Digital and event data loggers can be used to record digital signals such as switch closures, relays or solenoid actuation or the pulse output of devices like flow meters or encoders. When selecting a digital data logger, there are three important factors to consider: what kind of data are you trying to capture, what is the voltage of the signal and do you need to capture any other data at the same time?
What Kind of Data are you Trying to Capture?
There are three main types of digital recording and each has slightly different requirements for the measurement equipment:
- Digital State Data Logging – For some applications like run-time monitoring, all that is needed is to capture the state (on or off) of a digital signal at regular intervals. For example, you can sample the state of a current switch that is monitoring a motor every second and accumulate the total time that the switch is on each day, week, etc.
- Digital Event Data Logging – A slightly different application is when you want to record the time when a particular event occurs. Rather than logging the state of the input at a regular interval, a timestamp is recorded whenever the state changes to a particular value, i.e. the switch closed at 4:01 on 12/17/2017. This can be more effective for infrequent events by reducing the memory required to store the data.
- Digital Pulse Counting – The third type of digital data logging is recording the number of pulses, typically within a certain interval. A very common application of this is flow meters that provide a pulse output proportional to flow volume. The flow meter manufacturer will provide a specification such as 100 pulses per gallon of flow and the data logger can then measure the number of pulses in an interval and calculate the flow rate – 550 pulses in one minute = 5.5 gallons per minute.
What is the Voltage of the Digital Signal?
There are three common voltage ranges for the input signal for digital data loggers:
Voltage Free – Also known as dry contact, this is when the signal is a simple switch or relay contact closure with no external voltage present. In this case, the data logger must provide some sort of excitation, typically a low voltage. The low current signals across the switch so that it can sense when it closes.
TTL Level Signals – This is a digital signal that is typical of standard digital electronic equipment. The on and off signal levels are specified as part of the TTL Standard, low/off/0 is a voltage less than 0.8 volts and high/on/1 is a voltage above 2.0 volts. Anything in between the voltages is undefined.
Process Level Signals – Commonly found in industrial equipment, process level signals are associated with relays, solenoids, actuators and signaling devices. The most common value for process signals is 24 VDC.
Are There Any Other Signals That Need to be Captured?
For some projects, there are only one or 2 digital signals that need to be recorded at regular intervals or time stamped when they change state. For these, a simple digital data logger like the Lascar EL-USB-5 is adequate. If you need a more flexible system, consider a digital data logger that can be configured to operate in one of several modes (state, event time stamp or pulse countings) such as the Versalog-P and AEMC L452. Universal Input Data Loggers such as the dataTaker DT85, Grant Instruments SQ2020 and Delphin Expert Logger 100 offer digital inputs in addition to analog inputs with the ability to use a digital input as a trigger to sample other channels. These devices are well suited to applications that require logging a mixture of different signal types such as measuring a tank temperature or level and the state of a heater relay or fill valve.
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