HD2156.1 and HD2156.2 are portable instruments with LCD display. 

They measure pH, mV, redox potential (ORP), conductivity, liquid resistivity, total dissolved solids (TDS) and salinity using combined 4-ring and 2-ring conductivity/temperature probes. 

Temperature only is measured by Pt100 or Pt1000 immersion, penetration, contact or air  probes. 

The pH electrode calibration, as well as manual, can be carried out on one, two or three points and the calibration sequence can be chosen from a list of 13 buffers. 

The calibration of the conductivity probe can be performed automatically in one or more of the 147 µS/cm, 1413 µS/cm, 12880 µS/cm or 111800 µS/cm solutions. 

The HD2156.2 instrument is a datalogger. 

It stores up to 20,000 sets of three measurements composed of pH or mV, conductivity or resistivity or TDS or salinity and temperature: these data can be transferred to a PC from the instrument connected via the RS232C or USB 2.0 serial ports. 

The storing interval, printing, and baud rate can be configured by the menu. 

Both models are fitted with an RS232C serial port and can transfer the acquired measurements to a PC or to a portable printer in real time. 

The Max, Min and Avg function calculates the maximum, minimum or average values. 

Other functions include: the relative measurement REL, the AutoHOLD function, and the automatic turning off which can also be excluded. 

The instruments have IP66 protection degree.