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Minimise the risk of boating explosions Bilge Sniffer by John Clarke Messin’ about in boats is one of the world’s favourite leisure activities. Yet far too regularly, we see in the news stories about injuries and deaths when boats blow up due to fuel vapours in the bilges. This little sensor/alarm unit is designed to let you know about this danger before you turn the key . . . M otorboat explosions and fires are the largest single cause of marine property damage. The hazards are due to the presence of flammable liquids within poorly ventilated spaces, such as the bilge. Petrol is the major hazard, whether by leakage, spills, the use of poor storage containers or even downright stupidity. With the right mixture of petrol and air, a small spark (even from a switch), a cigarette or match, 36  Silicon Chip or even electrical failure can cause an explosion that can literally blow the boat apart. As an extra danger, the petrol vapour is heavier than air and tends to collect in lower areas of the boat. Consequently, a small leak or spill can present a significant hazard for explosion and fire. To blow up, the mixture of petrol vapour and air must be within a certain (and quite limited) range of fuel-to-air ratios. The lower explosive limit (LEL) is 1.4% by volume and the upper explosive limit (UEL) is 7.6%. Above and below these limits, the fuel will not explode. With diesel fuel, the explosion risk is significantly reduced – although it is still significant. This is due to diesel’s higher flash point, which prevents it from readily evaporating and producing a flammable mixture. The LEL for diesel fuel is typically 3.5% by volume siliconchip.com.au It’s very cheap insurance! Fit it to your boat and it could save it – or your life. It can automatically turn on bilge exhaust fans when explosive gases are detected. and the UEL is 6.9%; ie, a higher LEL and a smaller range than petrol. While the reduced range makes it harder for diesel fuel to explode, it can still happen. In the heat of summer, for example, temperatures within the boat compartments may be raised sufficiently to provide the right conditions for an explosion. In addition to engine fuels, boats often have propane gas cylinders for fuelling stoves, on-deck barbecues, and so on. This too can be a fire and explosion hazard. Propane will explode with an air/fuel ratio by volume of between 2.5% and 4.5%. Once again, because propane is heavier than air, a leak can create an explosive mixture in lower boat compartments. The bilge Ahoy, me hearties. Throw ’em down in the bilges. . . OK, pirate movies are about as close as the great unwashed ever come to a bilge. But what is it? It’s the lowest area of the boat, closest to the water. Often the bilge is actually under a floor (between the floor and the outer hull), so it can be almost inaccessible. In a large motor siliconchip.com.au boat, it’s almost certainly right under the engine. Being the lowest point in the boat, the heavier-than-air fuel mixture tends to concentrate here. Any ingress or leakage of water through the hull (and there almost always is some, especially in timber boats!) congregates there (and that water can really go “off”!). On a big ship, it’s where the rats love to nest. With a mixture of rancid water, oil, fuel and all manner of dirt, it can often be an unpleasant, foul-smelling place. All boat owners should be aware that the bilge is by far the most dangerous area of the boat, as far as explosive gasses are concerned. Even if the fuel is mixed with the bilge water there is no guarantee of safety: all fuels float on water and so vapours are still likely be released into the air. Ideally, boats should include bilge ventilation to expel any inflammable gases that will inevitably accumulate, even if correct refuelling methods are used and leaks/spillage minimised. However, ventilation on its own is usually not enough to guarantee safety. Exhaust fans should also be used to remove the potential explosive gases before starting the engine. Many boat owners have bilge exhausts that are run “as a matter of course” for perhaps a minute or so before the engines are started – just in case. But wouldn’t it be better to know for certain, one way or the other, if there are dangerous gases down below? That’s what the SILICON CHIP Bilge Sniffer is designed to do. What’s more, it can be set up to automatically start an exhaust fan if dangerous levels of hydrocarbons – that’s any fuel or gas – are detected. It could also be made to be part of the engine start or ignition circuit, locking out the engine if hydrocarbons were detected. That could be a boon for hire craft operators who cannot guarantee hirers will wait for gases to be cleared before starting the engine. The “on” time of any exhaust fan is important because bilge fuel levels might be above the upper explosive limit and therefore