Alphatemp Technology Ltd, Network House, 300-302 Cressing Road, Braintree, Essex CM7 3PG

Tel: +44 (0) 1376 344679 Fax: +_44 (0) 1376 348976

Email:  sales@enviromon.co.uk

PC Data Logger Guide

The current range of PC based data loggers from Pico Technology are USB based data loggers, some also have Ethernet connectivity.  These devices do not have any onboard memory and use the PC to store the data, they require a PC connection at all times to drive the logger and collect the data.  They are typically used for short to medium term logging, ranging from seconds to days.

The PC based data loggers use the PicoLog data logging software, this allows the user to view the data in graphical and table form.  The data can then be imported as a text or csv file and the graph view can also be saved as an image format.

Environmental Data Loggers

Using PicoLog recorder up to 20 devices can be connected to your computer in one instance of the software, this can be  a mixture of devices.

If PicoLog does not cover exactly what you want, the software SDK is also available with some programming examples to get you started.  With the SDK the maximim number of devices increases to 64 data loggers.

Understanding the Sepcification

When looking at the data logger specification table a number of important parameters come up:

  • Conversion Time
  • Sample rate
  • Resolution
  • Accuracy

To understand these parameters it is useful to know how a data logger works, essentially a data logger can be divided into 3 blocks, the front end, the digital side and the back end.

The front end of a data logger measures the real world signal from your sensor or signal, this converts it into a digital reading, the digital side of the data logger then keeps on reading (sampling)  the front end circuitry at regular intervals to see what the latest value is and the back end transfers it to your computer ready to be displayed on the screen.

Conversion Time:  This is the time that the front end of the data loggers (ADC) converts the analogue signal into a digital signal and is often measured in ms.

Sample Rate:  After the front end of the data logger has converted the data to a digital reading this is then sampled by the digital side circuitry of the data logger and is measured in samples per second (ks/S).

Resolution:  This is the smallest change in voltage (or whatever you are measuring) the data logger can resolve, this is a repeatable value.

Accuracy:  This is the degree of closeness that a measured values is to the actual value, this is a variable value.

Conversion Time Vs Sample Rate

Conversion time and sample rate are two different parameters but it is important to understand the relationship between them.  Some of the Pico PC data loggers give conversion time as a parameter whilst others don’t.  As a general rule slower devices will give conversion time in the specification, whilst the faster devices will give sample rate.

For sampling rate devices fall into three categories:

  • Real Time Continuous: Sampling and timing is dictated by computer – can sample for extended periods. Sampling rates can be up to 1kS/s however this is not guaranteed as this will vary form computer to computer and the resources available.
  • Continuous Streaming: Sampling and timing is dictated by devices that support this feature – can sample for extended periods. Sampling rates are guaranteed to be up to 1kS/s in PicoLog recorder software and higher if you wrote your own software.
  • Block Mode:  Sampling and timing is dictated by device, in this mode either memory is used to achieve very high sampling rates over much shorter periods.  Sampling rates can be in the MS/s when using PicoLog recorder, PicoScope (Oscilloscope Software) or writing your own software.

Most of the Pico PC data loggers have their sample rate set by set by the computer, i.e. Real Time Continuous, whilst the data can be read from the computer quickly the data logger cannot convert the analogue signal quick enough.  In cases like this the conversion time is specified.  On other devices the data loggers themselves have their own timing and can sample at higher rates and convert the analogues signal faster in cases like this conversion time is not defined and sample rate is specified.

#With the ADC20/24 the resolution can be adjusted to give different conversion times, so a lower resolution will give rise to a faster conversion time.

*The conversion time per channel means that with multiple channels the overall time will increase with more channels, so for a 3 channel device to see a change in readings it will take  an overall time of  nearly 1.5 seconds.

**For the USB TC-08 in voltage mode the above statement reads true, so for 8 channels voltage measurement the overall time to see a change in reading would be approximately 800ms.  However when measuring 8 thermocouples it will be slightly longer as the way a thermocouple data logger works is that it takes another reading form an internal temperature sensor which effectively makes it 9 channels, approximately 900ms between readings (closer to 1 second with overhead).

Resolution Vs Accuracy

Resolution and Accuracy are often confused however they are two very different parameters.

As described earlier resolution is the smallest change or increment that an instrument can detect, whilst accuracy is the degree of closeness that a measured  value of an  instrument or sensor is  to the actual real value.

An example would best describe the difference, if for example a data logger can detect the smallest change of a temperature at 0.1°C (Resolution)  and the temperature probe connected has an accuracy of +/-0.3°C.  If the source that is being measured  is 23.9°C the temperature probe itself has an accuracy of +/-0.3°C so the value that the logger will read will be between 23.6°C and 24.2°C.  If the logger reads it at 23.7°C and shortly after the temperature increases by 0.1°C i.e. from 23.9°C to  24.0°C , as the logger previously read the 23.7°C it will now read it at 23.8°C.

This is assuming the accuracy of the temperature remains constant, however the accuracy can vary within the range specified.  The example is a basic one but it gives an idea about the differences, in the example above the accuracy of the temperature probe was specified, however the data logger itself will also have an accuracy as well.