jondallimore
Posts: 251
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Temp sensor for Water Tank

Wed Mar 25, 2020 7:13 pm

Hello,

Looking for recommendations. I have a Pi 3 turning my immersion heater on and off via a contactor and relay board.

I would like to add a temperature sensor to measure the water temperature, probably by mounting the sensor against the copper tank where the water exits, and adjusting for the difference in temp at the outlet and at the bath tap itself.

Can anyone recommend a suitable sensor that is very small as I will need to insert it into a narrow gap between the insulation and outlet pipe.

thanks
Jon

PhatFil
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Re: Temp sensor for Water Tank

Wed Mar 25, 2020 8:34 pm

can you be specific about the size limitations. a small option would be a basic thermocouple since its just 2 dissimilar metals joined together. also small would be a pt100 or pt1000, or ntc thermisters these are available in some tiny sizes. however all these will require additional circuitry.

if small enough you could use a ds18b20 which would only require an additional 4k7 resistor. which would be ,my #1 choice.
Image

another option not requiring a hard wired connection could be a ble enabled bbq thermometer?? if following a standard ble format you should be able to pick up its advertised readings from your pi and such thermometers have their sensors placed at the tip of a sharp pointed probe which can fit into very tight gaps..

you can allways cut insulation away and replace it afterwards (bubblewrap and mylar used in alternate layers is one of the most effective budget insuklation options available 11/12mm (4 ply) equates to 50 or 75mm of expanded foam.. and mylar wraps crunchie bars :)

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neilgl
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Re: Temp sensor for Water Tank

Wed Mar 25, 2020 9:00 pm

+1 what Phil said and I tape my DS18B20 sensor to the copper outlet pipe close to the top of hot water tank. Enable the “1-wire” interface on the pi to read the sensor using python (or other).

jondallimore
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Re: Temp sensor for Water Tank

Wed Mar 25, 2020 9:09 pm

Hmmm... theoretically the DS18B20 is waterproof, so it could be dangled inside the tank. However I wonder how long it would last under water with regular heating and cold water flooding when the tank refills.

Something similar on the outside that doesnt need to be waterproof might work. There are no leaks, I put the tank in myself, its sound

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rpdom
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Re: Temp sensor for Water Tank

Wed Mar 25, 2020 9:28 pm

jondallimore wrote:
Wed Mar 25, 2020 9:09 pm
theoretically the DS18B20 is waterproof, so it could be dangled inside the tank.
The DS18B20 sensors are commonly available sealed in a metal tub to make them properly waterproof.
Unreadable squiggle

jondallimore
Posts: 251
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Re: Temp sensor for Water Tank

Wed Mar 25, 2020 9:32 pm

waterproof to a point. theres a seal in there somewhere, and any seal thats continually temperature cycled will eventually fail.
However, Ive just found 5 of them for £10 on amazon, so I may get those and use them elsewhere too. They would be fine outside I expect in the rain, and probably ok in the fridge if I can be bothered to do that eventually.

jondallimore
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Re: Temp sensor for Water Tank

Wed Mar 25, 2020 9:34 pm

from the pictures I can see, its a metal tube case with a bit of neoprene shrinkwrap around the join between the wire and the case. I would hope theres an o-ring seal inside there otherwise theres no chance it will remain waterproof when heated and cooled

PhatFil
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Re: Temp sensor for Water Tank

Wed Mar 25, 2020 9:46 pm

rpdom wrote:
Wed Mar 25, 2020 9:28 pm
jondallimore wrote:
Wed Mar 25, 2020 9:09 pm
theoretically the DS18B20 is waterproof, so it could be dangled inside the tank.
The DS18B20 sensors are commonly available sealed in a metal tub to make them properly waterproof.
even so i wouldnt place a 'naked' probe in liquid..

when using ds18b20 sensors in my microbrewery alongside pt100 sensors i use 2 methods:

#1 a thermowell a section of tube with its end plated off protruding into the vessel containing the liquid to be monitored allowing a probe to be inserted into the vessel.

a 15mm tank connector and some 15mm 10mm tuve a reducer and endcap can make one for about a fiver or you can buy one for £10-15..

Or if using a probe with a ss sheaf a sealing gland around the sheaf body can also work.

the weakpoint is the sheaf end and cable joint so just dont let that near liquid . It will fail eventually!.

here is a naked probe sealed into the pot wall with a gland in a brew kettle (right hand side under the recirc tube elbow.