offer little risk of explosion. If the fan sucks out only some of the gases, the level might be reduced to where it is bang (literally!) in the middle of the danger zone. September 2005  37 REG1 7805 +5V OUT IN GND 10 µF + 100 µF 16V D1 1N4004 10Ω +11.4V (nom) ZD1 16V 1W OPTIONAL 12V DASH SIREN K A +12V (nom) 0.5W 0V – +11.4V A +5V K SENSOR1 MG-3 HYDROCARBON SENSOR H A B H CALIBRATE 8 2 IC1a 2.2k 1 K A 5 6 IC1b 4 MG-3 BILGE SNIFFER/ALARM A 10k IN4004 A H SC 10k 7 LED B NC COM NO HYSTERESIS D2 1N4148 VR2 500k ALARM LEVEL VR3 10k 10k 2005 NC COM NO IC1: LM358 3 VR1 50k D3 1N4004 A 2.2k 10 µF 16V RLY 1 K λ 100 µF 16V LED1 B C E BC337 Q1 BC337 E B C GND K OUT B H A A K ZD1 1N4148 A K 7805 IN A K Fig.1: the hydrocarbon sensor’s resistance is monitored by op amp IC1a. When gas is detected, transistor Q1 is switched on, pulling in a relay. What you do from then on is up to you! The dash siren is optional – see text. Therefore, the sniffer is designed to keep removing gases while ever it senses any hydrocarbons; ie, until they are well under the lower explosion limit. The hydrocarbons that can be detected include iso-butane, propane and LPG, as well as petrol and diesel fuel. The sensor itself is encapsulated in a plastic housing with a stainless steel mesh cover which provides antiexplosion protection. It has an aluminium oxide ceramic tube that is coated with a tin dioxide sensitive layer, along with a heating element. Electrodes attached to the sensitive layer provide the gas sensing terminals. The semiconductor sensor changes its resistance between two measuring electrodes in the presence of the appropriate vapour, with the resistance decreasing with increasing gas concentration. Circuit details The circuit for the Bilge Sniffer is shown in Fig.1. Operation is quite 38  Silicon Chip simple: the sensor is monitored by IC1, which controls transistor Q1. This switches the relay at a preset hydrocarbon concentration. One of the electrodes from the sensor connects to the 5V supply while the other end connects to the calibration potentiometer (VR1) and series 10kW resistor. In operation, the sensor electrodes present a certain resistance at various hydrocarbon concentrations and this resistance sets a voltage due to the divider action of VR1 and the 10kW resistor. Typically, the voltage is around 2V at 0.1% hydrocarbon concentration when VR1 is set at 10kW. IC1a is an op amp connected as a unity gain buffer, which monitors the sensor voltage at the top of VR1 and the 10kW resistor. IC1a’s output drives the non-inverting input of IC1b which is wired as a comparator. Pin 6 of IC1b connects to the alarm level trimpot, VR3. Wired directly across the 5V supply, this trimpot can set the level anywhere between 0V and 5V. When the voltage at IC1a’s output goes above the voltage set by VR3, then IC1b’s output goes high, to a little less than 12V. Diode D2 then conducts and the pin 5 input of IC1b is pulled slightly higher than the pin 1 output of IC1a due to the voltage divider action of VR2 and the 2.2kW resistor connecting to pin 5. Built-in hysteresis, set by VR2, prevents IC1b’s output from “hunting”, or switching high and low at a rapid rate at the threshold where IC1a’s output equals the level set by VR3. The output of IC1b drives the base of Q1 via a 10kW resistor. The 10kW resistor from base to ground ensures that Q1 is off when IC1b’s output is low. Transistor Q1 drives relay RLY1. The common and normally open contacts close, turning on the ventilation fans. LED1 also lights to indicate whenever the relay is on. Diode D3 quenches the back-EMF produced by the relay coil when Q1 switches off. Note the relay should be a sealed (ie flameproof) type if it is to be used anywhere near explosive gases. siliconchip.com.au K K A 100 µF H 10 µF 500k 2.2k B VR1 50k K D2 C CN 4148 10k 100 µF 10 µF K A D3 Q1 siliconchip.com.au ON A 10k We have also shown an optional 12V siren wired across the relay coil. There would almost certainly be times when a LED might not be noticed (especially in daylight) and we would imagine that the skipper would like to know that there is something not quite right down in the bilges just as soon as it happened! Any small 12V warning siren or buzzer would be satisfactory here – they draw very little current for a lot of noise! No provision is made on the PC board for this – it would most easily be connected directly across D3. If the buzzer or siren is polarised, the “+” side would connect to the cathode of the diode. Of course, such a buzzer could also be switched by one of the sets of relay contacts if you didn’t need them for something else. Power for the circuit is provided from a 12V supply. This would normally be from the boat’s 12V battery. The supply is reverse-polarity protected using diode D1 and protected from transient voltages using a series 10W resistor and the 16V zener diode, ZD1. K rab/tod VR2 A SENSOR1 H A 2.2k ZD1 LED1 10k 10k K 0V A VR3 IC1 LM358 REG1 +12V NO RELAY 1 15090150 The 100mF capacitor filters the incoming voltage and REG1 regulates the voltage down to 5V for the sensor’s heater coil. This coil has a resistance of 33W, so the current drawn from the 5V supply is around 150mA. Note that both the 12V supply (and indeed the 11.4V supply) are labelled “nominal” because these could be higher, depending on the state of charge of the boat battery – up to about 14V. The 11.4V rail is simply 0.6V less than the supply rail (the 0.6V being dropped across D1). 