and 70.7 mA, the major effect of double the heating comparing to the 70.7 mA resulted in “pushing” the TCR curve sideways by 0.17 °C. In other words, doubling the power from 5 mW to 10 mW resulted in additional heating of 0.17 °C. The zero power curve is produced by “pushing” 5 mW TCR 0.17 °C in opposite direction.

Conclusions for TCR and power effects in Fluke 742A resistors.

100 mA in 742A-1 resistor A produced 0.34 °C of self-heating that should be considered as additional to the ambient temperature of the resistor.

From data analysis at 100 mA of current it takes 742A-1A approximately 6 hours to reach its terminal value. 3 hours are sufficient for 0.02 ppm stability window and 1 hour for 0.05 ppm stability window.

At 70.7 mA the corresponding times were 5 hours, 2.5 hours and 0.5 hours respectively.

When resistor is measured at 100 mA very quickly, its value initially corresponds to theoretical 0 mA TCR curve, after 6 hours of measurement it will be at 100 mA TCR curve and for intermediate times it will be in the middle between these two curves.

Specifically for resistor A used in this article: at +28 °C there might be as much as 0.2 ppm difference in R28 value, depending on measurement current (100 mA or less) and measurement time. At +23 °C, difference diminishes to 0.1 ppm and at +19 °C it is completely negligible at 0.02 ppm.

Specifically for resistor B, 100 mA produced approximately 0.5 °C of heating. β was also identical to manufacturer’s data and R23 and α both drifted.

The resistance value difference between different measurement currents (under 100 mA) and measurement times were 0.13 ppm or less regardless of the temperature (+18 °C to +28 °C).

