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Ablution Solution

  • Fyko van der Molen
    Fyko van der Molen on

    In another thread in the Caravan section on this forum relating to Water Meters I made a brief reference to a method some caravan parks use to determine the contents of remote storage tanks. There have been a few private inquiries which I will address here rather than answer individually.

    There is the general case of a farmer who establishes a small resort on a remote section of his land. Being off the beaten track it does not have an electricity supply. The ablution facility has a water tank, filled by a solar pump, and two solar water heaters, one for the showers, and one for the scullery.

    Since there is a reliance on an energy source that is not entirely predictable, there is potential for anxiety over whether there is sufficient water being supplied to the tank, and whether the water to be heated is hot enough to satisfy the guests. Since the resort is some distance from the farm’s admin area it is not convenient to repeatedly go there and back to physically check the status.

    Our solution is this:

    A controlling device, the master node (MotherNode), is connected to the establishment’s wireless LAN which is centred on a standard internet router, as found in any home. This device is made from a Raspberry Pi Zero (the only version of Pi that is in plentiful supply), which host a Node-Red control flow. We generally supply a basic workflow framework that is then user-customised to suit the particular requirement. Some skill is required for this but it’s something anybody can manage given a bit of perseverance.

    We supply a slave node (SplitNode Base), connected to the same LAN, that feeds data to the master node and receives commands in return. The slave node has an LCD display attached (C-Node) that shows all current system parameters.

    The slave node communicates with its distant brother by low-power FM radio. At the remote end of the radio link is the slave node’s brother (SplitNode Remote) that receives input from the sensors at the ablution facility, and sets outputs in accordance with commands from the master node. The sensors include an ultrasonic scanner (or a load cell that can weight things), two heat probes, and 2 switch inputs.

    The SplitNode Remote, (which is powered by its own solar panel, charge controller, and 7Ah battery), can read the height of the water level in the tank using the U/S scanner, the temperature inside the storage of both water heaters by using the heat probes, and two opto-isolated  switch inputs. These switches can monitor the state of door or gates, or anything that comes to mind. It also has two relays with a 10A switch rating that are under command of the master node.

    Although the relay outputs are available to do the master node’s bidding, it is usual to control the pump by a float switch in the top of the tank, which starts the pump when there is even a slight drop in water level. This makes the relay outputs available for some other purpose, if needed.

    The radio link, depending on local conditions can be up to 5Km long if directional antennas are used. The signal strengths at both ends of the radio link are continuously reported to the master node.

    To disseminate this data over a wider area the master node continuously updates a repository on the establishment’s web domain (as determined by the user), from where it can be reproduced on any device connected to the Internet, using the device’s Internet browser – no special app is required. So the farmer can be running errands in town and simultaneously be aware of the state of his resort.

    Unlike some other IoT systems this does not delegate control over ones resources to some far away cloud data centre in China, or who knows where. Everything stays in-house and under own control.

    Josef Konrad
    Josef Konrad on

    What technical expertise do I need to get such a system working? I can drive a computer but only at a superficial level.

    Fyko van der Molen
    Fyko van der Molen on

    For me there are only 2 qualities needed to make a success of a project like this:

    • ·        A willingness if not an eagerness to learn new stuff,
    • ·        Sufficient perseverance to keep going even though things don’t immediately work as expected or as advertised.

    A good place to start for many people goes like this:

    • ·        Buy a Raspberry Pi Zero from somewhere like They have hundreds in stock.
    • ·        If it’s your first time then also buy the bundle that contains: housing, power adapter, keyboard, mouse, and SD card. (The keyboard is cheap and nasty but good enough to start with). For a display you can use a small flat screen tv the has HDMI capability.

    There are several reasons for using Pi Zero:

    • ·        They can do the job,
    • ·        They’re available,
    • ·        They have built-in WiFi capability,
    • ·        They can run from a 2A power regulator.

    Once you have the OS setup install Node-Red as a system service. Raspberry Pi is very well supported by a variety of forums and special interest websites. You can find everything you need online if you look for it. Working with Pi and Linux is vet command line intensive, so keep a notebook to hand so you can jot down the line commands that work for you.

