Extech UM200 Precision Micro-Ohm Meter, 10 A, 1 uOhm

Extech UM200 precision micro-ohm meter with 10 A test current

The Extech UM200 is a handheld micro-ohm meter for measuring very small resistances with a high test current of up to 10 A. It resolves down to 1 µΩ on its 60 mΩ range, offers six ranges with three sub-ranges in each current range, and can measure both resistive and inductive loads such as transformer windings, motor windings and large busbar joints.

A five-digit backlit display, a comparator with 20 stored limit sets, memory for 3000 measurements, a cable-length function and a PC interface with software and USB cable make the UM200 suitable both for field maintenance and for documented acceptance testing. It is supplied with Kelvin-clip four-wire leads, a rechargeable lithium-polymer battery, an AC adapter and a soft carrying case.

Technical specifications of the UM200

The figures above come from the UM200 datasheet. Ordering variants include UM200-NIST (supplied with a certificate traceable to NIST) and UM200-KTL (replacement Kelvin test leads).

Why a 10 A test current matters on the UM200

At very low resistance, the voltage developed across the part is tiny. At 10 A through 100 µΩ the voltage drop is only 1 mV, which is easy to measure; at 10 mA it would be 1 µV and buried in noise and thermal voltages. By using up to 10 A on the lowest range, the UM200 gives a stable reading on joints and welds whose resistance is in the tens or hundreds of micro-ohms.

Many large machine windings and transformer windings are inductive, so the current takes time to settle. The UM200 is designed to measure inductive material, which means the technician can measure winding resistance without having to wait for an excessive settling time.

Using the comparator, memory and software of the UM200

• Comparator: select one of the 20 stored limit sets so that a joint or contact is flagged against its acceptance limit at once.
• Memory: store up to 3000 readings and recall them on the display, so a full switchgear survey can be saved on site.
• PC interface: transfer the stored readings by USB to the supplied software for reports and trending.
• Cable length: estimate cable length from resistance in feet or meters when the conductor material and cross section are known.

For routine checks that do not need 10 A, the Extech 380462 or 380580 are lower-cost milliohm meters. The UM200 is the choice when low resistance joints and large windings need a documented result.

Selection checklist for the UM200

• Confirm that you need resolution below 0.1 mΩ (the lower-cost meters stop at 0.1 mΩ).
• Confirm that a 10 A test current is acceptable on the equipment under test.
• Decide whether memory, comparator limits and PC reporting are required for your documentation.
• Decide whether the certificate-traceable UM200-NIST version is needed.
• Order spare Kelvin test leads (UM200-KTL) for heavy use.

Operation and care of the UM200

Charge the lithium-polymer battery before going to site, de-energize and isolate the circuit under test, and clamp the Kelvin leads with the voltage clips inside the current clips on clean metal. Use auto range for unknown parts and manual range for repeat tests. Never connect the instrument to a live circuit, and allow transformer windings to discharge before disconnecting the leads.

Keep the clip jaws clean, avoid kinking the leads, and store the meter in its soft case. A regular calibration check against a reference resistor is advisable.

Applications of the UM200 in Oman and the GCC

Power utilities and industrial plants use micro-ohm meters to check bolted busbar joints, breaker main contacts, bonding of earthing grids, cable splices and welded rail or tank joints. In oil and gas and petrochemical facilities the instrument is used on large motor and generator windings and on bonding between pipe flanges. High ambient temperature affects resistance, so temperature should be recorded next to each reading so results can be compared to earlier surveys.

Information to send with an RFQ for the UM200

• Part number UM200 and the required quantity
• Resistance range and the object under test, for example busbar joint, breaker contact or winding
• Whether UM200-NIST traceable calibration or spare Kelvin leads (UM200-KTL) are required
• Delivery location in Oman or the GCC and the required lead time
• Whether calibration documentation or a traceable calibration certificate is required

Documents and quotation

A datasheet for the Extech UM200 is attached on this page. It lists the ranges, test currents and accuracy for the model.

Send us the part number UM200, the quantity and the delivery location in Oman or the GCC, and we will reply with availability and a quotation. Seven Star LLC is an independent supplier in Muscat and is not affiliated with the manufacturer.