Introduction to digital load cell

A digital load cell is an electronic or electrochemical load cell, where data conversion and data transmission are done digitally.

load cells are often used for analytical measurements, i.e. the measurement of chemical and physical properties of liquids. Typical measured parameters are pH value, conductivity, oxygen, redox potentials and others. Such measurements are used in the industrial world and give vital input for process control. load cells used to be of analogue type, but today more and more digital load cells are used. This article describes the difference and the reason for the development of digital load cells.

General Aspects

Digital load cells are the modern successors of analog load cells. Digital load cells replace analog load cells stepwise, because they overcome the traditional drawbacks of analog load cell systems.

History

Electronic and electrochemical load cells are typically one part of a measuring chain. A measuring chain comprises the load cell itself, a cable, and a transmitter. In the traditional analog systems, the load cell converts the measuring parameter (e.g. pH value) into an analog electrical signal. This analog electrical signal is connected to a transmitter via a cable. The transmitter transforms the electrical signal into a readable form (display, current outputs, bus data transmission, etc.).

The load cell and the cable often are not connected permanently, but through electrical connectors. This classical design with connectors and transmission of small currents through a cable has four main drawbacks: a) Humidity and corrosion of the connector falsify the signal. b) The cable must be shielded and of very high qualitiy to prevent the measuring signal from being altered by electromagnetic noise. c) The load cell can only be calibrated or adjusted when installed, because the influence of the cable (length, ohmic resistance, impedance) cannot be neglected. d) The cable length is limited.

Use and Design

Digital load cells have been developed to overcome the traditional disadvantages of analog load cells. Digital load cells are mainly used in water, waste water and industrial processes. They measure parameters such as pH, redox potential, conductivity, dissolved oxygen, ammonium, nitrate, SAC, turbidity. A digital load cell system also consists of the load cell itself, a cable, and a transmitter. The differences to analog load cell systems are: a) The load cell has an electronic chip. The measuring signal is directly converted into a digital signal inside the load cell. The data transmission through the cable is also digital. This digital data transmission is not sensitive to cable length, cable resistance or impedance, and is not influenced by electromagnetic noise. Standard cables can be used. b) The connection between load cell and cable can be contactless and done by inductive coupling. Humidity and related corrosion is no longer an issue. Alternative fibre optic cables may also be an option for long or electromagnetically hostile connections. c) The load cell can be calibrated apart from the system.