Video: PDRM-10A

Video transcript

How the PDRM-10A measures resistances of a few micro-ohms, how its 4 leads go on, and how to use its modes and probes. A joint, a weld or a contact can look perfect and still hide a few extra micro-ohms. Under load current, they turn into heat. Only a four-wire measurement with enough current shows them. It measures from 0.01 micro-ohms to 200 ohms, with a resolution of 0.01 micro-ohms, choosing its own current up to 10 amps. 0.1% accuracy, in 860 grams.

The scale is chosen automatically. The 4 lowest scales use the full 10 amps. Higher up, the current drops to 250 milliamps and then 5 milliamps, so high resistances are not overdriven. On top, 5 sockets: yellow and green carry the current, red and black sense the voltage, and blue is a trigger input. On the front, power, GO for a new reading, MODE, and the backlight. The charger and data port are on the side.

At micro-ohm level, the leads and their contacts can weigh more than the joint. So the current flows through two leads at the ends, and two more, inside them, read the voltage drop. Only what lies between the inner pair is measured. Each lead goes into the socket of its own colour. Yellow and green clip onto the two ends of the object: they carry the pulsed current. Red and black go between them.

The reading then starts by itself. With dual-hand probes, each probe carries a current tip and a voltage tip. All 4 spring tips must bite, and the black dots on the probes must face each other: that keeps the voltage tips on the inside. Hold still until the reading is done. Kelvin clamps carry both leads in one jaw. Clamp them firmly and give them a slight twist to break through the oxide. The resistance is measured between the innermost jaw surfaces.

MODE steps through the measurements: standard, relative to a stored reference, relative in percent, motor mode for small windings, and OK or X against a reference. Held down, MODE opens the settings: mains frequency, language and contrast. Relative mode speeds up a campaign. Measure a good reference in standard mode, press MODE, and each new reading shows the difference from it.

The 9.5 micro-ohm sample gives a quick function check, but it is no calibration shunt. Exothermic welds in earthing grids have their own probe: Four fixed spring tips at a constant spacing, used with one hand. Press until all 4 are engaged, keep still, and compare each weld with a sound one. The accessories match the contact: standard leads with clips, dual-hand probes with replaceable tips, heavy-duty probes for circuit breakers, Kelvin clamps of two sizes,

the weld probe, and a tester for temporary earthing jumpers. Six rechargeable AA cells give more than 12000 readings at 10 amps. The charger works from 120 to 240 volts. Each reading can also be sent once over USB to a PC, for test benches. Next to the one amp PDRM-1A, the PDRM-10A brings 10 times the current and a 100 times finer resolution, down to a 20 micro-ohm scale: the choice for busbars, breaker contacts and heavy joints. Four rules.

Measure only on de-energised equipment, and treat every terminal as live. The instrument and its adapters are not insulating. No flammable gas or explosive areas. And only the approved batteries and charger. It is used for quality control of castings and welds, electrodes in aluminium plants, contacts, switches and fuses in maintenance, and earthing connections in substations. Low resistance measurement, from Amperis.