How to make a PT100 Switch Box

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Vossy1

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In my Brewery I use a PID unit to display temperatures from a PT100 probe which I use for various applications. I use the probe to measure things such as grain temp pre dough in, boiler temp for steeping hops, pre heating my heat exchanger prior to recirculation etc. I’m making a few brewery modifications introducing a fixed PT100 in the HERMS heat exchanger, one in the wort outlet and one in the coolant inlet of my gasketed plate heat exchanger. This means I want 4 PT100 inputs going to one PID unit.

The PID unit only has one set of PT100 probe input terminals at its rear, I needed to find a way of inputting 4 PT100 signals to the PID unit, through this one terminal. This can be achieved by switching the inputs from the PT100's, to the PID unit, via a switch box. This means that one PID unit can now display the inputs from 4 different PT100's, depending on the switch position at the switch box.

The parts are from Maplins
1 x LF11M Aluminium Chassis box £3.49
5 x JX02C 4 way Mini din line plug £6.45
5 x JX08J 4 way Mini Din Chassis Socket £6.45
1 x FF75S Rotary Switch £1.79
1 x YR65V Brushed Aluminium Knob £1.98
Component nut, bolt and washers (already had)
Wire BL09K (white) BL07H (red) both from Maplins

Tools required
Drill
Dremmel (to cut lug of chassis box rear panel, and file drilled burrs)
10mm Drill bit (to drill hole for switch spindle)
4mm Drill bit (to drill holes for socket bolts)
Soldering iron & solder
Cable strippers
Cable (I used some spare cable from a PT100 but Maplins BL09/BL07 does the job)





I marked the positions for the switch and sockets. I drew these on the plastic coat which the aluminium chassis box comes wrapped in. I don’t remove the chassis box plastic coating until I’ve finished the job, this prevents damaging/scratching the case. I then used a hole punch to mark the positions, so the drill bit wouldn’t slip. I drilled pilot holes for the switch and socket bodies with a 4mm drill bit, widened the holes with a 10mm drill bit. I inserted the sockets and marked the positions for their retaining bolts, removed the sockets, hole punched the marks, and drilled them with a 4mm drill bit. Finally I attached the sockets with some component bolts which I already had.

Next, I offered the chassis box top, to the base. The switch and bottom sockets are very low in the box and they rest proud on the base lug, preventing the box from closing properly. I marked on the base plastic coating where the lug on the base needed to be cut away to accommodate the switch and socket, and I cut these areas away using the dremmel.

The far right socket on the switch box acts as a feed to the PID unit on my HERMS control box. They are connected via a length of wire with 2 4 pin mini din plugs on it.


I noted the positions of the wires on PID PT100 input socket on the HERMS control box, then extrapolated these to the switch box output socket. Once I had done this it was time to start soldering. The rotary switch I’m using is 3 pole, 4 way. This means it can switch between 4 sets of 3 inputs. The centre of the rotary switch has 3 output terminals marked a,b,c. These terminals are soldered to the switch box output socket.
The outer edge of the rotary switch has 12 input terminals, marked 1 - 12.
1 - 4 act as input ‘a’, 5 - 8 act as input ‘b’ and 9 - 12 act as input ‘c’.
The sockets were soldered into their relevant positions which were 1,5,9……2,6,10…….3,7,11…and 4,8,12.
I then soldered the mini din plugs onto the PT100’s and the job was a good’un.
Total cost of project = £20.16*
* excludes solder, nuts, bolts and washers which I already had.


This is the way I made my switch box. It is not the only way. Using different parts can make the project a lot cheaper, as can buying the parts from another source, i.e., 3 way mini din plug = £0.23 at Rapid. I chose to make this box so I could remove any PT100 independently from the rest, allowing for total flexibility for maintenance or moving a component of the brewery, if required. For those eagle eyed amongst you, you may notice that 2 of the sockets are 6 pin, whereas the list of parts is for 4 pin sockets. I had 2 of the 6 pin sockets and plugs already, and chose to use them up. I thought leaving the parts list as per 4 might avoid any confusion. Apologies for any misuse of 'terms'...I'm no electrician and it probably shows :lol:

Many thanks to jb & eb for their help.
 

ni9e

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Looks good Vossy even though I have no idea what its for :lol:
 

A T

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Haven't you still got to reset the values on the pid controller every time you switch probes? :wha: I think i need more detail to understand how you will work this in you system :oops: I can understand how you can switch probes but doesn't the pid still run an element somewhere or is it just measuring temperatures? :wha:
 

Vossy1

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Haven't you still got to reset the values on the pid controller every time you switch probes?
Nope, I was pleasantly surprised when I plugged in 3 PT100's, left them for a while and they all read the same temp :cool:
I can understand how you can switch probes but doesn't the pid still run an element somewhere or is it just measuring temperatures?
No and yes :D
In my Brewery I use a PID unit to display temperatures from a PT100 probe which I use for various applications
The PID is basically a thermometer, it doesn't drive anything, just shows the temp from 4 different sensors :thumb:
 

Muddydisco

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Looks funky! :) But don't you get variations in the probes accuracy? The two Probes and 2 pids I run have major Differences in temp correction factor?
 

Lurch

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muddydisco said:
Looks funky! :) But don't you get variations in the probes accuracy? The two Probes and 2 pids I run have major Differences in temp correction factor?
Are you using rtds? There really shouldn't be much difference with those if the construction is the same and the lead length is the same too.
 

Muddydisco

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two probes both the same, length, size, etc and 2 identical probes but one was about 2 degrees different to the other and both were set around -10c from normal that was set in slushy ice water at 0 c with spirit thermo.
 

Vossy1

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Are the PIDS from different sources MD? The one I bought from Auberins was very different to my Chinese jobbies from e-bay, ie, the Auber one was very out :shock:
 

Lurch

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Righto then, the probes were obviously excellent!
With there being a fairly small change in resistance for a given temperature, even small differences in resistance or measuring equipment can have an impact. Hence as you found one PID can give a different reading to another. In your average controller a difference of a couple of degrees is not that huge, wherever a particular temperature is required then calibration is advised anyway.
 

RatAle

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Nice tidy work there V1 and a good way to use one PID for different temperatures around a brewery system. :thumb:

Got me thinking, I've seen ganged rotary switches in the past, they used to be in stereos for left/right channels. Due to the PID's low SSR control voltage/current another switch could be used to switch SSRs and hence elements. Would this be of use :?:

 

EskiBrew

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That should work Ratale - just be careful to put a resistor across the inputs of each SSR so that when it is not connected to the PID, the input is not left floating - I have heard stories of them spontaneously turning on if left unconnected due to noise pickup. :thumb:
 

BigYin

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sorry to drag this up, but saves me creating a new thread - I've been looking for advice on exactly this subject - what kit to get and how to set it up... I though about a sensor for the boiler, and one for the mash tun.. :hmm:

Not sure what probes to buy, where from, or how to fit them, or indeed what display units I'd need, or where best to get them from... :whistle:
 

Buster

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This may sound daft but I was told that you cannot alter the wiring on a PT100 as it will affect the resistance reading it sends to the PID so by adding switches and other wiring will this not affect how the probe works correctly.
 
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