Temperature is the property that characterizes the thermal state of two systems in relation to the direction of the heat flow that would arise between them.
Regulates the transfer of thermal energy or heat from one system to another. When two systems are in thermal equilibrium, no heat exchange occurs, when there is a temperature difference, the heat will tend to move from the system at higher temperatures to that at a lower temperature, until the thermal equilibrium is reached.
Temperature does not measure the amount of thermal energy of a system, but is connected to it.
The temperature sensors that Riels® Instruments offers are the following:
• Resistance Thermometers
• Temperature transmitters
• Analogue and digital thermostats
• ATEX Thermostats
• Flexible heating cable
• Digital thermoregulators
Resistance thermometers, also called resistive thermal devices (RTDs) are temperature sensors that exploit the change in resistance of some materials with changing temperature. Resistance thermometers are built from materials that meet the following specifications: high temperature field, high electrical resistivity and high temperature coefficient.
The metal materials most commonly used for the manufacture of resistance thermometers are Platinum (Pt) - it is used in nearly all applications, Nickel (Ni) is used occasionally in applications with modest variations in temperature, and, Copper (Cu) is used more rarely. Resistance thermometers are built in two different ways: wire wound - suitable for temperatures up to 300°, wire encapsulated - for temperatures also over 500°.
A resistance thermometer provides optimum measuring precision and a high temperature field (in the case of those in Platinum) but their structure makes them extremely shock and vibration sensitive.
Thermocouples are made up of two metallic conductors in different materials which, under the effect of the temperature, generate an electromotive force.
Thermocouples exploit the principle of thermoelectric effects: when the junctions in a circuit made up of two dissimilar metallic conductors are at different temperatures, a current produced by electromotive force will circulate - the intensity of which is directly proportional to the temperature difference between the two junctions. The materials that formthermocouples have been standardized over time and chosen depending on the performances and the industrial fields of use, on the basis of the following specifications: electromotive force generated, stability and repeatability, operating range, atmosphere, cost.
A thermocouple measures very accurately and has a good response time to loss of a minor repeatability of the measurement.
Temperature transmitters convert the signal originating from electric thermoelements such as resistance thermometers and thermocouples, into a normalized signal in continuous current 4÷20mA or into a voltage signal 0-10V.
The temperature transmitters can be fitted to an electrical panel, connected to the thermoelement by a cable or directly onto the head of the instrument. Its fitting onto the head of the resistance thermometer or thermocoupleinstead of the electrical panel has significant advantages. This type of fitting reduces the sensitivity to the induced disturbances from connection cables, reducing the electromagnetic and/or radiomagnetic interference in the plant. Thetemperature transmitters can be supplied Atex certified intrinsically safe.
The thermostats are comprised of a switch, the on-off action of which is controlled by a temperature change of a sensitive element that is part of the instrument itself. The function of a thermostat may be that of regulating a flow or it acts on a physical component to change the control, or as an electric switch to control an electric device that will more or less control in a varied manner the parts that have an influence on the temperature to be worked out. Thermostatscan function in two different ways: with automatic reset, where an electrical contact is switched over when the temperature descends below the set value, with max. reset, where an electrical contact switches over when the temperature exceeds the set value.
A thermostat is used in process plants, as well as in ventilation, refrigeration and heating systems.
The ATEX thermostats differ from normal thermostats as they have ATEX approval with a higher classification.
ATEX thermostats have very high protection against overpressure; they are also suitable for offshore applications, are shock and vibration resistant and maintain perfect setpoint stability over time.
A datalogger is a battery powered instrument, ideal for recording the temperature over long periods of time.
A datalogger consists of a sealed stainless steel tube that houses the electronics for the sensor and battery; this type ofdatalogger has to be recovered from the measuring point to read the data. There is another datalogger version that is connected to a receiver by a cable thus avoiding the need to recover the instrument.
A datalogger accepts analogue input signals from resistance thermometer and/or thermocouple probes.
Digital thermoregulators can be connected to any type of sensor, to any linear signal and even to non-linear signals.
Digital thermoregulators can perform single and double action regulation, as well as continuous and servomotor regulation. They foresee two methods of initial tuning, i.e. “one shot” and an automatic system which selects the best one on the basis of the process conditions. Digital thermoregulators are fitted with a password-protected customizable display that allows the operator to quickly access the information he needs and to hide it or prevent changes to it.
Digital thermoregulators are also provided with a Modbus protocol to facilitate connection to other peripheral devices.
Mechanical expansion thermometers exploit the changes caused by the temperature on the volume of solid materials, liquids, gas and fumes. As regards dissimilar materials, different function relationships are used: for solids there is elongation of different metallic materials with a dissimilar coefficient of thermal expansion, in liquids it is the raising of a column of liquid or pressure caused by the thermal expansion of the constant volume liquid, while for gases and fumes it is the pressure caused by thermal expansion of the constant volume gas. It is on the basis of these function relationships that thermometers are subdivided into two categories: column thermometers, which use the raising of a column of liquid depending on the changes of the temperature, and dial thermometers, which operate with the expansion of solids or gases and fumes. The manufacture of this latter type of thermometer can be of two types: with a bimetallic strip in expansion thermometers, or with a bulb-capillary in gas expansion thermometers. These thermometers are ideal for hostile environments and are highly accurate - however, they do not provide remote data transmission.
A flexible heating cable or self adjusting cable increases or decreases the dissipated power with the rising or lowering of the temperature, independently of the circuit length. As a result the cable has the following features: the energy consumption in relation to the real thermal conditions, reduced planning activity, the possibility to lengthen or shorten it to the desired length at the worksite without the power changing. A flexible heating cable is mainly used in antifreeze and heating plants, and in maintaining the temperature of tanks and piping, as well as in preventing the accumulation of ice and snow on ramps.
The flexible heating cable can also be supplied in the ATEX explosion proof version.