Duran QD255 Ultra DC Millivolt Potentiometer 2mV-400mV Range Benchtop Meter

Duran QD255 Ultra DC Millivolt Potentiometer 2mV-400mV Range Benchtop Meter

The Duran QD255 is a precision ultra DC millivolt potentiometer designed for accurate measurement and standardization of low-level DC voltage signals in the 2 mV to 400 mV range. Potentiometers of this type work on a null-balance principle — an internal reference EMF is adjusted to exactly oppose the unknown voltage, drawing zero current from the source at balance, which eliminates loading error and allows highly accurate readings independent of source resistance. The QD255 is housed in a compact portable steel case intended for both bench and field use.

Specifications & What's Included

• Manufacturer: Duran
• Model: QD255
• Type: Ultra DC Millivolt Potentiometer (null-balance type)
• Measurement range: 2 mV to 400 mV DC
• Front panel: millivolt dial scale (0–40 mV markings visible), SET ZERO galvanometer control, multiple range selector knobs, red push-button (galvanometer key)
• Form factor: benchtop / portable steel carrying case with lid latches and leather carry handle
• Lid-mounted laminated reference sheet: general operating instructions and reference data table (present and legible)
• Included loose paperwork: Omega Engineering Type J Iron-Constantan thermocouple temperature reference table (Temperature in Degrees C, Reference Junction at 0°C) and supplemental instruction sheet

Applications

DC millivolt potentiometers like the QD255 are classic precision laboratory instruments used to accurately measure, calibrate, and standardize low-level DC voltages and thermocouple outputs. Because the null-balance method draws no current from the device under test at the balance point, these instruments are especially valuable for measuring the output of thermocouples, resistance thermometers, and other low-output transducers without introducing measurement errors from source loading. They were widely used in industrial calibration labs, standards laboratories, and research facilities from the mid-twentieth century onward for calibrating temperature instruments, verifying thermocouple accuracy, and providing a traceable voltage reference. The inclusion of an Omega Engineering Type J thermocouple reference table with this unit reflects its typical application in thermocouple calibration work. Vintage potentiometers of this type remain sought after by metrology labs, educational institutions, and collectors of precision analog test equipment.