Image

while here is a copper pipe/tank connector diy thermowell in a plastic bucket hlt

Image[/url]

LTolledo
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Re: Temp sensor for Water Tank

Wed Mar 25, 2020 9:59 pm

I bought these a few weeks ago..... for a different project that I have yet to start....
DS18B20 waterproof sensor.jpg
DS18B20 waterproof sensor.jpg (32.17 KiB) Viewed 387 times

these were "marketed" as waterproof.... but my project doest not require on immersing it in liquid...
it may be waterproof up to a certain depth (am only assuming up to 1 meter)

additional water proofing may be done with careful and even application of silicone sealant on the mating parts
Last edited by LTolledo on Wed Mar 25, 2020 10:01 pm, edited 1 time in total.
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achrn
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Re: Temp sensor for Water Tank

Wed Mar 25, 2020 10:00 pm

jondallimore wrote:
Wed Mar 25, 2020 9:34 pm
from the pictures I can see, its a metal tube case with a bit of neoprene shrinkwrap around the join between the wire and the case. I would hope theres an o-ring seal inside there otherwise theres no chance it will remain waterproof when heated and cooled
I endorse DS18B20

There are different quality ones. I prefer the ones that have the sensor potted in a suitable resin within the stainless tube. I have a number of them in under-cover outdoor environments (three genuinely outdoors, a couple in unheated unsealed buildings, a couple in my garage). But I wouldn't put one inside a hot water tank.

I don't trust the ones that have plastic sleeving at the join to be waterproof - I think they are an 'enhanced robustness' option, but not a watertight.

For a hot water tank a sensor in a thermowell works much better than contact on the tank surface, in my experience - faster, more consistent response to tank changes. (Not a useful comment if your tank doesn't have a thermowell, I know).

For a pipe temperature then I cable-tie them to the pipe inside the lagging and it works quite well. It is worth putting thermal grease (or presumably some other suitable material - a heatsink gap filling pad probably works) between the sensor and the pipe. When I mount one to a pipe using thermal grease it gives a few degrees higher reading than done dry. It surprised my how much difference it made when I tested it (simply by mounting two adjacently on the same pipe, one greased, one dry).

jondallimore
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Re: Temp sensor for Water Tank

Wed Mar 25, 2020 10:55 pm

Agree with the thermowell idea. But don't have one and don't want to drill the tank to put one in cause I know the bugger would never seal. There is one in the immersion heater but that's for the main tank thermostat which needs to stay. I might see if there's enough space alongside it but not optimistic.
I think best option is the 5 sealed with shrink-wrap on Amazon, and insert next to the outlet pipe wrapped up with the pipe lagging, then test against water temp at the tap and establish an offset to adjust it in python if it's consistently off by more than a few degrees.

achrn
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Re: Temp sensor for Water Tank

Thu Mar 26, 2020 8:28 am

jondallimore wrote:
Wed Mar 25, 2020 10:55 pm
Agree with the thermowell idea. But don't have one and don't want to drill the tank to put one in cause I know the bugger would never seal. There is one in the immersion heater but that's for the main tank thermostat which needs to stay. I might see if there's enough space alongside it but not optimistic.
I think best option is the 5 sealed with shrink-wrap on Amazon, and insert next to the outlet pipe wrapped up with the pipe lagging, then test against water temp at the tap and establish an offset to adjust it in python if it's consistently off by more than a few degrees.
Yes agreed - I wouldn't fit a thermowell just for this - my cyclinder had a spare one.

I don't used the steel cased ones for indoor dry deployemnts - all the ones on my heating system are just teh bare plastic package type (that look like a small transistor, as in teh second post in teh thread). I dasiy-chain them using cat5 cable, mostly.

If you have long sensor strings, get a proper driver. The pi will handle a short cable with a few sesnors on it, but if you have lots on a long, unruly bus it's not so reliable. I use DS2482-based drivers. There are several suppliers of Pi boards that have these.

jondallimore
Posts: 251
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Re: Temp sensor for Water Tank

Thu Mar 26, 2020 8:36 am

Only considering the 5 steel ones as its a fairly good deal and I can then use them for other places, like in a fridge with condensation and outdoors in rain. I can always strip them apart if needs be.

Not going to daisy chain, just use one on the tank.

richrussell
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Re: Temp sensor for Water Tank

Thu Mar 26, 2020 10:11 am

I've got five DS18B20s on my heating setup (coal/wood boiler in kitchen with tank in bathroom airing cupboard), they're simply cable tied to the copper pipes then I've covered them with a layer of aluminium tape to protect them against being knocked and also help keep the heat in. I'm measuring temperature in and out of the boiler for the hot water and central heating pipes, as well as one about half way up the tank, which is stuck to the side of the copper tank with aluminium tape, then the square of insulation I cut pushed back in over the top.

I run a fairly simple bit of code to make a decision as to whether to run the radiator pump or not, as that's all the control I have over things, the boiler can't really be turned on and off (well, not without firelighters and kindling). OK, so I'm using an Arduino Nano not a Raspberry Pi, but the concept is the same.

The only issue I found with my method of attaching it to the tank is that the temperature at the top of the tank is often much higher than it is lower down, so you might need to consider where you mount your sensor depending on what you're after.

stevend
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Re: Temp sensor for Water Tank

Thu Mar 26, 2020 1:33 pm

Not applicable to the OP's problem, but for daisy chaining temperature sensors I use the DS1825. It has four inputs which I hard-wire to 0V or +ve to code a 4-bit ID into the device. That way the software just has to read the ID to know which device is which, rather than messing with 7-byte addresses. And 16 devices on a bus has always seemed to be enough. I also bus round a 5V supply, so that bus activity can continue while a device is making a temperature reading.
That solution does need a dedicated PCB, though.

I've also found that a thermal transfer pad between the sensor and a metal plate works well.

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