Thermopiles raise heat on gas detection
12 June 2008
Thermopiles are favoured because they use infra-red, IR measurement technology, are not sensitive to temperature changes, and are economical to use.
The main application of thermopiles is in non-contact temperature measurement. Temperature is one of the most widely sensed parameters and as a result, a sensor which can measure temperature accurately, cost effectively, and reliably is important.
In addition to temperature, use of thermopiles is increasing in gas detection applications, including C02 detection in HVAC (heating, ventilation and air conditioning) systems or medical applications.
Thermopile principle
Thermopiles consist of arrays of interconnected thermocouples, each of which is made of two dissimilar materials joined together at one end of the circuit. When the junction of the two metals is heated or cooled, a voltage is produced which is proportional to the temperature of the junction. This effect was first discovered by physicist Thomas Seebeck, and has since been the foundation of thermocouple operations.
The thermopile can detect surface temperature of an object through radiation rather than direct contact. The hot junctions of the thermocouples are exposed to infrared (IR) radiation emitted from the measurement surface while the cold junctions are connected to a heat sink. This incident IR radiation changes the temperature of the hot junctions and produces an output voltage proportional to this change.
The main difference between a single thermocouple and a thermopile is that thermopiles have a much higher output voltage range. This is because the output voltages of all the thermocouples are added to create the output voltage range of the thermopile. Thermopiles are used in many applications including medical instrumentation, gas detection and ear thermometers.
Thermopile interface
The output of a thermopile is in the mV range; therefore the output signal must be amplified. In this case the LMP2016, dual, high precision rail-to-rail output amplifier is used for this purpose; the signal is then converted to a digital output using the ADC122S021 (a two-channel 200ksps 12bit A/D converter from the National Semiconductor PowerWise family).
Since the output of thermopiles is usually very small, the amplifier which is used for amplification should contribute minimal noise to the thermopile signal. The source of this noise can be the op-amp’s offset voltage, the input voltage noise, the input bias current, or other non-ideal parameters. It is therefore key to choose an op-amp with superior performance in the required frequency and signal range. The LMP2016 amplifiers are suitable devices for this interface as they utilise patented techniques to measure and continually correct the input offset error voltage. In addition, they have excellent CMRR and PSRR ratings, and do not exhibit the familiar 1/f voltage noise.
The first half of the LMP2016 is used to level shift the signal. This is important since most applications operate on a single supply. The thermopile’s output is a bipolar signal, while ADCs can only handle positive signals. The second amplifier gains the thermopile’s output voltage; the output of the amplifier is centred on half the supply voltage to correspond to the ADC input voltage range. The precision voltage reference used in this circuit is the LM4140, again of the PowerWise family.
As mentioned previously, thermopiles measure the temperature difference between the hot and cold junctions of their thermocouples and their cold junctions are connected to a heat sink. This heat sink is usually the TO (Transistor Outline) package, which is a metal canister. The temperature of the package, and hence the cold junction, can be varied due to ambient temperature. Due to this, it is important to monitor the ambient temperature at all times. Some thermopiles offer a thermistor (NTC) integrated within the thermopile package to measure the ambient temperature.
Noise calculation
Input referred voltage noise is often specified in spectral density format with units in nV/√Hz. Using patented methods, the LMP2015/LMP2016 (single/dual) eliminate the 1/f noise present in other amplifiers. As demonstrated in figure 2, that noise which increases as frequency decreases, is a major source of measurement error in all DC-coupled measurements. For example in a conventional op-amp, the noise level at 1Hz is 300 nV/√Hz and 100nV/√Hz at 10Hz. This corresponds to a time domain noise in the frequency range 0.1Hz to 10Hz of 3.8μVpp whereas the noise with the LMP2016 which has no 1/f noise is 850nVpp (0.1Hz to 10Hz).
Other applications
The interface example described was related to temperature measurement, but thermopiles can also be used as gas sensors. These function on the same principle as remote temperature sensors; they both measure the incident infra-red radiation. Gas sensors use the NDIR (non-dispersive infra-red) technology based on the principle that gas absorbs infra-red light in a specific wavelength range. The law of Lambert and Beer gives the total absorption such that I = Io e–εcl, where I and Io represent respective radiation intensity (W/m2) after and prior to absorption, ε is the specific absorption coefficient, c [ppm] the concentration, and l [m] the length of the absorption path.
Gas concentration
By using a specific wavelength-selective filter, the ratio I/Io can be measured and the gas concentration determined. As represented in figure 4, a CO2 detector consists of an infra-red source, the thermopile which is the detector element including the wavelengthselective filter, and the electronics to process the signal.
As shown when discussing the thermopile’s interface, the op-amp used for the signal amplification can be the LMP2015 due to its good precision specifications. In this case, the step down DC/DC regulator that converts the 4.5V-12V input supply voltage to 3.3V is the LM2736.
Monitoring CO2
CO2 sensors can be used for HVAC systems to check the air quality and inform the control system that it is necessary to increase the quantity of fresh air. This is called DCV (demand controlled ventilation). The CO2 concentration in the HVAC is not directly harmful to the person (unless the concentration is very high), but monitoring the CO2 gives an indication of the air quality inside the room. When people exhale CO2 they also exhale micro-organisms such as particles, germs and gases which can be responsible for headaches, tiredness or discomfort. CO2 sensors are also used in the medical domain to monitor the quantity of CO2 exhaled by patients.
The thermopile has some properties that cannot be duplicated by other detectors. It shows an inherently stable response to DC radiation and is not sensitive to ambient temperature variations.
CARINE ALBERTI is applications engineer, National Semiconductor
Contact Details and Archive...
Related Articles...
Most Viewed Articles...