Fluke 742A resistance standards


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 resistance standards are required. Example of such unit is famous ESI SR104 or Fluke 742A series resistance boxes. 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.

  • These standards have good temperature stability, reducing need of airbath temperature control
  • Metrology grade +18 °C to +28 °C temperature rating
  • Best 1 year specification is ± 4 ppm
  • 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 I will collect information and benchmark some of the Fluke 742A units, that we have access to. This would represent performance that one can expect from such a device, given typical conditions. Table below shows available models and test result summary. Models for 1 Ω and 10 kΩ are especially popular to support artifact calibrations of Fluke 5700A, 5720A and 5730A calibrators and Keysight 3458A multimeters.

Unit S/N Results
742A-1 6270023 Details
742A-10 7170003 Details

Some additional custom values available from Fluke by special order. Fluke 742A standards are available for MSRP ~$3565 USD per model. There is also Fluke case designed to hold two 742A standards, Model 742A-7002. It’s 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

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 calsticker 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:

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 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 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 8927002 1.0000000 Ω -0.09-6 -0.036-6 Reps
742A-1 7792005 NA Ω 0.99999955 – 0.99999968 Ω NIMT & NMIA DCC -0.063-6 -0.013-6 KCDB APMP.EM-S6
742A-100 7798002 NA Ω 100.000215 – 100.000254 Ω NIMT & NMIA DCC -0.029-6 -0.023-6 KCDB APMP.EM-S6
742A-10k TS48981 NA kΩ 10000.2234 – 10000.2279 Ω NIMT & NMIA DCC -0.08-6 -0.020-6 KCDB APMP.EM-S6
742A-100k 8363002 NA kΩ 100.00023 – 100.00034 kΩ NIMT & NMIA DCC 0.134-6 -0.004-6 KCDB APMP.EM-S6
742A-1M 8363001 NA MΩ 0.9999998 – 1.0000010 MΩ NIMT & NMIA DCC 0.014-6 -0.013-6 KCDB APMP.EM-S6
742A-1.9 5325001 1.899997 Ω ? 0.060-6 -0.047-6 eBay listing
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-1k 5335007 1.0000006 kΩ ? 0.020-6 -0.020-6 eBay listing
742A-10k 7815006 9999.999 Ω ? 0.000-6 -0.032-6 Reps
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 6350011 10000.004 Ω ? 0.030-6 -0.025-6 bbs.38hot.net
742A-10k 7694007 10000.003 Ω ? -0.010-6 -0.006-6 EEVBlog forum
742A-19k 7852001 19.000015 kΩ ? 0.076-6 -0.018-6 eBay listing
742A-19M 6460005 19.00003 MΩ ? 0.110-6 -0.086-6 eBay listing
Author: Illya Tsemenko
Published: Sept. 15, 2020, 6:04 a.m.
Modified: Jan. 5, 2021, 9:46 p.m.