    Aside from the LCD tv everything above will cost less than R1000.

    Forever Power Supply. (Another R1000 expense).

    All models of Pi are famously intolerant of power glitches – they need clean, unbroken power or they will crash spectacularly often shredding the SD card on the way down. You make a Forever power supply out of a 12V 7Ah gel battery (ubiquitous in the burglar alarm industry), a 13,8V float charger from the Chinese shop, a 12V to 5V buck regulator, and a suitable housing. (A buck regulator is better than 95% efficient whereas a series regulator will waste about 60% of delivered power.)

    With this the 5V output will be serenely unaware of any load shedding shenanigans taking place at the input. The capacity of the battery is sufficient for longer than anything ESKOM will throw at you, but if it looks like multiple days of blackout you have time to shut the Pi down in an orderly manner.

    You now have a pretty bullet-proof master node.


    Read whatever you come across that explains about DNS, TCP/IP and UDP. This actually applies to most Internet users – you need to know where the engine sits in your car without needing to understand every gismo under the bonnet. Not essential, but it all helps to see the bigger picture.

    There are two types of people giving advice in the IT industry – The Simplifiers and The Complicators. The Simplifiers will try to help understand something, and the Complicators will try to confuse you in order to aggrandise themselves. They’re not difficult to tell apart.

    Josef Konrad
    Josef Konrad on

    I do have a farm, but with no campsite is on it as yet.

    I do also have a borehole with a sunpump and 5000L tank that’s about 2.5Km from the house, but it’s used for watering livestock. It was a windpump with a brick wall dam until a few years ago.

    Is there much power required for the radio link? And can I stop the radio draining the battery at night?

    Fyko van der Molen
    Fyko van der Molen on

    Keeping the tank (or last in the series of tanks if there is more than one)topped up is normally a function of a float switch, which acts directly onto the pump’s switchgear, and requires no other intervention – the tank is always near full but never overflows.

    When the tank is not kept topped off, and the water level drops faster than the pump can raise it, is when notification to the outside world is needed, for possible corrective action – either supplementing power, restricting outflow, or something else.

    The newest addition to the NodeStar family is the TextNode, which has the ability to send an  SMS text message, as defined by the user, to a user-designated number, when something important happens. Whatever events that trigger notification text messages are determined by the user, as are the destination phone numbers, and the text content itself.

    A load-shedding power failure might trigger a message to the housekeeper, for instance, while the fact of the generator’s diesel tank falling below 30% full could initiate an SMS to the maintenance manager. Any condition of any input can be a trigger – the user decides.

    There is a cost implication in sending these text messages since SMS credit must be purchased from the mobile phone network operator, but this disadvantage is offset by the convenience and countrywide coverage of the SMS system. If a target phone is out of range, for instance, the network holds the message until such time as it can be delivered – messages are never lost.

    Radio Power Saving

    The LoRa radio transmitters that we use radiate only 50mW of antenna power – about the same as a small LED, which is too small to justify the complexity of switching it on and off – actual transmission time, when energy is radiated, is also a small fraction of the whole.

    The very impressive range that such a low transmission power can achieve, up to 5Km, is due to the high gain of the antenna design we employ. These antennas can be easily made, according to exact calculated dimensions, by any handyman, using only common hand tools, in less than an hour. The cost of materials per antenna is less than R50, being a 3 metre length of single strand house wire, a metal grid, and some off-cuts of plastic conduit tubing.

    The directional bias of these antennas increases their gain by a factor of more than 100 over conventional omnidirectional dipoles.

    A picture of such an antenna can be found at



    Josef Konrad
    Josef Konrad on

    How does it detect load shedding? Does it have some kind of voltage meter?

    Fyko van der Molen
    Fyko van der Molen on

    A voltage transducer would be required to read the actual line voltage but, since this is an all or nothing power situation, we can use a simple relay with a 240VAC coil. When the power fails (or restores) the relay drops out (or energises)  and the resulting contact change signals the new state of the power, and a message is generated accordingly.

