Choosing a data logger

We field hundreds of calls and emails weekly addressing the same question: “Which of the data loggers you carry best fits my application?” Whether you are an engineer working out the details of your latest project or buyer filling a requisition, the number of available options when choosing your data logger can be overwhelming. For that reason, the majority of the calls begin with a simple question: “What exactly are you looking to accomplish?”

Consider your short- as well as long-term needs. Are looking to solve a specific problem (short-term) or are you are looking for a “generic” tool to handle current requirements as well as being flexible enough to handle future requirements (long-term).

In general, we try to ask specific questions to help us determine which products are most suitable for the customer including:

• How many inputs are required?
• What type of inputs are needed?
• How often do you need to take a reading?
• How much data needs to stored?
• How will the data logger be used?
• How will you communicate with the data logger?
• What other requirements does the application have?

How many inputs do I need?

Your first consideration is the number of inputs you plan on logging. Since data loggers are available in configurations capable of handling anywhere from one to literally hundreds of inputs, knowing what you need now and possibly in the future will have a significant impact on your choice. dataTaker’s DT80 Series loggers, for example, can be configures to monitor anywhere from a single sensor to over 300-plus inputs. The Grant SQ20xx Series data loggers are available in 8 to 32 channel models. Do you need a logger with external sensors or can you use a logger with a built-in sensor like the TandD RTR-51?

What type of inputs do I need?

Not all data loggers are created equal, so knowing the type of sensors or inputs you plan on using is critical. For instance, if you already have a thermocouple and you need to log temperature readings, the simple or inexpensive temperature data logger that caught your eye might not be suitable if it comes equipped with a dedicated RTD.

If all you need is to record data from 4 individual 4-20mAmp current-loops, a simple 4-channel logger dedicated to process current loops might work fine. A single, 0-5 VDC? Again, a simple dedicated data logger might do the trick. On the other hand, if you are mixing inputs (current-loops, voltages, pulses, etc) you will need a more flexible, sophisticated data logger.

CAS offers a diverse selection of data loggers compatible with most types of signal inputs. While some of these loggers are dedicated to a certain input type, others are user configurable for different types and infinite combinations of signal types. CAS offers data loggers for the following types of signal inputs: AC Voltage, Process Current, Bridge, Strain, Load, Pressure, Dew Point, Event (or State), Frequency, Level, Process Voltage/Current, PH, Relative Humidity, RTD, Shock, Acceleration, Sound, Temperature, Thermocouple, Pulse, Serial and more.

How often do you need to record a reading?

Most data loggers can handle recording at a rate up to about 1Hz (once per second). If you need a faster recording frequency, be aware that as the speed of the data logger increases, the price of the data logger does as well. Make sure that the recording rate you are specifying makes sense. In the case of a K-Type thermocouple, for example, the sensor/sample may take several seconds to change temperature. Recording such a temperature with the data logger at 5Hz would provide redundant or useless data.

How much data needs to be stored?

Depending on the application, you may need to only capture a few minutes worth of data or you may need to be able to store months worth of readings. You can determine the amount of data storage required by multiplying the number of channels by the sample rate and recording duration:

Total Number of Points = Number of Channels X Sample Rate X Recording Duration

Depending on the data logger there may be a limit based on the total amount of internal memory or the logger may offer the option of using external memory such as a USB memory stick to expand the available memory.

It also important to consider what is practical for the application and analysis. We often get users that want to record multiple channels of data at hundreds of Hz. The problem is that this will quickly fill the available memory necessitating more frequent downloads. Even worse, it becomes impractical to analyze the data; with a high speed data logger sampling at 100 Hz you can exceed the maximum number of rows in Excel in just a bit over 10 minutes!

In this case we recommend either:

1. If you are looking for trends, use the statistical capabilities of some of the data loggers to summarize the data over an interval.
2. If you are looking for anomolies use the triggering features of the data loggers to just capture a window around the event.

How will the data logger be used?

We offer data loggers that are designed for fixed installation and data loggers that are intended for more portable applications. When considering how the logger will be used keep in mind:

• How will it be powered? Battery, AC adapter, solar panel
• Will it be used in a lab or does it need to be protected from moisture and dirt?
• Does it need to be completely self contained so that it can be used in the field?

How will you communicate with the data logger?

Ultimately you have to retrieve the data from the data recorder. Will you bring the logger to the computer to download data or would you like to do it remotely? Communciation with the data logger for set-up monitoring and downloading data can be done in many different ways:

• Serial or RS-232 interface
• USB interface
• Ethernet interface
• Wireless including Wi-Fi and proprietary RF links
• Analog telephone (PSTN) modem
• Cellular, CDMA or GPRS/GSM modem
• Satellite modem

What other requirements does this application have?

In addition to basic data collection does the application require other features such as:

• Do you need alarms?
• Does the data logger have to perform real-time calculations on the measured data?
• Do you need a local display?
• Do you need output signals?