SPN-17 (Instrumentation): Minimalist heater/pump control system

“Instrumentation,” as a category, includes not only what one would expect— devices that gather information about temperature and similar parameters— but as well, devices that control, such as relays that turn on the pump for the HEx (heat exchange) units, or the electric heater.

In the case of [The_Cube], the situation is made more complex by the fact that we have two instrumentation systems, one developed (the “minimalist” system), and the second a more complex work-in-process that we will describe in another post.

The “minimalist” system (larger picture here), is about as simple and low-cost as we feel comfortable designing. It consists of a heater coil and a fountain pump, at its core, and these are supplemented by devices which detect when the heater and pump should be turned on or off. But note that while this system reacts to circumstances to keep the digester warm, it does not provide any information externally. It will not tell you, in and of itself, what the temperature of the digester is, for example, nor if the water level in the sump is low.

In this system, the heater (A) is plugged into a GCFI outlet (B; one where either plug can be independently powered), so of course when the proper plug in the outlet is powered, the heater is on. That plug is controlled by a relay (C; which has a heat sink under it, not shown), and when 12v is supplied to its control side, it will supply 110v AC on its supply side. Then further, there is a cascade of devices that each has a role in supplying that modest, controlling voltage:

• The first is a 12v power supply (D), which is connected to
• a float valve (E), that will be off if the sump does not have enough water. The float valve is connected to
• an over-temperature switch (F), which is usually on, but which will turn off if it encounters a temperature of 45°C (or more) inside the heater/pump container, because that indicates that the pump is not working. (The water heats up but has nowhere to go.) The over-temperature switch is connected to
• the first PID controller (G), which listens to a temperature probe (H) inside the digester. The PID is set such that if the digester temperature is too low, it will turn its switch on, sending the 12v current to
• the relay (C) we started by mentioning. which is connected to the 12v power supply (D), completing the circuit. When this whole squence completes successfully, power is sent to the outlet (B), and the heater coil (A) gets powered up, warming the water inside the heater/pump container.

Once the heater is on, the drama shifts to the pump. My thought was that the heater/pump container might well be filled with cool or cold water, because the sump is not as well insulated as the digester. So when the heater goes on, it takes a little time for the water in the heater/pump container to get warm enough that can (should) be sent into the digester. Otherwise, when the heater is turned on, and if the water is too cool or cold, that too-cool water is sent into the digester, cooling it down. That seems backwards to me, so this design prevents that.

To return again to the point, when the heater is turned on, the pump is not yet on. What happens then?

• There is a second temperature probe (I) inside the heater/pump container (complementing the one inside the digester). When that second probe registers that it has hit the temperature to which the second PID (J) has been set, it turns on the pump (K). Then, and only then, the digester is heated by the system just described.
• And by the way, the level of water in the sump is kept at an established level by the float valve (L) of the sort often used in water tanks for livestock.

Below is a table showing the parts described above. Note please that the listed parts have not been chosen based on lowest cost, best suitability, or any similar attribute. The table is best thought of as a means of being specific about parts that could be used, if you wish. But here’s the thing: the whole design can be seen as highly generalized. That is, it is by no means required to purchase Inkbird ITC-1000 PIDs, because there are many PIDs that would do the same job. The same thing is true of the GFCI plug, heater coil, and indeed, as regards this design, any and all of the parts listed. As such, the parts listed in the table could be used, if you wish, or if you want to replace them or if you want to save money, then you should do your own research, taking this as a point of departure. SPNs are provided (second column) for those who have plans for [The_Cube].

Note also, if you followed the link regarding PIDs (above), that these devices are fairly smart with regard to turning things on and off. What happens with really simple devices is that they turn on until a given target is hit, then turn off. But meanwhile, the inertia of the system is such that things continue to heat up. The PID not only responds to the fact that something is too cold and needs to be heated, but it also pays attention to the slope, if you will: how fast is this thing heating up? Then it modifies the power it switches so that, the slope moderates. This is similar, really, to the way we might take a right turn when driving. We do not approach the turn at full speed, but slow down as we approach so that we can make the turn without some adverse incident, like fish-tailing or crashing across the median.

I note as well that there have been many reports of the SSRs being manufactured at low quality by copycatters, businesses that make a copy that looks very much like the original. The great majority of these copycatters, however, have poor quality control, and so while you might save a few dollars buying from them, the probability is that your new SSR will fail, or even be dangerous (failing in the “on” position could mean that your heater coil will stay on, potentially setting your digester on fire).

There is one final thing to mention. Using hot water, as we do, to heat a small digester can be a more expensive option than heating more directly with electricity, for example by installing a hot water heating element directly in the digester. (I will be writing another blog post about this option.) The best reason for heating with hot water is that it preserves the options in terms of heat sources, since one can use electricity, solar, or biogas as a source to heat up the water that is then circulated in the digester. By contrast, using a heating element directly in the digester is cheap and effective, and if you will never want to consider using, say, solar hot water to heat the digester, then it may be a good choice.