2HVAC SensorsSensors measure the controlled medium and provide a controller with information concerning changing conditions in an accurate and repeatable manner. The common HVAC variables are temperature, pressure, flow rate and relative humidity.
3HVAC SensorsThe placement of a sensor is critical to achieve good control. In sensing space conditions, the sensing device must not be in the path of direct solar radiation or be located on a surface which would give a false reading such as a poorly insulated external wall.In pipework or ductwork, sensors must be arranged so that the active part of the device is immersed fully in the fluid and that the position senses the average condition.
4Pressure • Temperature Types of InputsDigital InputTwo position, a simple contact closure from a mechanical switch or similar deviceAnalog InputModulating, the signal is proportional and varies with the control variablePressure • TemperatureEvent • Level • Flow
5Types of Analog InputsTypical sensors used in electronic control systems are:Resistance sensors are ‘Resistance Temperature Devices (RTD’s)Voltage sensors could be used for temperature, humidity and pressure. Typical voltage input ranges are 0 to 5 Vdc (Volts direct current), 1 to 11 Vdc, and 0 to 10 Vdc, 2 to 10 Vdc.Current sensors could be used for temperature, humidity, and pressure. The typical current range is 4 to 20 mA (milliamps) or 0 to 20 mA
6Require an external source (voltage or current) of excitation Types of Analog InputsActiveRequire an external source (voltage or current) of excitationPassiveAre two-port devices that directly transform physical energy to electrical energy, generating output signals without the need for an excitation source.ThermistorsA photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation. The common, traditional solar cell used to generate electric solar power is a large area photodiode.Strain GageThermocouplePhotodiodeRTD
7Thermocouples (Passive) Thermocouples have two dissimilar metal wires joined at one end.Temperature differences at the junctions’ causes a voltage, in the mill volts (Seebeck Effect)The output is a voltage proportional to the temperature difference between the junction and the free ends
8ThermocouplesBy holding one junction at a known temperature (reference junction) and measuring the voltage, the temperature at the sensing junction can be deduced.The voltage generated is directly proportional to the temperature difference.
9ThermocouplesAdvantages: Widest operating range, simple, low cost and no external power supply requiredDisadvantages: Non-linear, low stability relative to other types, reference junction temperature compensation required
10Thermocouples – Various Types (Active) Type K = 41µV/˚CSeebeck Coefficient is Nonlinear
11ThermistorsThermistors 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 exhibit the characteristic of falling resistance falling with increasing temperature.
12ThermistorsThey are non-linear, and cannot be characterized by a single coefficient. Manufacturers commonly provide resistance-temperature data in curves, tables or polynomial expressionsLinearizing the resistance-temperature correlation may be accomplished with analog circuitry, or by the application of mathematics using digital computation
13ThermistorsAdvantages: Large resistance change with temperature, rapid response time, good stability, High resistance eliminates difficulties caused by lead resistance, Low cost and interchangeableDisadvantages: Non-linear, limited operating temperature range, may be subjected to inaccuracy due to overheating, current source required
14RTD (Resistive Temperature Device) RTD (Resistance Temperature Detector) is a temperature sensitive resistorIt is a positive temperature coefficient deviceCommon materials used are BALCO wire and PlatinumAdvantagesVery predictableStable output over a long period of timeEasy to recalibrateAccurate reading over relatively narrow temperature range
15BalcoA sensor constructed using a BALCO wire is an annealed resistance alloy with a nominal composition of 70 percent nickel and 30 percent iron.A BALCO 500-ohm resistance element provides a relatively linear resistance variation from –40 to 250°F.The sensor is a low mass device and responds quickly to changes in temperature.When 1000 ohms is measured across the BALCO element, the temperature is approximately 70°F.As the temperature increases, the resistance changes 2.2 ohms per 1°F.
16PlatinumRTD sensors using platinum material exhibit linear response and stable over time.In some applications a short length of wire is used to provide a nominal resistance of 100 ohms.However, with a low resistance value, element self-heating and sensor lead wire resistance can effect the temperature indication. With a small amount of resistance change of the element, additional amplification must be used to increase the signal level.
