Datos estructurales

and K a proportionality constant determined by calibration.

Output was monitored by a DVM across a load resistor (50 Ohm) resistor (giving ~20 mV in air). During tests the values were recorded manually as variations were small and changes were not rapid.

The 2FO electrochemical cell has a bulk flow cap to reduce interference from sudden fluctuations of pressure, and an operational range of 0-25% O2. To check for changes in

sensitivity the unit was calibrated on a monthly basis using high purity nitrogen (BOC) and air as 0 and 20.9% oxygen.

2.3.7 Fire Detector Temperature sensor Calibration

801PC and 801PH devices and some of the variants used for this study incorporate thermistor devices for temperature measurements and provide output in bit form. The temperature response of each device was calibrated before supply to UCLan in flowing air in a heat tunnel at TYCO Sunbury and in house method, recording bit output response during a 3 oC/minute temperature ramp. Figure 33 shows a plot produced from an example record. Temperature sensitivity for these devices is very stable.

98 !" #" $"" $%" $&" $!" $#" %" '" &" (" !" !" #$% &# '& #$ #()'(*+#&*($$,(-.$/. 0()'(*+#&*($*(1'%21($3%*$4567/$,(8"9(:$;$+,,*.$6<:$ 9=+2.$+85$%&#'&#>$?"#=$@A_{/B)"2.$*+)'$"2$0C9%$D(+#$} #&22(E$3*%)$FGA_#%$GH$A_/ 1E%'($IF.J$!"#1BA_/

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Figure 33 Example 801PC temperature channel calibration

At UCLan, validation of detector temperature sensor response was carried out on a continuing basis by referring the devices temperature response to that of a K type thermocouple itself checked periodically with ice (0oC) and steam above boiling water (100oC).

2.3.8 UCLan Enclosure NDIR for CO and CO

A general introduction to the application of the IR techniques and particularly NDIR was provided in chapter 1. A dual fixed wavelength NDIR device (Unicam 22PU NDIR) was used with the UCLan 2 m3 _{enclosure to monitor the carbon monoxide and}

carbon dioxide concentrations in the fire gases. Gas input to the NDIR system was from a fixed sampling point in the roof of the fire enclosure. As the NDIR system uses an aspirated sampling pump care was taken to ensure sampling did not interfere with the smoke transport to sensor/ detector locations.

The NDIR device sampled at a rate of 2 l min-1with a sampling time of 15 seconds. The sample path length was 10 mm. High purity nitrogen gas was used to provide a reference zero value and this was checked on a daily basis using the inbuilt instrument validation system. The span was checked on a daily basis using a maximum value was supplied by calibration CO/CO2 gas.

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An additional validation in the UCLan enclosure was carried out each week. This involved measuring the response of the CO and CO2 channels to 10 litres of CO/CO2

(6000ppm/4% in air) calibration gas fed into the enclosure at a rate of 1.7 litre min-1 (for 353 seconds). A fan was used to ensure the calibration gas was adequately mixed in the 2 m3 enclosure to give 30 ppm CO, 0.02% CO2. After gas injection and mixing the

instrumental readings for the CO/CO2 levels were monitored until the levels reached a

plateau.

Figure 34 The figure shows the validation checks used for the NDIR device in the 2m3 UCLAN fire chamber , The values are averages of regular checks (n=13)

2.3.8.1    UCLan  Enclosure  Calculation  of  CO/CO2  yield  by  NDIR    

The NDIR gives us a convenient method for defining yield and ratios of CO and CO2

formed in fires. The Ratio is expected to vary depending on fire type and the degree of fire development and may also be informative with respect to nuisance sources.

If at a given time CO concentration = a ppm, and increase in CO2 concentration = b

ppm.

Then CO/CO2 ratio = a/b (assuming volume ratios used for ppm definition equate to

molar ratios, which should apply well for these gases in air at atmospheric pressure. Using ppm values averaged over the test duration to calculate CO/ CO2 may provide a

value generally characteristic of fire type. Calculations based on a series point of time ppm values and yielding the corresponding ratios may be taken as indicative of fire condition development.

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One may attempt to calculate yields of CO and CO2 from combustion based on weights

of fuel before and after fires but the validity of that depends on the degree of gas mixing in the enclosure, a feature not controlled in these fire test emulation experiments. Values are not therefore presented.

2.3.9 UCLan Enclosure Humidity sensors

Humidity sensor : The moisture changes in the enclosure were monitored using a humidity sensor device (HONEYWELL HUMIDITY SENSOR, 2.54MM, SIP , HIH- 4000-001, Farnell stock code 1187547) which was coupled into a modified TYCO 801PH fire sensor (addr. 100, ser.no920000266) where the photo-sensor input is replaced by humidity sensor input. This allowed humidity sensor output to be monitored and logged in bit form simultaneously to the same files produced for the TYCO smoke detectors. A miniature fan (25x25x10mm, NMB, 1004KL-01W-B40-B00, Farnell stock code 1545794) was mounted into the detector cover to produce a significant airflow over the sensor, as response was otherwise excessively slow.

A three point validation of the humidity sensor range was performed using conditions corresponding to 0% RH, 41% RH and 100% RH (dry bottled air, and air bubbled through fine frit in salt solution and pure water). Using these values a response chart (figure 35) was constructed from which humidity readings could be calculated.

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Figure 35 Example of three-point calibration curve used for the analysis of humidity data Conversion of RH % to ppm values based on temperature and literature tables.

2.3.10 UCLan Enclosure Optical Density measurements

The BRE test room is equipped with on Obscuration measurement system as required by BS EN54/7. Such commercially produced equipment was not available for the UCLan measurements so an in house built obscuration detector was constructed for incorporation into the enclosure using an 890 nm IR LED (10o viewing angle) and a receptor separated by a 0.691m path length. The LED and photosensor were chosen to meets the specification of a light source from the BS-EN 54 standard – as below

The wavelength of the light was selected so it has the following specifications;

4. At least 50% of the radiated power shall be within a wavelength range from 800nm to 950nm.

5. Not more than 1% shall be in the wavelength range below 800nm. 6. Not more than 10% shall be in the wavelength range above 1050nm.

Prior to each test the obscuration device range was checked by measuring the output in free air and completely obscured by a non trans-missive sheet.

The analogue output from the unit was linked via a buffer amplifier to a TYCO 801PC detector (address 135) converted to provide A to D function so that it could be simultaneously logged onto the same data files containing the output of TYCO smoke detectors. The bit output for the system is linearly related to photosensor output and covers the full range between 0 and 100% transmission. The bit output is converted to Obscuration (%Obsc.m-1) using the path length 0.69 m using the expressions below;

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Output fully obscured = a bits (typically 4 bits) Output for clear air = b bits (typically 155 bits) Output for a smoke level = c bits

Transmission Fraction through smoke = Ts = (c-a)/(b-a) (18)

Equation 18 Calculation of the transmission fraction of light through smoke.