    In Cape Town you often get reprieves from the scheduled power cuts, but not always, so a lot of folks that work outside of their load shedding area like to get informed of the state of power back at the casa.

    The beauty of SMS texts is that they reach you wherever you are. If you’re in the Huguenot Tunnel when the power goes off the SMS will be waiting for you when you emerge.  Also you don’t need to run any special app to get the info, and you have a written record of all of the power cuts that happened, until you physically delete them.

    This is a screenshot of a typical SMS stream


    Josef Konrad
    Josef Konrad on

    How about sniffing for carbon monoxide gas in a workshop? Can it handle that?

    Fyko van der Molen
    Fyko van der Molen on

    We use the MQ range of gas detectors to sniff out carbon monoxide (CO) with MQ7 and propane/butane (LPG) with MQ5.

    I’ve not been able to find what the letters ‘MQ’ derive from, the best guess is that they represent the Chinese words for ‘sensitive’ and ‘gas’; possible since all these detectors come from China.  The entire range works on the same principle: a ceramic substrate is given a thin coating of tin dioxide (SnO₂) and heated by a local heating element. The normally very high resistance of the tin dioxide coating is reduced when it comes into contact with a combustible gas (CO and LPG are both combustible). This analogue output is digitised by circuitry designed to change the output from FALSE to TRUE when a determined analogue value is reached.

    Though they all work on the same principle they are optimised for the particular gas they are intended to detect. So an MQ7 will also detect LPG, but with reduces effectiveness, and an MQ5 will detect higher concentrations of LPG.

    LPG and CO have very different characteristics but are both dangerous for very different reasons. Carbon monoxide is generated by incomplete combustion and is highly toxic at concentrations above 100 ppm (parts per million). The gas is colourless, odourless, and tasteless. It has a similar density to air and will mix with air at all levels. It will kill you quietly while you sleep.

    LPG will accumulate at floor level because it is much denser than air, it stinks terribly so your nose will detect it well before the MQ5 does and you’ll take action accordingly. But at concentrations between 23,700 ppm and 95,000 ppm it will auto-ignite and demolish your caravan as well as those of your neighbours while you are relaxing on the beach.

    MQ detectors are covered by a fine mesh that is intended to prevent flashback, as pioneered by Humphry Davy’s safely lamp back in the 19th century. They should be mounted where the gas they seek is most likely to be found, i.e. near the floor for MQ5 (LPG) and at nose level for MQ7. The gas can only be detected when it touches the tin dioxide element, so there should be free circulation around the detector to give it quick access to the gas.

    When we monitor a gas detector with an intelligent node it is for information only. Any action to be taken in response to the presence of gas should not depend on long communication lengths and decisions made by algorithms – the response should be local, immediate and failure-proof. One of our most popular caravan products is a simple, dumb, and fool proof 2-detector ( 1 MQ7 and 1 MQ5) alarm system that sounds an un-mutable buzzer. Since there is no reason for gas to be present, so there is no decision to be made – an instant and totally dependable alarm must be sounded, your life could depend on it.


    Josef Konrad
    Josef Konrad on

    How long can the cable to the antenna be?

    Fyko van der Molen
    Fyko van der Molen on

    As short (and as fat) as possible.

    All antenna cables incur signal loss proportional to their length. Thinner cables lose more than thicker ones, despite having the same impedance. Impedance increases with transmitted frequency and does not depend on cable thickness. Signal attenuation increases as the inner and outer conductors get closer together. As frequency increases the cable diameter must increase also to avoid crippling signal attenuation.

    Thinner cables are a lot easier to use – they are light, flexible, and easily terminated, but they impose a harsh penalty in signal attenuation. On the one extreme you have RG174 which is less than 3mm in diameter but loses 105dB per 100m at 850MHz and, on the other end of the scale LMR-900 which is 22,5mm in diameter but loses only 5,6dB per 100m at that frequency. Somewhere in between you get RG6, the traditional TV cable, rated at about 18dB per 100m @ 850MHz.

