Tag Archives: Temperature Sensors

Measuring 16 Temperatures Remotely

In several cases, one cannot access the area where the temperature needs to be monitored. For instance, the temperature inside a kiln may reach a few thousand degrees, which is beyond the tolerance of humans. Environment chambers may need to be completely sealed off when operating, which means monitoring the conditions within has to be remotely accomplished. Simpler cases may also be considered, where the computer logging the temperature is in a central location, whereas the monitored sites are spread out in different rooms.

The ideal instrument should allow measuring temperatures over a local or remote network, with a built-in web-server to access the instrument, requiring neither programming or app. Measurement Computing has just the instrument and it is the WebDaq-316, a stand-alone temperature logger that allows the user to measure temperatures on 16 channels using J, K, T, E, N. B, R, or S type thermocouples. The user can access the instrument through its web-server over a local or remote network.

With 3 GB of internal acquisition memory, the WebDaq-316 acquires samples at the rate of 75 samples/sec. However, if the memory is insufficient for the job on hand, the user can add more by inserting two USB flash drives or an internal SD memory card. The measurement data can be transferred from the SD card, flash drive, or the internal memory. Alternately, the user can download the acquired data from the web-server as well. It is easy to import the data to an analysis software or a spreadsheet as the WebDaq stores data in CSV format.

The user can operate the WebDaq-316 in two modes—normal or high resolution. In the normal mode, the instrument works at 75 samples/sec or 78 Hz maximum, whereas in high-resolution mode, it can scan at less than one sample/sec across all channels. In the high-resolution mode, WebDaq-316 drops its bandwidth to 14.4 Hz, which also lowers its noise and gain error. That allows the 24-bit delta-sigma ADC on the instrument to operate at its peak efficiency.

The web-server has the ability to send SMS texts or e-mail messages. Therefore, the user can receive an appropriate notification whenever a temperature moves out of limits. Additionally, there are four programmable digital IO channels on the WebDaq-316. The user can make use of these IO channels to operate some local activities such as trigger an alarm or shut down equipment. As the IO channels are programmable, they can be inputs or outputs. As inputs, they can act as trigger depending on external signal, and as outputs, they can trigger alarms. The channels are available on terminal strips on the front panel, which makes all T/C and DIO connections easier.

The user can assign measurement operations through jobs and schedules. For instance, the user may want to change jobs or sample rates whenever a temperature crosses the limits, or return to a schedule of lower rate when the temperature returns within the limits. The user may also want to schedule jobs for triggering alarms or receiving notifications of such conditions.

Based on the Raspberry Pi compute module, the WebDaq-316 operates on a DC power source of 6 to 16 V. This allows vehicular operation as well.

Sensing Temperature with NTC Thermistors

Temperature sensors using NTC thermistors are built from sintered semiconducting ceramic material. Such materials contain a mixture comprising several metal oxides. The specialty of these materials is they possess charge particles, which allow current to flow through the thermistor, and display large changes in its resistance value even when the change in temperature is rather small. The manufacturing process allows standard NTC thermistors to operate effectively in the temperature range between 50 and 150°C, with glass-encapsulation type of thermistors going up to 250°C.

Thermistors come in a large variety of sizes and styles. These include glass encapsulated, customizable probe assemblies, surface mount types,, disc types, and chip styles. The large variety is necessary as individual attributes of each style gives them the ability to perform effectively in several different industries, while adapting to various and different application requirements.

For instance, industries using NTC thermistors for measuring temperature include telecommunications, medical, healthcare, military, aerospace, automotive, industrial, HVAC, among many others. On the other hand, applications for NTC thermistors cover time delay, volume control, circuit protection, voltage regulation, temperature control, temperature measurement, temperature compensation, and more.

Now, the availability of precision interchangeable NTC thermistors eliminates the necessity of individual calibration for each thermistor. Capable of accuracies of ±0.05°C, the interchangeable thermistors are now gaining popularity as industry standard. Their standard resistance values usually range from 2.25 kilo-ohm to 100 kilo ohm, with temperature coefficients of -4.4% per °C or -4.7% per °C.

Interchangeable NTC thermistors offer extreme accuracy when sensing temperature. This makes these versatile sensors an excellent choice for use in industries focussing on temperature measurement and control. These industries include, among others, aerospace, HVAC, automotive, industrial, and medical. Applications for interchangeable NTC thermistors include temperature sensing, temperature measurement and control, temperature measurement, and control.

Using interchangeable NTC thermistors in the industry offers several benefits and features. These include beta of 3435 K to 4143 K, RoHS compliance, dissipation constant of 1 mW/°C, fast thermal response times, wide range of ohmic values, a thermal time constant of 7 secs, and fast measurement times. Aging is slow as these NTC thermistors show less than 1% change in resistance even after a span of 10 years. That means, there is no need to recalibrate the system when replacing the thermistor. This certainly reduces operating costs and system downtimes.

Glass encapsulated NTC thermistors are a special breed that are hermetically sealed. This allows the micro-sized sensors to eliminate reading errors from moisture penetration. As they are hermetically sealed, they function effectively in extreme conditions of temperature, pressure, and other severe environmental conditions. The extreme operating conditions allow glass encapsulated NTC thermistors to target markets such as industrial, automotive, medical, and HVAC.

Glass encapsulated NTC thermistors are good for applications involving outdoors such as infrared lighting systems, medical such as those relying on airflow/respirators, industrial such as those including monitoring of terminal temperatures of battery packs while charging, common household appliances such as in coffee makers, ovens, and refrigerators, HVAC such as for temperature measurement and control.

Apart from greater accuracy and faster response times, Glass encapsulated NTC thermistors offer high precision resistance and beta value with a huge operating temperature range.