Infrared flame sensor element (pyroelectric, PbS/PbSe photoconductive and photodiode modules)

01

Choose the infrared band the element must see

InfraTec, a German maker of pyroelectric detectors, explains that a flame sensor's detector picks up the spectral radiation of burning organic substances such as wood, natural gas, gasoline and plastics. For a flame sensor, the absorption bands of the combustion products CO and CO2 are set between 4.0 µm and 4.8 µm, with a band around 12 µm for water, and the band-pass filter in the detector cap limits what reaches the chip to those bands. [1]

To tell a flame from the sun or arc-welding light, InfraTec names independent criteria: a flame has a flicker frequency of 1 to 5 Hz, a hydrocarbon flame produces CO and CO2, and burning produces water that can also be detected in the infrared. Evaluating the flicker together with the spectral information prevents false alarms, and InfraTec states that its standard pyroelectric detectors can detect a flame up to 100 m away without additional optics. [1]

A silicon photodiode module works at much shorter wavelengths. Easyelecmodule lists its 4-wire flame sensor module as detecting flames or light sources between 760 nm and 1100 nm, with adjustable sensitivity, digital and analog outputs, more than 15 mA drive and a 3.3 V to 5 V supply, for smart homes and small fire-prevention equipment. [5] Editorial calculation: 760–1100 nm is 0.76–1.1 µm, which lies outside the 4.0–4.8 µm CO and CO2 band InfraTec describes.

Give the supplier the fuel or fire type to be detected, the detection band and filter centre wavelengths, the reference channel if dual-band, the range and field of view, the response time needed, the supply voltage and output signal, the window material, the operating temperature, the package and whether a bare element or a module with amplifier is wanted.

02

Match the element type to response time and range

Winsen Electronics lists its RP911 pyroelectric flame sensor with detection bands at 2.7, 4.3, 4.4 and 4.48 µm and reference bands at 3.8 and 5.0 µm, a 5 × 5 mm window over a 3 × 3 mm sensitive element, an electrical time constant of 5 s and a thermal time constant of 150 ms, a working voltage of 2–15 V (3–5 V recommended), a field of view over 115°, a detection range over 50 m and a working and storage range of –40 °C to +85 °C. Responsivity without window is at least 500 V/W and specific detectivity at least 5 × 10⁸ cm·Hz½/W; Winsen notes the test amplifier gain was 80 dB. [2]

Its lead-salt photoconductive sensors are faster. The REF-X2212 PbSe sensor has a 1.2 × 5 mm sensitive area, a 20 µs response time, a 1–5 µm response range, sapphire or narrow-band filter windows at 2.7, 3.8 and 4.3 µm, a peak responsivity of 4 × 10⁴ V/W, a detectivity of 1 × 10¹⁰ cm·Hz½/W and a working range of –40 °C to 60 °C. [3] The RGF-L1212 PbS sensor has a 2 × 2 mm area, 200 µs response, peak responsivity of 4 × 10⁵ V/W, detectivity of 1 × 10¹¹ cm·Hz½/W and a –30 °C to 60 °C range; its table gives the response range as 1–3 µm but the sapphire-window central wavelength as 1–6 µm, so confirm the usable band with the supplier. [4]

Editorial calculation: flicker at 1–5 Hz has a period of 200 ms to 1 s, longer than the RP911's 150 ms thermal time constant, while the PbSe and PbS sensors respond in 20 µs and 200 µs, a factor of 10 apart. [1] [2] [3] [4]

03

Compare the published element figures

The response column mixes two different quantities: the Winsen pyroelectric part gives a 150 ms thermal time constant, while the lead-salt photoconductors give 20–200 µs response times, and InfraTec quotes the 1–5 Hz flicker band its electronics evaluate rather than a speed. Range figures also differ in basis — Winsen states > 50 m for its module, InfraTec up to 100 m without optics.

04

Chip, filter window, housing and amplifier

05

Range, response, band and supply data to quote

06

Send the fire, band and electrical data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Fire: [fuel; size; distance; field of view]
Band: [detection and reference wavelengths; window]
Element: [pyroelectric, PbS, PbSe or photodiode; sensitive area]
Response: [time; flicker evaluation]
Electrical: [supply; output; gain]
Environment: [temperature; housing; sealing]

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