24 or 48V boat supplies If you are really well-heeled, you might have one of those floating palaces with a 24V or even a 48V DC boat supply. . . in which case, you have even more to lose than those of us with little putt-putts and tinnies. You really need the Bilge Sniffer! Naturally, the Bilge Sniffer cannot be powered from these higher rails direct but it would be a very unusual “big boat” not to have a derived 12V(ish) supply somewhere for such things as two-way radios, entertainment R OS NES GPL/L O H O CLA 10Ω CN D1 C ON A COM CONTACTS 1 NC NO COM CONTACTS 2 NC Fig.2: component overlay and matching photograph below. The gas sensor is the large silver object at lower left. The relay needs to be a completely sealed type if you are going to mount the PC board anywhere near where explosive gases might congregate. Don’t worry about the unused holes in this PC board – we explain why in the text! systems, etc. We’re only talking a few hundred milliamps maximum so it shouldn’t cause any overloads! Failing this, you could add a suitable DC-DC converter. Construction The Bilge Sniffer is constructed onto a PC board coded 05109051, measuring 123 x 60mm. It has its four corners removed and shaped to allow it to fit into a plastic UB3 box measuring 130 x 67 x 44mm. Power is connected via a figure-8 lead that attaches to the boat’s 12V supply via an in-line fuse. Begin construction by checking the PC board. Check that the corners of the PC board have been shaped correctly so that the PC board can be fitted into the box. First, install the six resistors, making sure the correct value is placed in each position. You can use the accompanying resistor colour-code table to check the values or use a multimeter to measure them. When placing the regulator and IC, make sure they are oriented correctly. A heatsink is attached to the regulator September 2005  39 the power screw connector and relay contact connections. Connect the figure-8 power lead to the 12V terminals. We didn’t find any need to screw the PC board into the case – it sits snugly inside the four corner pillars and on the recesses in the moulded slots in the case side. When the lid is scrwed on, it virtually locks the PC board in place. Parts List – Bilge Sniffer 1 PC board, code 05109051, 123 x 60mm 1 UB3 box (130 x 67 x 44mm) 1 MQ-6 hydrocarbon gas sensor (Jaycar RS-5610) (Sensor 1) 1 12V 5A DPDT PC-mount sealed relay (RLY 1) 1 mini heatsink, 19 x 19 x 10mm 1 M3 x 10mm screw and nut 4 2-way PC-mount screw terminal blocks 6 PC stakes 1 200mm length of light-duty figure-8 polarised wire 1 150mm length of 0.8mm tinned copper wire Semiconductors 1 LM358, TL072 dual op amp (IC1) 1 7805 5V regulator (REG1) 1 5mm red LED (LED1) 1 BC337 NPN transistor (Q1) 2 1N4004 1A diodes (D1,D3) 1 1N4148 diode (D2) 1 16V 1W zener diode (ZD1) Capacitors 2 100mF 16V electrolytic 2 10mF 16V electrolytic Resistors (0.25W, 1%) 3 10kW 2 2.2kW 1 10W (0.5W) 1 500kW horizontal trimpot (VR2) (code 504) 1 50kW horizontal trimpot (VR1) (code 503) 1 10kW horizontal trimpot (VR3) (code 103) using an M3 x 10mm screw plus an M3 nut, with its flanges oriented so that they point towards the edge of the PC board. When mounting the electrolytic capacitors, ensure they are oriented as shown on the overlay diagram. Similarly with the diodes: solder them in place with the orientation shown. Terminal blocks for the power connection and relay contacts can also be installed, along with the trimpots. Use PC stakes and tinned copper wire (eg, resistor lead cut-offs) to hold the sensor in position above the PC board (see photo). The sensor can be mounted either way around, as the pin connections are symmetrical. The top of the sensor should be 40  Silicon Chip Testing The gas sensor itself is mounted up off the PC board, on PC stakes and tinned copper wire, so its “nose” pokes through the lid of the box. 35mm above the PC board to allow it to poke through the jiffy box lid. For the same reason, mount LED1 so that its top is 32mm above the PC board. Construction of the PC board is now complete. Check that your board agrees with both the overlay diagram and the photographs, and also check your soldering for bridges or dry joints. Don’t worry about the large number of unused holes in the PC board – this board will also be used for another project in a coming issue (the holes are primarily for a LED bargraph and driver