    So if your minimum practical length is only a metre or two use RG174, if it’s up to 10 metres use RG6, and if it’s longer than that consider rearranging your equipment layout because the heavy cables are very expensive and very difficult to route and terminate.


    Josef Konrad
    Josef Konrad on

    What is the range of these gas sensors?

    Fyko van der Molen
    Fyko van der Molen on

    Gas sensors don’t detect remotely – they detect in the layer that touches the heated tin dioxide element. From its source the gas diffuses through the surrounding air and, on reaching the detector, penetrates the two layers of fine mess covering the sensor. The purpose of the mess is to prevent a flash back to the open air should the heated detector ignite the gas that it is sensing.

    The maximum auto-ignite concentration for propane is 9,5%. Assuming the propane/air mixture is at 9% there will be about 90 grams of gas in every cubic meter of air at Highveld altitude. The calorific value of 90 grams of propane is 4,5 megajoules which, if burned entirely will increase the temperature of that cubic metre of air by 4500 degrees C – say to 4525 C. The pressure of that air will then increase from less than 100KPa (on the highveld) to 1,1MPa, or 11 Bar, Ask anyone in Loveday Street what that feels like.

    You can place a gas detector either in the place where it is likely to be coming from, or in the place where its effect will be most felt. Since there is absolutely no valid reason for propane to be loose around the caravan or tent, you should place the sensor low down near the floor (propane is nearly twice as dense as air) and close to the distribution hoses, be it for the fridge or cooker or geyser. Pure propane is odourless as well as colourless but the impurities it carries and the butane mixed into it means it really stinks.

    Carbon monoxide on the other hand is legitimately generated in small quantities during normal combustion. So placing the sensor in the flue duct will generate a lot of false positives. Outside of the flue duct however there should be no CO in any measurable concentration, so you could site the sensor in any place that people are likely to breathe it – probably closer to the likely source than further away. The main thing is to protect people while they sleep.

    CO has a similar density to air so it can be found equally distributed between floor and ceiling.  It will auto ignite at concentrations above 2,5%, which is 25,000 ppm but, since it is toxic above 100 ppm, you’ll be in the happy hunting grounds due to poisoning long before it blows you up.

    Remember that the gas has to drift to the sensor’s location and then penetrate the mesh before it can be detected.

    Josef Konrad
    Josef Konrad on

    I see there’s a flow meter available now.

    Would that be more suitable for the water tank? seeing as I get some spurious readings with the depth sensor.

    Fyko van der Molen
    Fyko van der Molen on

    The two factors most likely to confuse the ultrasonic echo sounder when measuring tank depth are:

    •  Echoes received from the sidewall of the tank,
    •  Reflections from splashed water droplets.

    Both become more prevalent as the water level in the tank becomes lower. To avoid splashing, and also because it’s more energy-efficient, the tank should be bottom-filled. This also stops the sediment from being churned up. If bottom-filling is not possible for some reason, then a downpipe should be used inside the tank to lower the water in a controlled fashion. To stop potential siphoning a small vent hole can be drilled in the top of the downpipe.

    Due to a general reluctance to cut a hole in the top of the tank the ultrasonic sensor is often mounted on the tank’s lid. When that lid is not in the centre of the tank top but near to the edge then it is possible for the sonar pulse to reflect off the inner wall of the tank before the water’s surface, and so give a false measurement. To counter this one can extend the barrels of the sonar detector as shown, or you can tilt the sensor to aim at the centre of the tank’s floor; or both.

    This should all have been explained to you by the distributor.

    In the caravan context we are forced to derive the tank content remaining from the measured outflow because the shape of the tank (broad and flat) precludes any other method of doing so.  It’s a relatively small matter for the user to zero the outflow count when the tank is filled, and thereafter it is possible to report the remaining contents with a high degree of accuracy, either on the screen of a phone or on a dedicated display unit.

    The flow counter that you refer to in the NodeStar system was added at the request of a major international user who wanted to distribute specific quantities of liquid nutrient into a trough at a remote location several times per day. Since the flow count can be reset programmatically the whole process can be automated with all the variables controlled from the home office.  It was not intended to compete with or replace the ultrasonic echo sounder.

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