Definition:

Data-logging can be defined as the capture and storage of data for use at a later time.

How the data is collected:

Sensors are used to input the data into the data-logging equipment. Almost any physical property can be measured with the correct sensor.  The data logger collects the data at regular intervals (the logging interval) over a set length of time (the logging period).

There are two categories of sensors:

• Digital sensors - these are either on or off i.e. a light gate sensing something breaking a light beam. Such sensors can often be connected directly to a computer as the data output is already digital.
• Analogue sensors - these measure some physical quantity by converting it into a voltage. The voltage signal is then converted into digital form by an interface and either stored or transferred directly to a computer.  The vast majority of sensors are of this type.

One advantage of data-logging is that it can be done in places that are hostile for humans such as inside a nuclear reactor or a volcano. Such data logging equipment therefore has to be very robust so that it can resist extremes of temperature or radiation for example.

Another advantage of data-logging is that regular data can be collected over very long time periods or with very short time intervals that would be impossible or impractical for a human operator.

• Example: data-logging equipment can record hundreds of readings in less than a second (imagine asking a human to take 100 pressure readings inside a room as an explosion actually takes place).
• Example: data-logging equipment can take readings at regular intervals, 24 hours a day, for months or even years on end. (Imagine asking a human to measure the temperature of the cooling water flowing into a river from a power station very half hour, 24-hours a day for  years on end!)

Many sensors produce analogue signals which must be converted to digital signals so they can work with computers. An analogue to digital converter changes the varying voltage from the sensor into binary pulses which are the digital equivalent of the voltage value.

How the data is stored:

The data that is logged is usually stored in RAM memory or on some form of backing storage as it is collected.

Some data-logging equipment is designed to be linked directly to a computer (this could be a wireless link). This would be suitable if an experiment is taking place in the laboratory for example.

If you wanted to record data out in the field then battery powered data-logging equipment would be needed that could measure and store the data until the unit is collected. The equipment would then be connected to a computer so the data can be down-loaded. This data collection could still be done out in the field if a portable computer was used to collect the data.

How the data can be displayed:

Once downloaded to a computer, spreadsheet software can use formulas to analyse the data and produce graphs/charts to display it more clearly.

A simple analysis might just be to find the MAX, MIN and AVERAGE values of the data that is downloaded.

Examples of data-logging in use:

1 - In a hospital:

The sort of life-support systems found in intensive care units will make extensive use of data-logging equipment. The patients will have sensors attached to them which will continuously measure data such as temperature, blood oxygen level and heart rate.

Advantages:

• 24 hour monitoring takes place without having to use a human operator who would need breaks, could lose concentration or could get tired and make mistakes.
• Alarms would be set off automatically if any measurements exceed a safe value.
• A continual record can be stored or printed out of everything that is being measured.

2 - In an experiment:

In the video clip below you can see a ball being dropped. It would be impossible for a human to continuously and accurately measure the height of the ball as it bounces. Data logging equipment can record the movement and software can then plot it against time on a computer screen and store it for further analysis.

The video clip shows the movement of a bouncing ball. The movement was recorded with data-logging equipment and displayed on a graph.

Advantages:

• Many accurate reading can be taken per second.
• The data is digital so it can be transferred straight to software which can display it or analyse it further.

3 - Recording over an extended period of time:

In an experiment during the last UK eclipse, light sensors and temperature sensors were used to continuously record the light intensity, the temperature in the sunlight and the temperature in the shade.

The data-logging took place with a logging interval of about 7 minutes and a logging period of four hours.

Spreadsheet software was then used to produce a graph from the data that was collected.

Advantages:

• Many accurate reading can be taken at regular intervals over an extended period of time without risk of 'human-error'.
• The data is digital so it can be transferred straight to software which can display it or analyse it further.

Summary:

Examination Tips:

If you are asked a question about data-logging equipment, be sure to cover the following points:

• If the question describes measuring something then make sure you mention the sensor that would be used.
• If you are asked to explain how data-logging equipment could be used then make sure you mention setting the logging interval and the logging period.
• If you are asked about analysing the data that has been logged then mention downloading the data to a computer and using software such as a spreadsheet to analyse the data (MAX, MIN, AVERAGE etc.) and produce graphs to make the data easier to understand.
• If the question describes a hazardous environment then mention the reduced risk in using data-logging equipment, rather than using a human to do the measuring.  Using data logging the data can be collected safely, either by a cable or wirelessly.

Example questions:

1. Scientists want to take measurements of the temperature inside the crater of an active volcano. Describe how they could use data-logging equipment to do this and what advantages it would offer.
   • Answer: Data-logging equipment could be placed in the crater, connected to various sensors to record things such as temperature and gasses released.  The logging period would be set for a long time such as months or years. The readings would probably not change that quickly so a suitable logging interval might be hourly or daily.  Data-logging equipment would take the readings more safely, accurately and more reliably than a human could.
2. How could a computer be used with the data-logger?
   • Answer: The data could be downloaded to a computer using a wireless link to reduce the risk to humans. Software such as a spreadsheet could then analyse the data using formulas and present it in the form of graphs to make it easier to understand.