IC). Using the front panel artwork as a guide (a photocopy temporarily stuck to the lid is ideal), drill or ream a 16mm diameter hole in the box lid for the sensor and a 5mm diameter hole for LED1. Also drill out a hole in each end of the box adjacent to the terminal blocks suitable for the wiring entry to Apply power to the Bilge Sniffer and check that there is +5V at the output of REG1 with respect to the ground. Similarly, check that pin 8 of IC1 is around 12V. Before further use, the Bilge Sniffer should be left powered up for about 24 hours. This allows the heater inside the hydrocarbon sensor to burn off impurities that have collected onto the sensing element during manufacture and while in storage. The Sniffer can be tested after a few minutes of purging by waving an open bottle of air and petrol over the sensor. Just a couple of drops of petrol in an empty 2-3 litre milk bottle will be sufficient (but take care as this mixture can be explosive!). With VR1 set mid-way, exposure to this mixture should see the voltage across the sensor (measured at pin 3 of IC1a) rise to around 1V or so. Setting up The Bilge Sniffer should be set up so that the relay switches on when it detects even small amounts of hydrocarbons in the air. In this way, the The shaped PC board slots into the recesses in the vertical mouldings inside the jiffy box and is held in place when the lid is screwed on. siliconchip.com.au exhaust fan can be operated to keep the remnants of hydrocarbons to a minimum. First, set VR3 so that the ALARM wiper is at 200mV and set VR1 to its maximum (fully clockwise). This will provide maximum sensitivity for the sensor. Now set VR2 to mid setting and test that the relay switches on when the sensor is exposed to the milk bottle petrol vapour and that the relay switches off not long after the bottle is taken SILICON away (ie, when the sensor is exposed to fresh air). CHIP www.siliconchip.com.au Set trimpot VR2 more anticlockwise if the relay does not Fig.3: same-size artwork for the front panel and the PC board. The panel artwork is also useful as a template for drilling the holes in the lid for the sensor and LED. switch off. The common and normally K A open relay contacts are wired in series with the exhaust fan wiring so that the fans will run when these contacts close. For a belt’n’braces approach, a dot/bar NO separate switch can be installed in parallel with the relay contacts so C that the fans can be run manually NC before starting the boat’s engine. Note that each relay contact is rated at 5A, which should suit all but the highest-volume fans. There are two sets of contacts which can 05109051 be wired in parallel if more current than the 5A is required – or additional fans powered. Ignition cutout If you find that the exhaust fans a false sense of security and becomes run continuously, you may need to It has been suggested that the Bilge too complacent to actually check the increase the alarm level by turning Sniffer could be incorporated into bilges before starting out (every good VR3 further anticlockwise. This will the boat’s ignition circuit so that the boat owner should know the value of cause the fans to run less often. motor couldn’t even be turned over if the “nose test” before starting engines Trimpot VR2 may need to be turned – you should never rely on electronics hydrocarbons were present. further anticlockwise to reduce hysFrankly, we are in two minds about alone!). teresis and cause the fans to run for a If you do want to wire the Bilge this – especially in the “what if” deshorter time. But before you do this, Sniffer to disable the engine(s) on a partment. make sure the Bilge Sniffer is not trying What if the relay (or sensor) failed fuel vapour alarm, you would wire the to tell you that you have a problem! and you couldn’t start the boat when boat’s ignition switch (not the starter To run the fans for a longer time, set or the high voltage line!) via the “noryou needed to? VR2 more clockwise – but make sure What if the Bilge Sniffer sensed a mally closed” sets of contacts. that the fans will switch off when the This would have the motor powered fuel spill and operated correctly, cuthydrocarbon levels are reduced. ting the engines – but at the time the in the normal way but when hydrocarSetting VR2 too far clockwise will boat was, say, crossing a bar? What bons were detected, the relay would pull cause the fans to run all the time once in, opening the normally closed contacts would be the worst evil? SC the alarm level has been reached. What if the owner gets lulled into and shutting off the engine. BILGE SNIFFER NO C NC ALCOHOL/LPG SENSOR Resistor Colour Codes 1 1 1 No. 3 2 1 siliconchip.com.au Value 10kW 2.2kW 10W 4-band code (1%) brown black orange brown red red red brown brown black black brown 5-band code (1%) brown black black red brown red red black brown brown brown black black gold brown September 2005  41