Calibration procedure and results for xDevs/Fluke 792X 10V DC Reference

Intro

This article provide calibration method, with verification procedure for prototype xDevs.com 10V DC Voltage Reference, assembled in standard Fluke 792A Power Pack chassis. Reference has permanent battery backup power with integrated charger, to maintain internal oven temperature and prevent output power cycle hysteresis effect.

Charger/power supply is originally manufactured Fluke 792A Power Pack, only difference is added ultra-stable 10V reference module, mounted in own shielded enclosure, internal to the power pack. Reference is based on proven Linear LTZ1000A super-zener design, build details are shown in this article. Reference was running on constant power since January 2018.

Front panel of Fluke 792A Power Pack was modified to install additional four 5-way low-thermal copper connectors. These connectors provide 4-wire +10 VDC output. Both current and sense terminals are present, and DC reference capable sourcing or sinking currents up to 20 mA, while remote sense helps maintaining output accuracy within 1 ppm or better.

Output definition:

  • RED terminal – FORCE HIGH +10V
  • WHITE terminal – SENSE HIGH +10V
  • BLACK terminal – FORCE LOW / COMMON
  • BLUE terminal – SENSE LOW

*For high-impedance loads, such as 8½-digit DMM or Fluke 5720A calibrator output terminals sense must be shorted with force, using heavy gauge copper bars.
Bars are included with standard when it’s shipped to calibration.*

Specifications

Reference is portable and designed to fit into transport case, such as Pelican 0340 or similar. It contains all necessary hardware and only require AC 110/230 VAC mains power for continuous operation.

xDevs.com/Fluke 792X Specification
DC Output 4-wire with dedicated sense and force terminals
Nominal output +10.00000 VDC
Calibration accuracy ± 2 ppm
Annual stability ±2 ppm, ±1 °C from calibration temperature
Transfer stability ±0.3 ppm, ±0.5 °C for 1 hour
Temperature coefficient less than 0.1 ppm/°C
Calibration temperature +23 ±1 °C
Adjustment range None, fixed output only
Overload protection indefinite with shorted output
Maximum output current ± 30 mA , both source or sink mode
Loading for 1 ppm error 100 µA with sense-force short on output or 20mA with remote sense
Battery type 4 × 6V Panasonic sealed VRLA 4.5 Wh
Battery life at least 72 hours with 792A power output in standby
Dimensions 18 × 22 × 31 cm
Weight 9.1 kg (20 lbs)
Operation temperature +18 °C to +28 °C

Battery operation

For most critical use cases, such as reference calibration or long-scale DMM verification xDevs.com 792X supports fully floating operation with internal battery power. Floating battery operation prevents ground loop currents and ensure lowest possible noise for +10 V output. Battery power is also important enabling factor for calibration against cryocooled Quantum Voltage Standards, such as NIST PJVS.

To maintain battery charge unit has original Fluke 792A power pack charger. Unit provides two modes of operation, exactly like standard Fluke 792A Power Pack. Linear power supply can provide ±11 VDC output at the LEMO port for use with Fluke 792A Transfer unit. While unit is in OPERATE (WHITE LED is ON) – mains power is disconnected by relay from the battery charging circuit. If battery charge level too low, WHITEOPERATELED will be turned off and “LOW BATTRED LED will be activated instead. If this happens, connect AC mains power AND switch unit to STANDBY MODE.

When unit is switched to STANDBY mains power will be connected to charger, and batteries will be charged until full. Low-Dropout Regulators provide stabilized power for the +/-11 V output supply only when unit is in OPERATE. However LTZ1000A-based reference is powered directly from the internal batteries and POWERED ALWAYS, disregarding what is the mode of 792A power output.

Caution! There is no low battery shut-down function in xDevs.com 792X reference. If unit left on battery power or OPERATE mode (WHITE LED ON) for more than 72 hours, batteries will be depleted. There is no automatic switching to charging, even if mains power is connected!

Do not leave standard unpowered after LOW BATT red LED is active. This condition will invalidate +10 V output calibration history and cause catastrophic damage the unit and require battery replacement!

Calibration and traceability reports

This reference was calibrated by Taiwan’s National Measurements Institute in August 2019 . NIST PJVS system was used as a primary traceable reference during the calibration.