17Humidity Sensor - Resistive Relative humidity based on a layer of hygroscopic salt, such as lithium chloride or carbon powder deposited between two electrodesBoth materials absorb and release moisture as a function of the relative humidity, causing a change in resistance of the sensorAn electronic controller connected to this sensor detects the changes in resistance which it can use to provide control of relative humidityAn older method that used resistance to determine relativehumidity depended on a layer of hygroscopic salt, such aslithium chloride or carbon powder, deposited between twoelectrodes (Fig. 9). Both materials absorb and release moistureas a function of the relative humidity, causing a change inresistance of the sensor. An electronic controller connected tothis sensor detects the changes in resistance which it can use toprovide control of relative humidity.
18Humidity Sensor - Capacitive Changes in capacitance can be used to determine relative humidityThis sensor measures the capacitance between two conductive plates separated by a moisture sensitive material such as polymer plasticAs the material absorbs water, the capacitance between the plates decreases and the change can be detected by an electronic circuitA method that uses changes in capacitance to determinerelative humidity measures the capacitance between twoconductive plates separated by a moisture sensitive materialsuch as polymer plastic (Fig. 10A). As the material absorbswater, the capacitance between the plates decreases and thechange can be detected by an electronic circuit.
19Pressure SensorsDiverse electrical principles are applied to pressure measurement.Those commonly used include capacitance and variable resistance (piezoelectric and strain gage)
20Pressure Sensors - Resistive A method that measures pressure by detecting changes in resistance using a small flexible diaphragm and a strain gage assemblyThe strain gage assembly includes very fine (serpentine) wire or a thin metallic film deposited on a nonconductive baseThe strain gage assembly is stretched or compressed as the diaphragm flexes with pressure variations.The stretching or compressing of the strain gage changes the length of its fine wire/thin film metal, which changes the total resistance.The resistance can then be detected and amplified.
21Pressure Sensor - Capacitive A fixed plate forms one part of the capacitor assembly and a flexible plate is the other part of the capacitor assemblyAs the diaphragm flexes with pressure variations, the flexible plate of the capacitor assembly moves closer to the fixed plate and changes the capacitance.
23Testing an Analog Input – Thermistor Use an ice bath to test thermistorsSolutions allows a known temperature for the testCrushed ice works bestFill container with as much ice as possibleThen fill with water (distilled water if possible)Let solution stabilize for several minutes to allow ice an water to stabilize at 32˚F
24Testing an Analog Input – Thermocouple There are two steps to checking thermocouples.The first is to check for a short on its terminalsAnd the second, to make sure that voltage tracks with the temperature.
25Testing an Analog Input – Thermocouple Put the meter in ohms or continuity mode; on a good thermocouple, you should see a low resistance reading. If you see more than a few ohms, you probably have a faulty thermocouple
26Testing an Analog Input – Thermocouple The second test requires a meter that can measure down to tenths of millivolts ( V). A meter that can measure hundredths of millivolts ( V) makes it even easier to do this check, because the added resolution shows very small temperature changes.Connect the meter to the terminals of the thermocouple. Grabbing the end of the thermocouple should cause the voltage to increase slightly, since you're warming it up. As you release the junction, the temperature (and voltage) should drop
27Testing an Analog Input - 4 to 20 mA / 0 to 10 vDC Manometer
28Testing an Analog Input - 4 to 20 mA / 0 to 10 vDC Check its Wiring Per the Manufacturers Wiring Diagram
29Check Setting of Sensor, if applicable Sensor should be set for 0 to 1“ operationFollow Manufactures GuidelinesSet PJ3 at L position and PJ5 at N positionZero out the sensor by removing any hoses from the two barbs and holding in the zero button for 4-5 seconds
38Selecting Replacement Sensors Normally best to use an OEM replacementIf resistive based, determine type and characteristicsIf current or voltage, determine the required range and required output signal
39Read the controller’s manual Getting HelpRead the controller’s manualCall the manufacture