Monitoring Thermal Dispersion of Buried Cables

Universal 8-Channel Datalogger Logs Several Values


Recently CAS DataLoggers configured a continual monitoring system to record thermal dispersion of medium-voltage buried  cables using an OMNIAlog data logger from Next Industries. This system has been configured to log temperatures, soil moisture,  and current at a 5- foot depth.

Multi-Value Monitoring:

The customer needed to measure multiple values including:

  • Temperature at 4 points, from  -10°C  to  +100°C (14°F to 212°F)

  • Soil  moisture at 1 point

  • Current of the buried, medium- voltage, unsectioned cable  (0  to  500A)

Standalone Datalogger:

CAS DataLoggers provided the customer with an NI-480 OmniaLog 8-Channel Datalogger from Next Industries. This datalogger  allows connecting with up to 8 sensors of any type with an analog output (voltage or current), Thermistors, RTDs, Thermocouples, and RS485 signals.

No separate software is needed since the datalogger is supplied with an internal webserver. The OmniaLog is fully-configurable  by simply connecting a PC via Ethernet cable and using a browser. The OmniaLog is a high-accuracy datalogger which can log for  months in standalone mode (unattented), powered from lead batteries. In fact, its sensors are activated and powered only for the minimal time it takes to perform measurements. Alternately, the sensors can be preloaded with a 'warm up' option from the logger's interface.

Temperature  sensor:

A  Type-T thermocouple (Constantan copper, slightly more accurate than Type K ones) was designed just for this application. This sensor is mineral-insulated and metal-jacketed, being highly resistant to earth corrosion. It is supplied with a 10-foot cable junction. This sensor is read without any special configuration by the OMNIAlog. The wiring scheme to the logger's terminals is  directly supplied from the GUI (Graphical  Interface).

Current sensor:

The maximum cross-section of the cable's conductors is 240mm2. The customer specified that the cables are medium voltage  and that a single-phase reading is enough for their purposes. For this purpose, it was necessary to install a toroidal openable  TA with a 10cm diameter. The chosen TA is made by IME and is combined with a self-powered transmitter with a 4-20 mA output.

The user simply configured the sensor type as 4-20  mA Signal  on  the OMNIAlog datalogger, providing the conversion constant between the signal and the engineering unit, which is Ampere in this case. In this way, the 4mA received by the transducer are equivalent to 0A current passing through the cable, and the 20mA received by the transducer are equivalent to 500A current  passing through the cable, if the conversion is considered linear. Non-linear transformation curves can also be set up on the OMNIAlog. Then the user sets the engineering unit as Ampere. The wiring scheme to the OMNIAlog terminals is directly supplied  from  the  GUI (Graphical  Interface).

Humidity sensor:

For humidity measurements, the customer only needs a guideline, not a very accurate measurement: in other words, users want  to make a rough comparison such as "Today the soil is more humid than yesterday," or to identify a rainy day.

In order to do so, users could simply use a sensor with 2 electrodes spaced at a fixed distance, measuring the change of resistance between them in different soil conditions. However, users are instead employing a sensor widely used in agriculture, available at a low cost, which uses the principle of electrical resistance. In this case the output signal is a voltage (0V for very wet  to 2.4V for dry). Since the transformation curve is not linear, another device is needed to 'straighten' the signal - making it linear  from 0 to 2.8V.

V signal (voltage) is set as the sensor type on the OMNIAlog and the sensor has to be powered at 5V. Centibar (or kPa) is set  as the engineering unit, which is used in this field to understand thesoil moisture degree. 0V is equivalent to 0 centibar, and 2.8V  are equivalent to 240 centibar. The wiring scheme to the OMNIAlog terminals - including the sensor power supply  - is directly supplied from the GUI (Graphical Interface). In this case it is best to set up a time to warm up (preheat the sensor) of at least 1  minute before data acquisition begins.


Recording time: 6 months

Sampling time: 1 reading collected every 15 minutes

Detection depth: 1.5 meters (nearly 5 feet)

Possibility to power the data logger: No

Data transmission: Internal memory or GSM network


With a single datalogger it is possible to collect:

  • 4 temperature measurements

  • 1 current measurement on a medium voltage cable

  • 1 soil moisture measurement

The OMNIAlog collects data every 15 minutes for the entire 180 day period.  Data is downloaded by simply connecting a laptop  PC via Ethernet cable and querying the datalogger.  In the future, the datalogger will be configured to send data to an online  server, where it is remotely accessible  via  a  GSM  secure  data  transmission  module. For more info on the Next Industries NI-480 OmniaLog 8-Channel Logger, or to find the ideal solution for your application-specific needs, contact a CAS Data Logger Applications Specialist at 800-956-4437 or visit our website at

Contact Information:

CAS DataLoggers, Inc.

8437 Mayfield Rd.

Chesterland, Ohio 44026



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