Any type of ventilated box will work, maybe an old thermostat with guts removed, or a small electronic project box from Radio Shack or Allied Electronics, or you can have your own custom made from wood, metal, or plastic.Ok here is question was looking at the RTDs was wondering if any suggestions of placements of the probe in a room. preferably on a wall, and a way to camouflage it?
Yes, as many cards as the rack has room for. If you get the DL-205 9-slot rack, with a 250 CPU, 2 digital Input cards, 2 digital Output cards, then you would have room for four of the 4-channel Thermocouple or RTD cards (16 total temperature inputs). Normally I would say that you need about twice as many digitial inputs as ouptuts, but I assume that most of your inputs will be the termperature measurements....Also planning on using an AD 205 PLC with a 250 processor, and was looking at the RTD input card. It looks like the maximum number of probes per card is 4, so can you use multiple cards in a rack?
I agree with Dan. Thermocouples would be much cheaper to buy (or make), and will provide enough accuracy for HVAC.
I have welded up many thermocouples using a homemade rig. Many thermocouples used to be made this way. Use a fused circuit, wear gloves and goggles, and don't try this if you are not agile with steady hands.
Pour a half-inch of mercury in the bottom of a quart mason jar, then add 2" of mineral oil floating on top of the mercury. Twist your bare T/C wires tightly together for 1/8" to 1/4" length, forming a small ball at the end. Wire-nut both the other ends to the hot black wire in a 120 volt cord, drop the 120 volt white neutral into the mercury near one side of the jar, apply power, then lower the T/C wire ball into the mercury (about 1.5 to 2" away from the neutral wire). The small arc will blow away the mercury, killing the circuit. It will weld, then you pull it out, with the oil quenching and protecting the weld from oxidation. It works every time.
Be warned again, the lowest AC voltage you can input into the DL-205 is 110 volts AC. You WILL NOT be able to connect it to your existing 24 Volts AC HVAC control circuit (thermostat circuit) without using a relay. See this list:
http://web1.automationdirect.com/adc/Shopping/Catalog/PLC_Hardware/DirectLogic_205/AC_I-z-O
If you do want to tie to your existing controls without relays or diodes, you can actually get a DL-105 that has 6 inputs that accept 24 VAC. The model F1-130DA is one such, and has 10 inputs, 8 outputs, and a built-in 24 VDC auxiliary power source:
http://web1.automationdirect.com/static/specs/f1130da.pdf
If you want interface directly to 24 VAC with many Inputs and Outputs, then consider buying an Alerton Bactalk VLC (a PLC made for HVAC control systems), or one of the other Building Management System controllers. Learning to program and set up one of these systems would be a valuable trick in your tool bag.
http://www.alerton.com/Products/BACtalk/Field_Controller_Level/VLC-550.asp
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