Well known fact that good stable resistor is not cheap thing. Most of common resistors we see in electronics every day are not great for long-term stability and often rated for temperature coefficient ±100 or even ±250 ppm/K. TCR 100 ppm means that every 1°C of temperature change resistance would change 100 ppm or 0.01%. While it’s very little change for most of applications and circuits, for precision work it’s not suitable at all. Let’s imagine you want use resistor to make calibration voltage source for 5½-digit DMM. Even cheap benchtop DMMs can have accuracy about 0.05%, usually much better. This means if we use resistor which may drift 0.01%/°C we will fail the calibration, with temperature change only 5°C.
So for tasks when we need high precision, for example to verify performance or to adjust benchtop DMM one need stable and known resistance elements. One of such devices example is Leeds & Northrup 40**-B series. These are available today on secondary market for very reasonable prices, since primary labs switches to use of much higher performance devices, such as ESI SR104 or even Quantum Hall Effect standard devices.
Resistor we will look at today is 10000 Ω nominal, Model 4040-B manufactured by well respected Leeds & Northrup which is 4-decade resistor. It is designed to be operated in oil bath and had factory accuracy better than ± 50 ppm from nominal at rated temperature +25 °C.
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Exterior and design
Resistance element is submerged in hermetically sealed metal oil can. Copper “horns” are only terminals available externally, with two thumb-nut screws to allow wiring connection.
Back in early years horn ends were amalgamated with pure metallic mercury to provide very low ohmic connection to calibration system, so beware of possible hazardous contamination on terminals of such resistors, especially if buying from former calibration lab. This resistor was acquired on eBay back in 2016, to use as a cheap travel standard. There was no certificate or previous history.
Center cap has a hole for thermometer to allow accurate resistance standard inner volume measurement to account for, and possibly compensate thermal instability when resistor used on temperature different to calibration point.
We didn’t bother to clean resistance body before measurement, to see how good data can we get with lack of care.
Performance and verification
Measurements were performed with Keysight 3458A 8½-digit DMM in 4-wire resistance mode. To mitigate tempco 4040-B standard was placed in peltier active air bath, programmed at +23.00 °C. Connection to DMMs front terminal was done with 4-conductor PTFE 1 meter long shielded AWG18 cable with bare CuTe spade forks. Guard terminal is floating and not connected at the resistor end.
Thermal chamber have mixed air with active Peltier-based heat/cool control loop. Details about DIY chambers like this are presented in this xDevs.com article
DMM configuration used for measurement:
def init_inst(self): # Setup HP 3458A self.inst.clear() self.inst.write("PRESET NORM") self.inst.write("OFORMAT ASCII") self.inst.write("TARM HOLD") self.inst.write("TRIG AUTO") self.inst.write("AZERO ON") self.inst.write("LFILTER ON") self.inst.write("NRDGS 1,AUTO") self.inst.write("MEM OFF") self.inst.write("END ALWAYS") self.inst.write("OHMF 10E3") self.inst.write("OCOMP ON") self.inst.write("NPLC 50") self.inst.write("NDIG 9") self.inst.write("DELAY 1") # 1 second delay to mitigate OCOMP accuracy issue due DA
Then meter was triggered every 5 seconds and readings were stored in DSV-file.
This resistor was measured back in June 4, 2017 using calibrated HP 3458A. All measurements during that period performed at +25.0 °C.
Green line shows measurement samples from 3458A. Clear wobble is apparent due to air-conditioner system cycling, as DMM was not in controlled air-bath environment.
For additional measurement of temperature coefficient DCC bridge Measurements International 6010B was used.
As reference resistance Fluke 5720A calibrator was used, configured to source 4-wire 1000.0210 Ω output. DCC sourced 316 µA current into L&N 4040B under test, while standard 1000.0210 Ω had 10 times larger current. Calibrator resistance output was calibrated few days before this test directly against ESI SR104 primary resistance standard, which in turn has verified stability better than 0.1 ppm per year.
About 4000 points were captured during temperature sweep from +17.4 °C to +30.5°C in chamber with DUT resistor.
This measurement revealed rather high (for a standard) temperature coefficient α = +8.15 ppm/°C and β = -0.57 ppm/°C2. Sweet-spot temperature where TCR curve is most stable is around +30.1 °C.
Same data presented in time scale format:
Resistance measurement at ppm-level uncertainty is very tricky business, and even old standards still can have pretty good results at modest 4½ or 5½-digit level, especially when temperature control/monitoring is added.Discussion about this article and related stuff is welcome in 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.
Modified: Feb. 15, 2022, 8:48 a.m.