Calibration history so far:

xDevs.com 792X Calibration Date Assigned value Uncertainty Shift Shift/year Days
Internal calibration 01/12/2018 10.000002 VDC 2 ppm Reference Reference 0
TW NMI calibration (NIST PJVS Quantum system) 08/08/2019 9.9999838 VDC 0.03 ppm -1.82 ppm -1.16 ppm +573
TMI calibration 03/03/2020 9.9999751 VDC 0.39 ppm -2.69 ppm -1.26 ppm +208

Recommended calibration methods

This section outline output calibration procedure. Procedures use direct comparison between xDevs.com 792X, called later the Unit Under Test (UUT), and a certified and traceably calibrated Fluke 732A/B/C or Datron 4910 standard called Standard Reference (SR) to calibrate the 10V output. There are no means for output adjustments for xDevs.com 792X reference and calibration include measurement comparison only.

Table below shows complete list of the equipment you will need to calibrate the 792X output.

Equipment required Manufacturer / Model number Calibration period
DC Nanovoltmeter Keithley 2182A Within 1 year from last calibration
DC Nanovoltmeter (alternative) Agilent or Keysight 34420A Within 1 year from last calibration
Standard reference Fluke 732A or 732B or 732C +10 V output, Less than 1 ppm uncertainty, 95%
Standard reference (alternative) Datron 4910 or 4911 +10 V output, Less than 1 ppm uncertainty, 95%
Standard reference (alternative) Fluke 7004N or 734A/C +10 V output, Less than 1 ppm uncertainty, 95%
Low thermal EMF cable Fluke 5440A-7002 2 terminal + guard Alternative pure copper spade lug cable is OK
Low thermal EMF cable Fluke 5440A-7002 2 terminal + guard Alternative pure copper spade lug cable is OK

Before proceed, leave xDevs.com 792X UUT and Fluke 732 SR powered on with the BAT switch set to [|] and the standard plugged into AC mains line power for at least 24 hours. Make sure UUT is in STANDBY mode, indicated by left RED LED. This ensure stabilized oven temperature in standards and fully charge the internal battery.

xDevs.com 792X is shipped with two heavy gauge copper bars between SENSE and FORCE terminals. Do not remove the shorts for calibration procedures.

Make sure both standards are located away from any equipment that generate heat or airflow, such as calibrators, DMMs or lab power supplies.

Ensure absence of forced airflow or drafts near both UUT and SR output terminals to avoid excessive noise due to thermal EMF.

Avoid the use of ordinary, nickel-plated, banana plugs for equipment interconnections. These will cause errors that negatively affect calibration accuracy. Use only copper connectors that have small thermal emf’s with respect to copper posts on standards.

Use the Fluke 5440A-7002 Low Thermal EMF Cable Set or alternative low-thermal EMF wiring. Use #24 AWG or larger bare copper Teflon®-insulated connecting wires. It is preferable to use shielded, twisted pair cable. Avoid splices or any adapters between standards. Use low thermal emf copper spade lugs. Crimp the lug onto the wire and solder the connection.

Loosen the top of the binding post, insert the lug, and tighten the binding post on the lug with fingers only.

xDevs.com 792X Calibration procedure – using solid-state reference

Inspect standards, ensure there is no physical damage. Preserve packaging, and let standard sit for at least 24 hours after transport. Mains power should be provided to unit, and “CHARGELED should lit red.

Keep included shorting bar between SENSE and FORCE terminals on 792X standard connected at all times.

1. Disconnect mains power cable from xDevs.com 792X UUT. Switch power button until white “OPERATELED is activated.
2. Connect calibrated Fluke 732 SR and DC Nanovoltmeter as shown in figure below.

3. Monitor the meter readings and allow the thermals to stabilize. 10 minutes is reasonable time frame for this operation.
4. Average the meter readings over a period of at least 1 minute. Record these values. Recommended to take median of 5 last readings.
5. Reverse the HI (RED) and LO (BLACK) (positive and negative) leads on the meter.

6. Monitor the meter readings and allow the thermals to stabilize. 10 minutes is reasonable time frame for this operation.
7. Average the meter readings over a period of at least 1 minute. Record these values. Recommended to take median of 5 last readings.
8. Subtract the readings from step 4 from the readings from step 7 and divide this result by 2. This is the difference EMF between the two devices. The value could be negative.
9. Add the value from step 7 to the value from the certificate for the calibrated Fluke 732 (SR). This is the value of the xDevs.com 792X UUT.
10. Disconnect all test equipment. Connected mains power cable back to xDevs.com 792X and 732 standard. Make sure to switch xDevs.com 792X to CHARGE mode with RED led on.

