Basics of Temperature Sensing

There are many ways to sense temperature for use in industrial controls.  There are both discrete (on/off) and analog sensors.  And there are contact and non-contact types.  This article will provide some basic information on some of the available temperature sensors on the market. Temperature Switch/Thermostat: These switches provide a discrete signal of whether the target temperature is above or below a set point.  Two common types are the bi-metallic strip and the liquid-filled.  These switches are mechanical devices that operator a set of electrical contacts for a control signal or PLC input.  They typically have a mechanism to adjust the set point and the dead band (the +/- range of the set point).  Some disadvantages of temperature switches are that they respond slowly and are susceptible to vibration and mechanical shock.  Thermocouple:  A thermocoupleconsists of two dissimilar conductors that contact each other at one or more points.  They will produce a millivolt signal proportional to the actual temperature, and thus are an analog sensor.  Thermocouples are classified in temperature ranges using a letter – for example type J and K. Thermocouples are relatively inexpensive, but will require a special thermocouple input card or a signal conditioner to interface with a PLC.  [See the Undercontrol issue from May 2014 for more information on signal conditioners. ]   RTD:  Resistive Temperature Devices contain a resistor that changes resistance depending on temperature.   These devices are also analog.  There are several types including thin-film, wire-wound and coil element.  They are more accurate than thermocouples and inexpensive.  Thermistor: Thermistors are typically not used directly in industrial controls, but since Amphenol makes thermistors here in St. Marys a brief description is included in this article.  Thermistors differ from RTDs in the material used to make them.  A thermistor is generally made from a ceramic or polymer, while an RTD is made from pure metals. Signal response is different, as RTDs are applied over greater ranges, while thermistors achieve better precision in a limited range.  PTC (positive temperature coefficient) thermistors will increase resistance with increased temperature, while NTC are just the opposite.   Non-Contact: Infrared Temperature Sensors use a lens to focus infrared energy on to a detector where it is converted to an electrical signal.  These sensors are very useful in measuring moving objects and where fast response time is required.  Typical sensor units will generate an analog signal (4-20ma/0-10V) proportional to the actual target temperature.

 Electrical Safety in the Workplace and the Purchase of Used Equipment

NFPA 70E – The Standard for Electrical Safety in the Workplace, has a requirement for labeling electrical enclosures that require access for troubleshooting, maintenance, or other servicing while energized.  Article 130.5(C) lists the requirements for these labels, which includes information to determine proper PPE, nominal system voltage, and arc flash boundary.  The PPE information and the arc flash boundary are determined by the line side characteristics of the enclosure, not by the load side characteristics.  So it is important to remember that used-equipment that still has the Arc Flash labeling attached from the previous location, is likely wrong and misleading to those who service it.  The risks could be less than or greater than stated on the label when installed in the new location.  Along these lines, remember that temporarily installing a stand-by generator to service equipment can also change the required PPE and the arc flash boundary.