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Old 01-20-2012, 01:30 PM   #1
DownRightAft
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Default 2 RTD temp probes, 1 PID

I am building a single tier, 2 vessel electric brewery. I am using an Auber 2352, and an RTD temp probes. I would like to use two probes, 1 in the HLT/BK, and another in the MLT. I would only be using 1 probe at a time. I would like to use 1 PID. Is there a switch available that I could use to select which probe connects to the PID? The probe has 3 wires, so the switch would be 2 position, switching all three wires. I was just going to unplug one probe, and plug in the other, but I would rather incorporate a switch on my control panel. Thanks for the help.


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Old 01-20-2012, 02:03 PM   #2
rosienej
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Originally Posted by DownRightAft View Post
I am building a single tier, 2 vessel electric brewery. I am using an Auber 2352, and an RTD temp probes. I would like to use two probes, 1 in the HLT/BK, and another in the MLT. I would only be using 1 probe at a time. I would like to use 1 PID. Is there a switch available that I could use to select which probe connects to the PID? The probe has 3 wires, so the switch would be 2 position, switching all three wires. I was just going to unplug one probe, and plug in the other, but I would rather incorporate a switch on my control panel. Thanks for the help.
I haven't done this, but looking at the manual, you might be able to use a SPDT switch and switch the W to terminal 5 connection on the PID between the two RTDs

When switching a signal, if you have the power on bad things might happen if you disconnect one sensor (let the input float) before you connect the other sensor. If you plan to "hot" switch the two sensors you might want to check with the vendor.


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Old 01-20-2012, 09:05 PM   #3
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yep, a SPDT switch would work.
just so you know, two of the three wires on your RTD sensor are looped together, essentially a 'jumper'. its so that the controller can factor in the lead wire resistance into its temperature conversion. RTDs work on resistance of the probe, so the legnth of wire attached to them will change the measured probe resistance. the 3rd wire is used to remove the variable of wire legnth.

since we dont need a 100th of a degree of accuracy, and really 1 or .5 degree is good enough, (and some PIDs dont even use this wire) you really only need to connect two of the wires on the probe. that gives you more options for switches.

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if you have the power on bad things might happen if you disconnect one sensor
possible, but ive done that several times with a few of my cheapo (lightobject.com) PIDs and it works fine. that does not mean that it is best practice, but it shouldnt damage anything. an RTD probe is just a resistor, so if the PID is normally sensing a resistance of, say, 10kohm, and all of a sudden the resistance goes up out of measurable range (when you disconnect the probe its essentially infinate resistance between those wires), it should just report an out-of-range error.

any problems you might have would only be from the PIDs ability to sense when the resistance is back within measurable range. you may need to power cycle some PIDs before it picks it up again.
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Old 01-21-2012, 11:04 PM   #4
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Quote:
Originally Posted by audger View Post
yep, a SPDT switch would work.
just so you know, two of the three wires on your RTD sensor are looped together, essentially a 'jumper'. its so that the controller can factor in the lead wire resistance into its temperature conversion. RTDs work on resistance of the probe, so the legnth of wire attached to them will change the measured probe resistance. the 3rd wire is used to remove the variable of wire legnth.

since we dont need a 100th of a degree of accuracy, and really 1 or .5 degree is good enough, (and some PIDs dont even use this wire) you really only need to connect two of the wires on the probe. that gives you more options for switches.


possible, but ive done that several times with a few of my cheapo (lightobject.com) PIDs and it works fine. that does not mean that it is best practice, but it shouldnt damage anything. an RTD probe is just a resistor, so if the PID is normally sensing a resistance of, say, 10kohm, and all of a sudden the resistance goes up out of measurable range (when you disconnect the probe its essentially infinate resistance between those wires), it should just report an out-of-range error.

any problems you might have would only be from the PIDs ability to sense when the resistance is back within measurable range. you may need to power cycle some PIDs before it picks it up again.
That was just the EE in me coming out.... Best to design a Make-before-break matching the input.... but I think it is likely a simple Wheatstone bridge....
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Old 01-23-2012, 06:38 PM   #5
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but I think it is likely a simple Wheatstone bridge....
probably. the wheatstone bridge has been around forever so is commonly used. almost all low budget resistance-sensing circuitry is exactly that...
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Old 01-23-2012, 08:03 PM   #6
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Here's how I accomplished this.

That would be 4 NO/NO contact blocks on a 3way toggle switch.
The 4th is to control which element is firing.

Here's the front panel, just for reference on the switch.
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Old 04-22-2013, 09:36 PM   #7
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ikonis, can you tell me more about that multi-contactor switch you've got in your picture? I want to do this exact same config.

All the selector switches I've seen come with 2 blocks. Did you find one with 4 blocks or can you buy the regular 2 block switch an somehow attach 2 extra blocks?

Also how do you wire it up? It seems like you'd need 5 blocks to do this.

Thanks for the help.
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Old 04-22-2013, 09:39 PM   #8
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Here's a box with switches for 4 sensors

http://www.thehomebrewforum.co.uk/vi...hp?f=30&t=4701


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