Database of labels/results from other 742A from public information

Model and serial Factory value Last measured value α β Reference
742A-1 4596007 0.9999995 Ω 0.9999837 Ω ±0.5-6,k=3, 50mA, +20°C, 27.JUL.2018 by SIMT 0.160-6 -0.068-6 bbs.38hot.net
742A-1 4935018 1.0000467 Ω Pressure coefficient -0.17-9/hPa, Current coefficient -0.0131 ppm/10mA -0.08-6 -0.044-6 AFRIMETS.EM-S1
742A-1 5435027 1.0000004 Ω ? 0.069-6 -0.042-6 etoysbox.jp
742A-1 5465010 1.0000011 Ω ? -0.073-6 -0.051-6 eBay listing
742A-1 5715009 1.0000005 Ω ? -0.040-6 -0.043-6 eBay listing
742A-1 5825005 1.0000002 Ω 1.00003507 Ω ±0.4-6,k=2, 100mA, +23.0 °C, 9.APR.2022 by DCC -0.060-6 (-0.109-6 current) -0.037-6 (-0.040-6 current) xDevs unit
742A-1 6270023 1.0000131 Ω 1.00004374 Ω ±0.46-6,k=2, 100mA, +23.0 °C, 27.OCT.2020 by IET -0.02-6 -0.032-6
742A-1 6855020 0.99999680 Ω -0.14-6 -0.029-6 eBay listing
742A-1 6915029 1.00002333 Ω ±0.17-6,k=2, 100mA, +23.0 °C, 12.DEC.2018 by PI -0.087-6 -0.0219-6 eBay listing
742A-1 7681001 0.9999995 Ω Fluke 1R0199 05.FEB.2001 -0.160-6 -0.007-6
742A-1 7791006 0.9999991 Ω Fluke 1R0331 09.MAY.2001 -0.007-6 -0.028-6 eBay listing
742A-1 7792005 NA Ω 0.99999955 – 0.99999968 Ω NIMT & NMIA DCC -0.063-6 -0.013-6 KCDB APMP.EM-S6
742A-1 8140001 1.0000018 Ω 16-Jul-2002, Fluke PSL label -0.015-6 -0.018-6 Fluke 5700-7002 ACAL kit
742A-1 8927002 1.0000000 Ω -0.09-6 -0.036-6 Reps
742A-1 9553013 1 Ω ? 0.119-6 -0.038-6 eBay listing
742A-1.9 5325001 1.899997 Ω ? 0.060-6 -0.047-6 eBay listing
742A-1.9 9558011 1.9 Ω ? 0.054-6 -0.029-6 eBay listing
742A-10 4920006 10.000470 Ω Pressure coefficient -0.178-9/hPa, Current coefficient 0.0027 ppm/1mA -0.02-6 -0.036-6 AFRIMETS.EM-S1
742A-10 5320005 10.000003 Ω Fluke PSL, 1991 0.090-6 -0.062-6 eBay listing
742A-10 7170003 NA Ω 10.000121 Ω 0.110-6 -0.038-6 eBay auction
742A-10 9558012 10 Ω ? -0.037-6 -0.045-6 eBay listing
742A-25 9566001 25 Ω ? 0.106-6 -0.048-6 eBay listing
742A-100 5485007 100.00682 Ω Pressure coefficient -0.185-9/hPa, Current coefficient 0.0013 ppm/0.1mA -0.009-6 -0.044-6 AFRIMETS.EM-S1
742A-100 6560003 100 Ω 100.001125 Ω ±0.95 ppm 0.040-6 -0.044-6 eBay listing
742A-100 7798002 NA Ω 100.000215 – 100.000254 Ω NIMT & NMIA DCC -0.029-6 -0.023-6 KCDB APMP.EM-S6
742A-100 9566002 100 Ω ? 0.003-6 -0.006-6 eBay listing
742A-1k 5505003 1000 Ω 1000.0613 Ω ±0.3 ppm, 11/24/2021, 3.16mA 0.050-6 -0.018-6 MMLab
742A-1k 5335007 1000.0006 Ω ? 0.020-6 -0.020-6 eBay listing
742A-1k 7733002 1000.006 Ω Pressure coefficient -0.185-9/hPa, Current coefficient 0.0012 ppm/0.1mA 0.05-6 -0.022-6 AFRIMETS.EM-S1
742A-1k 9566003 1000 Omega; ? -0.031-6 -0.016-6 eBay listing
742A-10k TS48981 NA Ω 10000.2234 – 10000.2279 Ω NIMT & NMIA DCC -0.08-6 -0.020-6 KCDB APMP.EM-S6
742A-10k 3316003 10000 Ω ? 0.126-6 -0.009-6 eBay listing
742A-10k 4690001 10000.4403 Ω ±0.33-6,k=2, 0.3mA, +23.0 °C, 12.DEC.2018 by PI -0.01-6 -0.034-6 eBay listing
742A-10k 5065038 10000.134 Ω Pressure coefficient 0.148-9/hPa, Current coefficient -0.0647 ppm/0.01mA +0.04-6 -0.020-6 AFRIMETS.EM-S1
742A-10k 5820007 10000.006 Ω ? 0.000-6 -0.024-6 xDevs.com unit
742A-10k 5435022 9999.997 Ω ? 0.050-6 -0.010-6 etoysbox.jp
742A-10k 5565006 9999.996 Ω ? +0.120-6 -0.049-6 EEVBlog forum
742A-10k 5680003 10000.004 Ω ? -0.020-6 -0.027-6 Listing
742A-10k 6350011 10000.004 Ω ? 0.030-6 -0.025-6 bbs.38hot.net
742A-10k 7240002 10.000038 kΩ 0.04-6 0.012-6 eBay listing
742A-10k 7633009 10000.000 Ω Fluke 1R0129 05.FEB.2001 0.000-6 -0.006-6 Listing
742A-10k 7694007 10000.003 Ω ? -0.010-6 -0.006-6 EEVBlog forum
742A-10k 7815006 9999.999 Ω ? 0.000-6 -0.032-6 Reps
742A-10k 7815007 10000.001 Ω Fluke 1R0403 26.APR.2001 0.105-6 0.000-6 Listing
742A-10k 8153001 9999.990 Ω 17-Jul-2002, Fluke PSL label 0.069-6 -0.011-6 Fluke 5700-7002 ACAL kit
742A-10k 9525002 10 kΩ ? 0.039-6 0.004-6 eBay listing
742A-19k 7852001 19.000015 kΩ ? 0.076-6 -0.018-6 eBay listing
742A-19k 9558013 19 kΩ ? 0.033-6 -0.017-6 eBay listing
742A-100k 3316004 100 kΩ ? -0.008-6 0.002-6 eBay listing
742A-100k 8363002 NA kΩ 100.00023 – 100.00034 kΩ NIMT & NMIA DCC 0.134-6 -0.004-6 KCDB APMP.EM-S6
742A-100k 9558014 100 kΩ ? 0.055-6 -0.014-6 eBay listing
742A-1M 3316004 1 MΩ ? 0.049-6 -0.018-6 eBay listing
742A-1M 8363001 NA MΩ 0.9999998 – 1.0000010 MΩ NIMT & NMIA DCC 0.014-6 -0.013-6 KCDB APMP.EM-S6
742A-1M 9558009 1 MΩ ? 0.074-6 0.005-6 eBay listing
742A-10M 3291001 10 MΩ ? 0.257-6 -0.015-6 eBay listing
742A-10M 9558007 10 MΩ ? 0.121-6 -0.036-6 eBay listing
742A-19M 6460005 19.00003 MΩ ? 0.110-6 -0.086-6 eBay listing
742A-19M 9558008 19 MΩ ? 0.365-6 -0.034-6 eBay listing


The author would like to express our thanks to the Measurements International for excellent calibration services on our primary 10000 Ω ESI SR104 standards and to Nikonoid for temperature/power effect testing, analysis and cross-verification of data sets. Also special thanks goes to long-term xDevs.com supporter Todd M. for participation in interlab comparisons for resistance and voltage.

Discussion is very welcome thru comment section or at our own IRC chat server: irc.xdevs.com (standard port 6667, channel: #xDevs.com). Web-interface for access mirrored on this page. If you have information and performance reports of Fluke 742A standards not mentioned or listed in this article, feel free to provide them and we will add them with next article update. We are looking forward to add another 742A units in dataset as it becomes available.

Author: Illya Tsemenko
Published: Sept. 15, 2020, 6:04 a.m.
Modified: May 9, 2022, 1:53 p.m.