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Addendum: improving the pressure sensor As detailed in the last two articles, the Water Tank Level Meter uses a pressure sensor to measure the water level. Here’s a few tips on improving the set-up plus an improved method for mounting the pressure sensor externally. A S ORIGINALLY DESCRIBED in November 2007, the Water Tank Level Meter used a pressure sensor that was mounted inside the case (ie, on the PC board). This sensor was connected to a tube that was then inserted into the tank, with one end close to the bottom. The resultant air pressure within the tube thus provided a measurement of water level. An alternative method was subsequently described in the December issue and this involved mounting the pressure sensor in a sealed box at the bottom of the water tank. The electrical output from the sensor was then fed back via a cable to the Level Meter. Since publication of these articles it has come to our attention that the “tube in tank” method is only valid for short-term water level measurements. Unfortunately, the measurements will become inaccurate after an extended period of time. This is due to some diffusion of the air into the water, resulting in loss of pressure. As a result, our first measurement technique (ie, where the sensor is mounted on the PC board) is no longer recommended for long term monitoring. By contrast, the in-tank sensor measurement technique described in the December article is suitable, because this is not affected by pressure loss due to the diffusion of air into the water. Making it better Assuming that you do mount the pressure sensor inside the tank (see pages 86-87, Dec. 2007), there are a few things you should do to improve reliability. First, a short squirt of silicone water repellent (eg, Selleys Water Shield) should be directed into both sensor ports, to improve water protection at the sensor’s gauge. In addition, a few drops of mineral oil should be placed in the tube, so that an air pocket and oil trap is formed just above where the tubing is clamped to the box. This is to prevent direct contact between the sensor and the water at port 1. Note that there should be a small amount of air left between port 1 and the oil. The oil repellent action of the silicone spray is also helpful here. Note that mineral oil is available from pharmacies as baby oil. Water in the vent tube One problem is that the vent tube for port 2 may ultimately contain water in the lower portion of the “U”-shaped section in the sealed box at the bottom of the water tank. This is due to water condensation from the air and if enough water condenses to close off the tube, this will lead to inaccuracies in the pressure reading due to incorrect pressure at port 2 with barometric changes. As a result, if the water level readings appear to be inaccurate, the con- This external sensor assembly is designed to connect directly to the outlet at the base of the water tank via a T-piece. Refer to Fig.1 for the assembly details. siliconchip.com.au January 2008  89 Fig.1: follow these diagrams to build and install the external pressure sensor. As shown, the sensor is mounted in a waterproof box and connects to the tank’s outlet via a T-piece made up using a metal tube and a cable gland. Don’t forget to drill a small hole in the underside of the box, so that atmospheric pressure is applied to port 2 (P2) of the sensor (see text). densation will have to be drained from the tube. To do this, it’s simply a matter of removing the sensor assembly from the tank and tipping the water out. Having said that, this degree of condensation is unlikely to occur except in very humid climates. If necessary, the effect can be minimised by placing 90  Silicon Chip a water-absorbing desiccant (eg, silica gel) within the tube. Alternative sensor placement Another recommended technique for water level measurement has the sensor located outside the water tank. This arrangement is shown in Fig.1 and involves connecting the pressure sensor input directly to the outlet connection at the base of the water tank. The main advantage of this scheme is that because the sensor and its wiring are now located outside the tank, there is no need to fully seal the inside of the box. siliconchip.com.au Parts List 1 IP65 sealed enclosure 64 x 58 x 35mmmm (Jaycar HB-6120 or equivalent) 1 31 x 26mm sheet of 18g Aluminium 2 3-6mm waterproof cable glands 3 M3 x 6mm screws 2 M3 x 20mm screws 4 M3 nuts 1 4-way pin header 1 50mm length of 4mm PVC tubing 1 4-way sheathed cable (length to suit application) 1 set of fittings suitable for water tank 4mm tubing connection The connection to the tank’s outlet can be made using a “Tee connector”. This can be obtained from an irrigation supplier (eg, a T-piece as used for drip irrigation) or you can fashion your own fitting. An alternative is to use a small metal tube inserted into a metal tap fitting, which would then accept the PVC tubing from the sensor. This metal tube can be brazed, soldered or glued in place. If you are using 25mm or larger poly pipe at the tank outlet, then a