QJ57P DC Resistance Measuring Instrument | Accurate & Fast

The Kelvin method uses four separate wires to connect to the object under test. Two wires (current leads) supply a precise current through the object. The other two wires (voltage leads) are placed directly across the measurement points and measure the voltage drop. Since the voltage leads carry virtually no current, the resistance of the leads and the contact points is not included in the voltage measurement. This completely eliminates a major source of error, which is absolutely essential for accurately measuring low resistances (less than 1Ω), where the lead resistance could be larger than the resistance of the sample itself. The QJ57P DC Resistance Measuring Instrument is built around this principle.

The electrical resistance of most conductive materials, especially metals like copper and aluminum, changes significantly with temperature. For copper, resistance changes by about 0.4% for every 1°C change. Without correction, this can lead to massive errors. The QJ57P DC Resistance Measuring Instrument features a dedicated input for an external PT100 temperature probe. When the probe is attached to the sample, the instrument measures the real-time temperature and uses built-in formulas to automatically normalize the resistance reading to a standard reference temperature (e.g., 20°C). This ensures consistent, comparable results regardless of the ambient factory temperature.

A higher test current is beneficial when measuring very low resistances, such as those found in large busbars, transformer windings, or thick cable joints. According to Ohm's Law (V=IR), a higher current (I) produces a larger voltage drop (V) for a given resistance (R). This larger voltage signal is easier for the instrument to measure accurately, improving the signal-to-noise ratio and leading to more stable and repeatable readings. The 10A capability of the QJ57P DC Resistance Measuring Instrument is ideal for these heavy-duty industrial applications.

The instrument's chassis is constructed from CNC-machined, anodized aluminum. This provides several advantages over a plastic or simple sheet metal case:

• Durability: It is extremely robust and resistant to physical damage in industrial environments.
• EMI Shielding: The conductive aluminum enclosure acts as a Faraday cage, shielding the sensitive internal measurement circuits from external electromagnetic interference (EMI) from nearby motors or power lines, which helps maintain measurement integrity.
• Thermal Stability: The aluminum body helps to dissipate heat from internal components, contributing to long-term stability.

To maintain peak accuracy and ensure compliance with quality systems like ISO 9001, we recommend an annual calibration cycle. Each new QJ57P DC Resistance Measuring Instrument is shipped with a calibration certificate valid for one year, traceable to national/international standards. We offer a recalibration service, or it can be performed by any accredited calibration laboratory equipped with the necessary standards.

Yes, absolutely. This is a key feature. Measuring the DC resistance of a highly inductive load like a transformer or motor winding is challenging for standard ohmmeters. The inductance resists the change in current, so it can take a long time for the test current to stabilize. The QJ57P DC Resistance Measuring Instrument has a specialized "inductive measurement mode" that uses a higher voltage to charge the inductor quickly and employs a sophisticated algorithm to detect when the reading is stable, significantly cutting down measurement time from minutes to seconds.

The QJ57P provides the accurate, traceable data needed to comply with a wide range of industry-specific standards, including but not limited to:

• ASTM B193: Standard Test Method for Resistivity of Electrical Conductor Materials.
• IEC 60076-1: Power transformers - Part 1: General (for winding resistance tests).
• IEC 60228: Conductors of insulated cables.
• SAE AS50881: Wiring, Aerospace Vehicle (for bonding & grounding checks).