UV vs IR Flame Sensors: How They Differ and Which to Choose

The flame sensor is the “eye” of the burner controller: it reports whether a flame is present so the controller can hold or cut the gas. There are three common technologies: ultraviolet (UV), infrared (IR) and the flame rod (ionisation). UV responds fastest but is weaker in smoke; IR suits oil flames and longer distances but can be fooled by the hot background of a furnace; the flame rod works on gas flames only. The right choice depends on the fuel, the environment and how the flame is viewed.

Key points

• A flame emits UV (about 1% of the spectrum), visible light and infrared (about 90% of the spectrum). Each type of sensor “sees” a different part of it.
• UV: fastest, suits gas flames, loses performance in heavy smoke or at distance; use a self-checking type when the burner runs continuously.
• IR: better at long range and with oil flames; red-hot furnace walls can disturb the signal.
• Flame rod: sits directly in a gas flame, needs a stable flame and a large enough grounding area.
• A burner flame sensor is different from a fire flame detector, which gives an alarm for fires in an area.

What a flame sensor does

In a combustion system the flame sensor is connected to the burner controller. At start-up it confirms the pilot flame and then the main flame; while running, if the flame is lost for longer than the flame failure response time (FFRT, from under 1 second to about 4 seconds depending on the standard and design), the controller closes the shut-off valves. Conversely, if the sensor reports a flame when there should be none, the controller must lock out and refuse to start.

What a flame emits

A flame radiates over a wide band made up of ultraviolet, visible light and infrared. Ultraviolet makes up only about 1% of the total spectrum, is most abundant in the first third of the flame and increases as flame temperature rises. Infrared makes up about 90%, mostly in the last two thirds of the flame. Hot furnace parts such as refractory also emit infrared when above about 1000°F (roughly 540°C). Each fuel gives a different spectrum:

UV sensors

A UV sensor uses a quartz detector tube filled with gas and containing two electrodes. When ultraviolet from the flame enters the tube, the gas ionises and the tube pulses; the controller counts the pulses to estimate flame intensity. The tube responds only in the short-wavelength band of about 200 – 300 nm (“solar blind”) so that light from outside the flame does not disturb it.

• Strengths: the fastest response of the technologies; works well on gas flames, especially premixed gas.
• Limits: performance drops with distance to the flame and in smoky surroundings; it can respond to ignition sparks, welding arcs, halogen lamps and very hot refractory (above about 2000°F, roughly 1100°C).
• “False flame” fault: an ageing UV tube can pulse with no flame present. The controller only detects this when the burner shuts down and restarts, so for burners that run continuously a self-checking type is used, with a shutter that blocks the UV periodically.

CAD’s range includes two Honeywell UV sensors: the C7024E1001 (24 VDC solid-state self-checking type used with the R7824C amplifier; the shutter blocks UV 12 times a minute to test the tube; can be mounted horizontally, vertically or at any angle) and the Minipeeper C7035A1080 (end viewing, NEMA 4 and 4X enclosure, field-replaceable UV tube, 5 psi maximum pressure, used with the R7249A, R7290A, R7749B and R7849A/B amplifiers).

IR sensors

An IR (and visible light) sensor cannot rely simply on whether radiation is present, because a hot furnace also emits infrared. It measures the flicker frequency of the flame, typically with a PbS (lead sulphide) photoconductive cell; flame flicker can range from about 5 Hz to over 200 Hz. By analysing flicker the sensor can tell a burning flame from a steady hot background.

• Strengths: better at long range; suits oil flames and non-premixed gas flames; less weakened by smoke than UV.
• Limits: low-frequency background radiation from furnace walls and other heat sources can saturate the photoconductive cell (“washout”), so the sensor can no longer track flicker; hot gas moving across refractory can also create false flicker. Aim at the ignition zone, avoid viewing the furnace background and adjust sensitivity if the controller allows.

Flame rod (ionisation)

A flame rod is a small metal rod placed in a gas flame. The flame makes the gas slightly conductive and the current through it is only a few microamps (typically 2 – 4 µA); a rectifying circuit lets the controller reject false signals caused by a short to ground or leakage. The rod is used on gas flames only, needs a stable flame and a grounding area considerably larger (generally about 4 times) than the rod. For example, the Kromschröder BCU 370 accepts both an ionisation signal of 1 – 28 µA and a UV signal of 1 – 35 µA.

UV, IR and flame rod compared

Continuously running burners and self-checking sensors

With a UV sensor, the dangerous fault is a tube stuck in the “flame present” state. A self-checking sensor has a shutter that blocks the UV periodically; the controller is designed to expect this interruption, and the test time must be shorter than the FFRT so it does not cause a nuisance shutdown. The applicable standard may require a self-checking type when the burner is not allowed to cycle for a specified period; check the specific requirements of NFPA 85, NFPA 86 or the project standard.

How to choose a flame sensor

1. Fuel: premixed gas leans towards UV or a flame rod; oil towards IR; non-premixed gas can use either depending on conditions.
2. Environment: smoke, dust and oil vapour weaken UV; a hot furnace background disturbs IR; arcs or sparks nearby disturb UV.
3. Distance and view angle: aim at the root of the flame, through a sight tube or a correctly sized flange; where the flame is hard to see, a swivel mount can help.
4. Operating mode: intermittent or continuous (whether a self-checking type is needed).
5. Compatible controller and amplifier: each sensor works only with certain amplifiers, as the product pages list.
6. Installation conditions: temperature, pressure at the sight tube, enclosure rating, cable length and type.

See the flame sensors group and the burner controllers of Honeywell.

Do not confuse with a fire flame detector

Besides sensors that supervise a burner flame, there are fire flame detectors used to detect an unwanted fire in an area such as a fuel store, pump station or process area. They do not control a burner; they give an alarm or an emergency shutdown signal. For example, the Honeywell FS24X infrared flame detector is a multi-spectrum detector (IR/IR/IR/visible) that detects both hydrocarbon and non-hydrocarbon fires; per its product page it has a detection time of 3 – 5 seconds for a 1 sq. ft. n-heptane fire at 30 m, an IP66 rating and SIL 2 capability. See also CAD’s Fire & Gas Detection solution.

Need to replace or select a flame sensor for an existing burner? CAD supplies genuine equipment with full CO/CQ documentation. Send the model / specifications and CAD will quote within 24 working hours: contact CAD.