Calculate expanded uncertainty, using DC Nanovoltmeter results, calibration reports and instruments specifications, as outlined by ISO 17025 and GUM.

Keep xDevs.com 792X standard plugged to mains 110 VAC power AND red CHARGE LED activated until it’s ready to ship.

xDevs.com 792X Calibration procedure – using Quantum JVS

UUT can be calibrated directly against SI Volt realization by JVS system, when operated floating and has output noise and stability similar to industry standard Fluke 732B.

1. Disconnect mains power cable from xDevs.com 792X UUT. Switch power button until white “OPERATELED is activated.
2. Connect JVS array output LO and UUT LO together with low thermal EMF cable.
3. Connect DC Nanovoltmeter as shown in figure below, CH1 between JVS HI and 792X UUT HI.
4. Program JVS output to +10V output. Note meter readings and adjust JVS output for smallest differential on 10mV range.
5. Monitor the meter readings and allow the thermals to stabilize. 10 minutes is reasonable time frame for this operation.
6. Average the meter readings over a period of at least 1 minute. Record these values. Recommended to take median of 5 last readings.
7. Reverse the HI and LO (positive and negative) leads on the meter.
8. Monitor the meter readings and allow the thermals to stabilize. 10 minutes is reasonable time frame for this operation.
9. Average the meter readings over a period of at least 1 minute. Record these values. Recommended to take median of 5 last readings.
10. Subtract the readings from step 6 from the readings from step 9 and divide this result by 2. This is the difference between the two devices. The value could be negative.
11. Correct for thermal EMF differential from JVS system. Calculate final value of the xDevs.com 792X UUT.
12. Disconnect all test equipment outputs. Connected mains power cable to xDevs.com 792X. Make sure to switch xDevs.com 792X to CHARGE mode with RED led on.

Calculate expanded uncertainty, using DC Nanovoltmeter data, calibration reports and instruments specifications, as outlined by ISO 17025 and GUM.

Now calibration of the UUT xDevs.com 792X is complete.

Final assembled reference, ready for use as working standard

Here is finally assembled DC Voltage reference, reusing Fluke 792A Power Pack to provide battery power to the xDevs.com FX 10V module, as well as charge battery. DC reference outputs are full 4-wire kelvin connection with Low Thermal posts, with current source/sink capability over 20 mA. In normal operation SENSE HI and SENSE LO posts should be shorted to CURR HI / LO posts to close the output feedback loop.

Charger / power supply for 792A AC/DC transfer unit is also allowed independently, using original LEMO port and cable.

Back and side of the unit as fully shielded, just like original Fluke 792A, no alterations made on these sides.

This DC reference is constantly powered after assembly on since January 2018.

Calibration results and performance tracking

Here I will do traceable calibrations to other references of similar grade, and estimate annual stability of finished 792X project.

Calibration date Method Measurement result Uncertainty Conditions Accredited
January 2018 Direct 3458A+3458B, K2002 measurement +10.0000134 V ±2.2 ppm Battery power No
March 2018 Direct 3458B +10.0000066 V ±2.3 ppm Battery power No
May 2018 Direct 3458A +9.99998983 V ±2.3 ppm Battery power No
October 2018 Direct 3458B +9.9999885 V ±2.4 ppm Battery power No
December 2018 Direct 3458A, corrected +9.9999862 V ±2.9 ppm Battery power No
March 2019 Direct 3458s,2002 +9.9999793 V ±1.8 ppm AC power No
June 2019 K155 vs F732B SI +9.9999779 V ±0.6 ppm Battery power No
August 2019 NIST PJVS SRI6000 transfer @ CMS ITRI +9.99998382 V ±0.03 ppm Battery power NMI
March 2020 HP34420A vs F732B +9.9999728 V ±0.4 ppm Battery power Yes
October 2020 3458A transfer from 732B +9.9999716 V ±0.44 ppm Battery power No
November 2020 Transfer vs D4910 system +9.9999698 V ±0.3 ppm Battery power No

Same data in graph form:

Based on linear approximation, annual drift of this Fluke/xDevs.com 792X DC reference is -0.44 µV/V.

Author: Illya Tsemenko
Published: Nov. 6, 2019, 5:34 a.m.
Modified: Nov. 25, 2020, 8:28 p.m.

Comments