T-piece can be made by first drilling a hole in the side of the pipe close to its end, to accept a 3-6mm cable gland. This hole needs to be positioned close to the end so that access is available to flatten down the tube at the mounting area and to provide access to the gland nut inside the tube. The seal between the gland and poly pipe can be improved by using an ‘O’ ring (as shown in Fig.1) or by using a silicone sealant that’s suited for wet areas (or you can use both). A 3mm OD metal (or hard plastic) tube then needs to be placed inside the PVC tube so that the gland will not close off the PVC tube when it is tightened down. Metal tubing this size can be salvaged from a telescopic antenna. As shown, the sensor is mounted on a small aluminium plate within a sealed enclosure. This baseplate is made up using sheet aluminium measuring 31 x 26mm and is attached to the two central internal mountings posts using M3 x 6mm screws. The sensor itself is attached to the baseplate using siliconchip.com.au two M3 x 20mm screws and M3 nuts. As shown in Fig.1, the port 1 connection comprises a 3mm PVC tube that connects to the T-piece in the water tank’s outlet. Port 2 vents to the atmosphere. A 4-way cable (eg, telephone cable) is connected to the four sensor pins and exits from the top or side of the box through a cable gland. Note that you must orient the sensor so that port 1 is connected to the tubing. As shown, the sensor is mounted with pin 1 (the notched pin) to the left. Make a note of the wire colour used for each connection to the sensor. At the other end, this wiring connects to the socket in the Water Tank Level Meter via a 4-way pin header connection. Make sure that this connection is made with the correct orientation and don’t get the wiring mixed up. Making it water-tight Note that if you are using a flat 4-way cable, it will not form a watertight seal within the gland. This can be fixed by applying a small amount of silicone sealant around the wire at the entry and exit points of the gland, so that the box is waterproof. In addition, a small hole must be drilled in the box to allow the air pressure to vary inside the box for the sensor’s P2 port. This hole should be drilled so that it is in a bottom panel when the box is mounted in position, to keep water out. A hole size of just 1.5mm is all that’s required. The same recommendations we made above for the in-tank sensor installation also apply here. These are to improve reliability from the sensor – ie, use a squirt of silicone water repellent into both sensor ports and place a few drops of mineral oil in the tube so that air and oil can then be trapped just above where the tubing is clamped in the gland. As before, the oil is to prevent direct contact between the port 1 sensor and the tank water. Don’t overtighten the cable gland for the port 1 tube – you don’t want to close off the tube completely. Mounting it in place The box can be mounted on the side of the tank, so that the port 1 tube sits vertically. This keeps the mineral oil floating on top of the water. Note that it’s important to keep the port 1 sensor input as low as possible, so that it sits just above the tank’s outlet. That way, the full range of the tank can be measured. Also, try to prime the tubing with water up to the gland before attaching to the tank T-piece, so that the initial calibration will be correct. If there is too much air, you may need to recalibrate after the air has diffused into the water. Note that the box has two mounting points that are effectively outside the box’s sealed section but are still covered by the lid. So to access the mounting holes, you have to remove the lid. The unit can be mounted on brackets or directly onto a wall or the tank. To mount it to the side of the tank, first mount two lengths of 19 x 19 x 70mm hardwood, spaced apart to match the box’s mounting holes. This timber can be secured to the tank using builder’s adhesive or silicone sealant. The box is then attached using suitable wood screws into the timber (but not so long that they can penetrate the wall of the tank). As with the in-tank sensor, temperature compensation is not required and the unit should be left at the zero compensation setting. As for the Water Tank Level Meter itself, this can be mounted in any convenient location, even if it is exposed to sunlight (since the temperature sensor no longer has to correct for the air temperature in SC the tube). Looking for real performance? • Learn about engine management systems • Projects to control nitrous, fuel injection and turbo boost From the publish ers of systems • Switch devices on and off according to signal frequency, temp­erature & voltage • Build test instruments to check fuel injector duty cycle, fuel mixtures and brake & temperature Intelligent turbo timer I SBN 0958522 94 Mail order prices: Aust. $A22.50 (incl. GST & P&P); Overseas $A26.00 via airmail. See www.siliconchip.com.au for ordering details. -4 TURBO BO OST & nitrous fuel cont 9 78095 8 522946 $19.80 (inc GST) NZ $22.00 (inc GST) rollers How engine management works January 2008  91