Input Devices and Sensors
One of the most common properties measured in the HVAC control world is temperature. Human comfort, computer room requirements, and a host of other considerations make temperature measurement necessary to HVAC control strategies.
Types of Temperature Measurement Devices
Several temperature measurement technologies exist for use with DDC control systems. The most common utilize resistance temperature detectors (RTDs) and thermistor based devices.
Resistance Temperature Detectors- RTD
Resistance Temperature Detectors (RTD's) operate on the principle that the electrical resistance of a metal changes predictably and in an essentially linear and repeatable manner with changes in temperature. The resistance of the element at a base temperature is proportional to the length of the element and the inverse of the cross sectional area. RTD's are commonly used in sensing air and liquid temperatures in pipes and ducts, and as room temperature sensors. DDC systems may accept RTD inputs directly, or a transmitter with voltage or current output may be used.
RTDs are typically characterized by their resistance in Ohms () at 0 C and by their temperature coefficient of resistance (commonly know as "alpha"). Alpha is expressed in terms of /( C) and is the slope of the line representing the resistance of the element between 0 C and 100 C. The resistance of a RTD can be expressed mathematically by the following equation (source i):
R(T) = R0 [1 + A(T - T0)]
- R(T) = the resistance at temperature T
- R0 = the resistance at reference temperature T0
- A = temperature coefficient of resistance (alpha)
- T0 = a reference temperature (usually 0 C)
RTDs with R0 resistance from 10 to 2000 are readily available. Currently, the most commonly used RTDs in HVAC applications are sensors with an R0 resistance of 100 , 500 or 1000.
The accuracy of a RTD sensor is typically expressed in percent of nominal resistance at 0 C (R0). RTDs are relatively accurate when compared to other sensing devices and have good stability characteristics. RTDs with accuracies of 0.2% to 0.01% are commonly available.
RTDs are constructed in thin film, thick film, totally supported and "bird-cage" configurations. They can be made from many materials, some of which include platinum, tungsten, silver, copper, nickel, nickel alloys and iron. Currently, the most common RTDs (used in the HVAC field) are constructed in film type configurations with platinum, nickel or nickel iron.
Since the resistance of the sensor is the property being measured, the resistance of all elements of the circuit, including the sensor leads, affects the measurement. With RTD's and particularly those with lower base resistance values, the resistance of long leads can amount to several percent or more of the sensor circuit. This can result in significant error. One option for correcting this problem is to locate a transmitter at the sensor. The other way is to compensate for the lead resistance by the method of wiring.
Three different wiring methods are used, involving two, three and four wires. These are applied based on accuracy requirements for the application. The circuit diagrams in Figure 2.9 show the various methods. Two and three wire configurations commonly use a Wheatstone bridge circuit to create an output voltage that is proportional to the RTD resistance. The two-wire method provides the lowest accuracy, but is adequate for non-critical measurements. The three-wire method provides better accuracy because the lead resistances L1 and L3 cancel when the leads are of identical length. The effect of L2 is small as long as the bridge is balanced or a high impedance voltage measuring technique is used. The four-wire circuit is the most accurate, and uses a constant current source to cancel the effect of unequal length leads. A high-impedance voltage measurement circuit is used so that the current flow in the measurement leads is negligible.
Thermistors are commonly used for sensing air and liquid temperatures in pipes and ducts, and as room temperature sensors. The term "thermistor" evolved from the phrase thermally sensitive resistor. Thermistors are temperature sensitive semiconductors that exhibit a large change in resistance over a relatively small range of temperature. There are two main types of thermistors, positive temperature coefficient (PTC) and negative temperature coefficient (NTC). NTC thermistors are commonly used for temperature measurement.
Unlike RTD's, the temperature-resistance characteristic of a thermistor is non-linear, and cannot be characterized by a single coefficient. Manufacturers commonly provide resistance-temperature data in curves, tables or polynomial expressions. Linearizing the resistance-temperature correlation may be accomplished with analog circuitry, or by the application of mathematics using digital computation.
The following is a mathematical expression for thermistor resistance (source ii):
R(T) = R0 exp[b (1/T - 1/T0)]
- R(T) = the resistance at temperature T, in K
- R0 = the resistance at reference temperature T0, in K
- b = a constant that varies with thermistor composition
- T = a temperature, in K
- T0 = a reference temperature (usually 298.15 K)
Because the lead resistance of most thermistors is very small in comparison to sensor resistance, three and four wire configurations have not evolved. Otherwise, sensing circuits are very similar to RTD's, using the Wheatstone bridge (Figure 2.10).
Other Temperature Input Devices
Other temperature measurement technologies are available for use in DDC control systems. Solid-state sensors are available for space, duct and pipe applications. These sensors provide a milli-volt level voltage signal used in a two-wire configuration, or a micro-amp level current signal used in a three-wire configuration.
Thermocouples are available for space, pipe and duct application. Thermocouples operate on the principle that when two dissimilar metals are joined at both ends and one of the ends is at a different temperature, a voltage that is proportional to the temperature of the junction is produced. This principle requires that the leads be made of the same metals in order to achieve reasonable measurement accuracy. The signal level from a thermocouple is in the milli-volt range such that transmitters are often used to overcome the effect of the leads. Although in widespread laboratory and industrial use, thermocouples are not widely use in commercial HVAC control applications. The American National Standards Institute has standardized thermocouple types. Common types are listed in Table 2.1.
Infrared Temperature Sensors that sense the wavelength of radiation emitted from the surface of an object without being in physical contact with the object are available with voltage or current outputs that are compatible with DDC systems.
Table 2.2 is a comparison of the most common temperature measurement technologies applicable to DDC control systems for HVAC. The comparisons made are general in nature and not intended to be all inclusive for each sensor type.
RTD's, thermocouples, thermistors, and solid-state temperature sensors are all small devices with similar mounting techniques used for all of the types. Sensors for pipe and duct mounting are commonly sheathed in a stainless steel sheath of 1/8 to 1/4" diameter (larger and smaller diameters are available). Wiring may be exposed or contained in various types of enclosures. Sensors for liquid piping systems may be mounted with direct immersion into the fluid or installed in a tubular sheath called a thermowell or well to allow removal without draining the piping system and to reduce the likelihood of leaks. Sensors installed in wells should be installed with a heat transfer compound filling the space between the sensor and the well to insure good thermal contact between the measured fluid and the sensor.
In measuring the temperature of air in large ducts, it is often desirable to use an averaging element because the air temperature can vary significantly over the cross section of the duct. RTD and thermistor sensors have been developed that accomplish this using multiple sensors installed in a single flexible tubular element. The element is typically arranged in a serpentine fashion so as to obtain representative measurements over the entire cross sectional area of the duct. Very large ducts or air handling unit casings ften require multiple sensors that are customarily wired in parallel-series arrangements. Averaging elements are commonly applied downstream of mixing dampers, and following large or multiple heating or cooling coils.
Sensors for outdoor air applications should be located in normally shaded areas to prevent the heating effects of solar radiation. These sensors are usually provided with a shield or hood to reduce the effects if exposed to direct sunlight and prevent direct contact with precipitation.
In adverse or outdoor environments, it is sometimes desirable to enclose sensors in aspirated cabinets to prolong their life and reduce maintenance. Aspirated cabinets typically include a filtered air intake and an exhaust fan to provide positive airflow through the enclosure. Flush mount wall sensors, wire guards or locking guards are also used to protect sensors in areas subject to vandalism.
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