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I'm a plumber and I'm not able to find a commercial product that fits my needs, so I'm wondering if it would be feasible to build it from scratch... I'm familiar with basic understanding of electronic circuits, but cannot find an answer by myself.

I'm designing a heating system powered by a fireplace, and a big hot water tank to store heat for use when fire is off. Heat will be usable in the 55-80°C range, and the water is expected to be fairly well stratified, so temperature sensors placed at different heights would give a good figure of the energy content of the tank.

When fire is on, boiler will push water at 80°C into the top, so the tank will start to "fill up" from top to bottom. When water at this temperature reaches, let's say bottom quarter, the operator needs to be warned that the tank is "full" to stop feeding the fire. If water at 80° reaches the bottom, water out of the boiler will be at a higher temperature, which is undesirable.

On the other hand, when the fire is off, the heating circuit will push water at about 60°C into the bottom. When all water in the tank is at this temperature, it is "half full", and it will take this water again down to about 45°, at that time the tank is "empty". The operator should know the amount of usable energy in the tank with a look to plan lighting the fire again.

So, I need a way of displaying the measurements of the sensors in an easy to read form, accuracy is not important. I was thinking about converting it to a voltage that could be shown by an analog voltmeter with a custom drawn scale, but I have no clue how to sum up the results of the multiple sensors. Other suggestions are also appreciated. The system just has to be easy to implement, reliable and easy to read.

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    \$\begingroup\$ You need to supply information about your sensors including type and number. \$\endgroup\$ – Barry Nov 20 '19 at 21:03
  • \$\begingroup\$ Do you need just 5 thermometers and 5 gauges or do you want to combine the N temperatures into a single number? \$\endgroup\$ – filo Nov 20 '19 at 21:05
  • \$\begingroup\$ If there's a pump anywhere in your system, just actively circulate water. That'll keep it stirred, and the temperature in the tank will always be fairly constant throughout. Then you just need a single thermometer. Even though I'm an engineer, and I tend toward geeky complicated systems, I'd be tempted to just use a plain old mechanical thermometer (or thermostat, if you need to take some action when the temperature hits a threshold). \$\endgroup\$ – TimWescott Nov 20 '19 at 22:13
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    \$\begingroup\$ Decide which temperature sensors are appropriate for the application first. They don't all behave the same way and thus their outputs aren't processed or averaged the same way. For example, thermistors, thermocoupoles, RTDs if you want to even go with an electronic temperature sensor. Don't forget to look at the signal conditioning circuitry for each one...based on that alone you may decide it's not worth the effort to use that particular sensor. \$\endgroup\$ – DKNguyen Nov 20 '19 at 22:16
  • \$\begingroup\$ In addition to all the advice of DKNguyen, you need to decide on the physical packaging. In industrial temp sensing there is a wide array of packages/implementation including sensors in stainless steel tubing, sensors in NPT fittings, etc. Have you thought about what packaging you want? Just as an example, follow this link: omega.com/en-us/search/?text=temperature+sensor \$\endgroup\$ – xstack Nov 21 '19 at 4:18

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