Makes perfect sense

WHILE OPTICAL detectors remain dominant, true growth has come from multi-sensors as installers and specifiers realise the benefits to be had, most notably a reduction in false alarms. 

The typical fire alarm multi-sensor comprises optical smoke and heat sensors, although other variations exist. They vary dramatically in design, from basic models where there is limited cross evaluation of the sensor values, through to highly sophisticated devices featuring advanced algorithms to assess the variation in values from each sensor in order to determine the nature of the potential fire. Consequently, their ability to detect fires whilst discerning false alarm sources is a very mixed bag indeed. Assuming all multi-sensors will perform to the same level would be a mistake.

Multi-sensors: the benefits

That said, most multi-sensors offer a number of benefits over single sensor detectors. There’s the obvious time and cost benefits of being able to install one device in place of two and the aesthetics of only having one alarm on the ceiling. Multi-sensors can also make the job of specification much easier. But top of the list, and the main reason for the ever-increasing popularity of the multi-sensor, is the reduction in false alarms.

We’re all familiar with the problems that stem from false alarms: they are estimated to cost UK businesses around a billion pounds per year! Reducing false alarms is therefore a major task the fire industry has been attempting to address over the years and one of the reasons for the development of the multi-sensor.

Customer feedback across different manufacturers’ multi-sensors has been mostly positive in their ability to reduce false alarms. Last year’s Briefing Paper from the Building Research Establishment (BRE) - ‘The performance of multi-sensors in fire and false alarm tests’ – has provided concrete evidence of this. Following testing of 35 different optical heat Multi-Sensor detectors (representing the full range of those available in the marketplace at that time), the BRE estimates that potentially 38.1% of observed false alarms could have been reduced if multi-sensors had been present. That means a potential reduction in UK business losses of £381 million per annum; a huge sum!

Multi-sensors: the limitations

‘The performance of multi-sensors in fire and false alarm tests’ BRE Briefing Paper concludes: “this research has shown that, the use of multi-sensor technology has the potential to reduce certain types of commonly encountered false alarms. However, the extent to which this can be realised depends on the particular implementation of features designed to improve false alarm immunity. It cannot be assumed that use of simply any multi-sensor detector will impact significantly on the occurrence of false alarms from every form of fire-like phenomena.”

Whilst offering improved performance over standard optical detectors, multi-sensors have limitations. In fact, the BRE research demonstrated that, although delayed somewhat, in all cases alarms were given from non-combustion materials such as steam, dust or aerosol. The delay might allow for any transient false alarm sources to disappear before the fire threshold is reached, but it does not entirely stop a false alarm in these instances. Furthermore, to ensure this delay, some multi-sensor detectors have been configured (or can be configured by the installer) to be less sensitive which is a compromise that is not always acceptable or ideal.

It should also be noted that not all known causes of false alarms could be tested by the BRE. False alarm tests covering long-term dust build up, condensation, cigarette smoke, synthetic smoke and insect ingress were not possible due to difficulties with developing repeatable tests. These are some of the major causes of false alarms.

Multi-sensors with a difference

So, the multi-sensor doesn’t provide the complete solution to false alarms and the quality and price vary dramatically. Still, they do perform better than single sensor alarms. But what if you could combine this principle of ‘two is better than one’ by adding another factor into the mix?

Nittan EV-DPH Multi-Sensors do exactly that by integrating dual optical alarm technology, developed by Nittan, with heat sensors to provide a very sensitive, but truly discriminatory multi-sensor device. The dual optical alarm technology is based on particle light scattering theory, but instead of just using the standard Infra Red light, found in optical alarms, it also employs blue LED to provide a more accurate measurement of particles within the chamber. Using a complex algorithm to calculate the ratio of these light sources, which operate at different wavelengths, the detector can determine the particle size and thus distinguish between smoke and non-combustion products such as steam, aerosols and dust. This is where most typical multi-sensors struggle.

EV-DPH Multi-Sensors have the ability to reduce false alarms caused by all these mediums, without the need to lower the devices’ sensitivity to actual combustion products. The technology has been further developed to make the detector more sensitive to flaming fires by shifting the alarm threshold level when the ratio of the two scattered light wavelengths indicate that the particle size present is similar to the smallest size defined as a smoke particle.

Dual optical alarm technology

Dual optical alarm technology is not as well known as its counterparts, yet it has been proven to be highly effective in many installations, significantly reducing false alarms, in many cases to zero.

At the University of East Anglia’s (UEA) campus, dual optical alarms replaced the existing smoke detectors throughout the student accommodation in a bid to remove issues with false alarms caused by steam. More than 2500 Nittan dual optical smoke detectors were installed by Global Fire Services Ltd. throughout the 24 accommodation buildings spread across the 320 acre UEA campus.

The University was experiencing around 20 false alarms every week caused by steam escaping from the bathrooms and activating the smoke alarms. Each alarm event has to be checked and verified by security personnel before Fire and Rescue Services will attend. A reduction in false alarms is therefore not just convenient, but also has significant cost and time benefits.

Director of Global Fire Services Greg Smith comments, “Originally the plan was to relocate the existing detectors further away from the bathrooms. Whilst this would provide the same level of fire protection, it would be quite a costly solution as it would not only involve moving the devices and cables, but also redecorating afterwards. We were also concerned that the steam would affect these devices in the new location.” 18 months after installation and not a single false alarm due to steam had been generated.

Although a very different site, dual optical alarms installed in a pub / live music venue in Southampton have proven similarly successful, this time addressing false alarms caused by vaping.

E-cigarettes feature a cartridge containing nicotine in a solution of either propylene glycol or glycerine and water. When you suck on the device, a sensor detects the air flow and starts a process to heat the liquid inside the cartridge, so it evaporates to form water vapour. In an enclosed venue where there are a number of people vaping, this can have the same effect as the steam from the bathroom at the UEA student accommodation, leading to false alarms.

The legendary Frog & Frigate in Southampton is renowned for being a very lively, busy venue and vaping is permitted. Venue owner Derek Gardener explains the problem, “We have relaxed, fun vibe here and are happy for people to use e-cigarettes inside. However, it has led to the alarms activating, especially when people are vaping right underneath them. We can’t stop people from vaping underneath them and equally we didn’t want to ban vaping.”

Derek turned to Fire and Safety Testing for a solution. Owner Derek Fay explains, “Many venues such as this resort to using heat alarms instead of smoke alarms, but the local fire officer would not allow this. He suggested a multi-sensor, but the same issue remains as it still looks for smoke and will false alarm. I had read about Nittan’s dual optical detectors resistance to steam in a magazine article and thought they might offer a solution here as the problem was very similar. We swapped the existing alarm devices out of the conventional system with the Nittan ones and there hasn’t been a single false alarm since; that was about six months ago. It’s worked beautifully.”

Conclusion

The benefits of multi-sensors are becoming better known, backed up by valid research and, with unit costs falling, are becoming all the more popular. However, assuming all multi-sensors will provide better detection and improved resistance to false alarms is not necessarily the case. Devices with dual optical sensors warrant a closer inspection and should be considered, especially where water vapour, or other non-combustible mediums, are an obvious false alarm factor.

Barry Sargent is product support manager at Nittan. For more information, visit www.nittan.co.uk