Ohmmeter PDRM-1A

Fully automatic 1 A digital micro-ohmmeter for resistances from 1 µΩ to 200 Ω

The PDRM-1A measures very low resistances by the four-point Kelvin method, selecting its own range and reaching an accuracy of 0.1% of reading that places it among the most precise instruments in its category.

Advantages

Category-leading accuracy
The instrument resolves 1 µΩ over 4¾ digits with an accuracy of 0.1% of reading ± 2 counts, using three test currents — 1 A, 100 mA and 5 mA — that adjust automatically to the range.

Four points, no contact resistance
Two leads inject the current at the ends of the joint while another two read the voltage drop directly across the section under test, so contact resistance stays out of the measurement; the microprocessor also cancels offset and parasitic voltages.

Stable readings near live busbars
Heavily filtered inputs keep the reading accurate even in the presence of strong magnetic fields, a condition frequently found in substations and switchgear rooms.

Built for field and bench work
At 0.86 kg and 203 × 114 × 51 mm, the sealed, shock-resistant enclosure operates from -20°C to +50°C, and the backlit LCD display can be read in dark locations.

Rechargeable autonomy
Six rechargeable NiMH AA batteries deliver over 4000 measurements at 1 A and more than 10,000 at the lower currents, with automatic shutdown after 2 minutes of inactivity.

Applications

• Checking earth connections and power busbars using the relative measurement mode against a stored reference.
• Verification of welded joints made with high current and of mechanical connections.
• Testing circuit-breaker contacts and power fuses.
• Quality control of castings and electrodes, and non-destructive verification of exothermic welds.

About the product

The differential mode stores the reading from a reference connection — for example, an earth connection known to be in good condition — and compares every new measurement against it, displaying the deviation; this speeds up inspection campaigns across large earthing grids. Low current injection protects applications with small measuring contacts, and the display backlight switches off after 30 seconds to save battery power.

The kit includes the micro-ohmmeter, the Kelvin clamps, the batteries, the manual and the calibration certificate. Available accessories: dual hand probes with replaceable spring-loaded tips, 35 mm and 19 mm Kelvin clamps, a dedicated probe for exothermic welds, a cable-elbow accessory, a 9.5 µΩ reference resistance, and a nylon pouch or rigid transport case.

Technical specifications

Frequently asked questions

Why does the instrument measure at four points?
Because the current and the voltage use separate pairs of leads, so the resistance of the leads and the contacts stays outside the reading circuit and does not distort the result.

Can it be used near energised equipment?
Yes. The filtered inputs keep the reading stable even with strong magnetic fields present, which makes the instrument usable in live substations.

What is the differential mode used for?
It stores a reference measurement and compares every new reading against it, showing the deviation; this is the recommended method for inspecting a series of earth connections.

How much autonomy do the batteries provide?
The six rechargeable NiMH AA batteries exceed 4000 measurements at 1 A and 10,000 measurements at the 100 mA and 5 mA currents.

Is it suitable for exothermic welds?
Yes. With the optional dedicated probe, the instrument verifies exothermic connections non-destructively, a common requirement in substation earthing grids.