DC resistance measurement is one of the functions offered by almost every benchtop multimeter. So why would you need a separate resistance meter?
One of the best digital multimeters available on the market offers a lowest resistance range of 1 Ohm. This is certainly not bad and more than sufficient when developing circuits or testing circuit boards. However, when measuring the electrical resistance of conductive materials or weld contacts, you're working with values that are more in the mOhm or µOhm range. That's why HIOKI 's entry-level benchtop resistance meter model - the RM3544 - has a lowest resistance range as low as 30 mOhm. Its bigger brother, the RM3545, has a lowest resistance range as low as 10 mOhm and a resolution of just 0.01 µOhm.
Another aspect is the measurement time: multimeters are not necessarily optimized for high speed measurements; in contrast, HIOKI resistance meters have been designed to perform resistance measurements even in mass production environments where speed is one of the key factors. For example, in lithium-ion battery manufacturing, the quality of one of each spot weld connection to the busbar is typically tested by measuring the contact resistance. The RM3545 can make such a measurement in the smallest resistance range of 10 mOhm in just 41 ms (FAST mode). In the higher resistance ranges, the measurement time is as low as 2.2 ms.
A very useful feature - which is hard to find even in some high-end multimeters - is the CONTACT CHECK function before the actual measurement: if activated, it will only start the measurement after making sure that the probe has the correct contact with the device under test. This is a feature that is certainly not critical in a lab environment, but is really useful in production where it helps prevent false PASS measurement results caused by insufficient probe contact with the DUT. (Author's Note: CONTACT CHECK is available on all versions of the RM3545, but not on the RM3544/RM3544-01 ).
Let's go back to the example with the battery module busbar welds: The mechanical positioning of the DUT to the test probe or test fixture takes the longest time in the testing process. Once properly seated, you naturally want to perform as many tests as possible at once before having to reposition the DUT. This is where switching units (multiplexers) come in. Especially with a high number of tests to be performed, dedicated multiplexers, which we have written about in more detail here, provide the necessary number of measurement channels. However, if the required number of channels is twenty or less, then such a dedicated multiplexer unit is not necessary. In this case, the RM3545-02 offers two free slots on the rear panel in which Z3003 switching cards can be installed. Each of these cards offers 10 measurement channels for four-wire measurements, and they can be added to that model of resistance meter very easily in a plug-and-play fashion. (Author's Note: Switching cards can only be added to the RM3545-02 model.)
A four-wire measurement usually means matching four connectors on the meter. However, if you look more closely at the RM3544 or RM3545 model, you will see five connectors. This is because the fifth connector is a protection connector, designed to reduce noise, which becomes non-negligible as test currents decrease, i.e. in the high resistance range. Technically speaking, this additional functionality represents the ground shielding of the measuring probe. The described protection is included in HIOKI probes with the designation L2101, L2102, L2103 and L2104. Of course, you can also use probes with RM3544 and RM3545 that do not connect to this protection connector, such as the one marked L2100. Especially if you intend to measure low resistances, then the test currents are high and the effect of noise is small. On the other hand, in the range of resistances on the order of MOhm, the effect of noise will already be significant and the measurement results may be distorted without this shielding method.
Similarly, the Z2001 temperature sensor is not just a nice addition that allows you to measure the resistance as well as the temperature. The main benefit is that the measured resistance value can be corrected by taking into account the temperature coefficient for the conductor material and the actual ambient temperature. This is particularly useful when the measurement cannot be performed in a temperature controlled environment. The Z2001 temperature sensor is a standard accessory for the RM3545 models and an optional accessory for the RM3544.
Another very handy accessory is the LED Comparator Attachment L2105, which indicates the results after comparison with pre-set resistance value limits (in the instrument's memory) using green and red LEDs - eliminating the need to look at the display. In addition, since the LED does not illuminate in the event of a disconnected circuit (i.e. no contact between the probe and the DUT), this small optional accessory can also be used to ensure proper connection to the DUT.
And finally, there is the instrument from the opening picture; after all, if you have an application that requires a low resistance measuring instrument, but at the same time it is not possible to use a benchtop resistance meter, then the HIOKI RM3548 is the ideal solution. It is portable, yet has the smallest resolution of just 1 µOhm and offers the smallest measuring range as low as 3 mOhm. The RM3548 does not have the protective connector described above, as this portable meter is typically used for lower resistances. As mentioned, lower resistances mean that higher measurement currents are predominantly used for measurements, so grounding the probe shield is not necessary. If we compare the benchtop version of the RM3545 with the highest possible range of 1200 MOhm next, we see that the largest resistance range of the RM3548 meter is "only" 3.5 MOhm.
Like the RM3544 and RM3545 desktop models, the RM3548 supports the optional L2105 LED accessory. And like the desktop models, the RM3548 offers a temperature compensation function. Just note that the temperature sensor for the RM3548 is labeled Z2002, so it is a different sensor than the one on the desktop models.
And a final word? Without a doubt, multimeters offer a huge range of features that make them one of the most important testing tools on the lab bench. Yet there are applications where dedicated resistance meters can make a truly significant difference (and benefit) in measurement.
By Kai Scharrmann, HIOKI EUROPE GmbH
