Need help for plc and thermocouple

[beng]

hello all.! im just new here.. im searching how to know if my plc has its input module for thermocouples till i reached this page.. my problem is that im doing my thesis which is a ventilation system for my school powered by solar panel, plc will be the one to control my four duct fans which will ventilate the school hallway, to make my thesis project smart i want to add a temperature sensor into it. When the temp in the school hallway reached the predetermined temp. that i specified (lets say 39degrees celcius), the duct fans then will turn on, && will be automatically off when the temp. got low.. now the prob is i dont have knowledge bout temperature sensors or any kind of sensor.. my technical adviser told me that i will be needing thermocouple.. i did my reasearch and found out that yes it is a sensor but when i look at the pics it was just 2 pieces of wires,, are those wires the temp. sensor itself or just parts that i will be needing to assemble the temp. sensor? i badly need help for this one.. It would be a great help if there would be someone who can help me answer my questions.. thanks..

 

[roadwhore]

http://en.wikipedia.org/wiki/Thermocouple

You will need a thermocouple input module in the PLC that you use for your project.

 

[keithkyll]

The world of temperature sensors and technologies is massive.
A thermocouple is a poor choice for this application. Use a home thermostat for the lowest cost. Digital input. Search terms: Mechanical thermometer, bimetal temperature switch.
If you need to know temperature in the PLC, then you need analog input. The sensor will need additional electronics to amplify the signal. Search terms: RTD, npn transistor temp sensor.

 

[Mickey]

Here is another link to help answer your questions.

http://www.omega.com/techref/

 

[danw]

When the temp in the school hallway reached the predetermined temp. that i specified (lets say 39degrees celcius), the duct fans then will turn on, && will be automatically off when the temp. got low..

That statement defines what a thermostat, the thing on the wall in your house or school room or office, does.

Re-read Keithkyll's recommendations above.

 

[Lancie1]

When i look at the pics it was just 2 pieces of wires. Are those wires the temp. sensor itself, or just parts that I will be needing to assemble the temp. sensor?

The end point where those wires are joined is the thermocouple. A thermocouple is nothting more than a welded joint between two different types of wire. This creates a voltage source that varies with temperature.

If you have some thermocouple extension wire, then you can make your own thermocouples by welding the ends together.

 

[beng]

Hi sir..im searching for thermocouple at the moment.. and yea i think this would be a better choice compared to thermocouple.. but as i see here thermostats are programmable.. thus if im going to use thermostat then would it be possible not to use plc.. and just use thermostat alone?

 

[Lancie1]

...as i see here thermostats are programmable.. thus if im going to use thermostat then would it be possible not to use plc.. and just use thermostat alone?

Yes, but then your project would not be as "smart" as you stated was a goal:

..to make my thesis project smart i want to add a temperature sensor into it.

A PLC allows custom programming such as time delays before switching off the system, thus giving better control of the system. However, thermostats are the traditional control method for a heating system.

..my problem is that im doing my thesis, which is a ventilation system for my school powered by solar panel...

I think for a thesis and school project, I would use the PLC to save and display data that would not be available with a programmable-thermostat control. For example, use the PLC to calculate how much electric power is being generated by the solar panel and how much money is being saved, and show that on a large real-time graphic display panel. You could show the current amounts and the total amounts since start of the project.

 

[danw]

Thermostats are programmable as real time clock, calendar based On/Off calls for heat or cooling. Their outputs are fairly robust, easy-to-use relays. Because of their widespread useage in consumer (residential) and commercial buildings for HVAC (Heating ventilating, air conditioning) they are very inexpensive. They are so inexpensive that you could use multiple thermostats to stage the fan control. One fan on at X°, 2nd on at X° + Y°. The use of a thermostat screams "Ease-of-use" in my book.

But doing stuff like defining deadband/hysteresis (difference between on and off points) is a PLC or digital controller task. Data logging and energy calculations require a sensor whose output can be recorded, maybe multiple sensors AND software.

Getting temperature values into a box where you can save the data and do calculations might be more viable on as PC task than a PLC task. There are lots of (relatively) inexpensive datalogger boxes that digitize thermocouple signals and save the data in something like .csv format that can be crunched in Excel (or any spreadsheet).

You need to define exactly what needs to be done before deciding on the hardware and software.

 

[Tom Jenkins]

Your instructor may want you to use a T/C (thermocouple) to give you exposure to analog inputs. However, I'd check with him on whether it has to be a T/C or not. There are other types of analog temperature sensors. For this application either an RTD or a temp transmitter with a voltage or analog output would be better choices. T/Cs are best at high temperature ranges.

See: http://www.omega.com/pptst/TX94A-SS.html or http://www.omega.com/pptst/RTD-870_RTD-880.html or http://www.automationdirect.com/adc...ors_-a-_Thermometers/Temperature_Transmitters

Regardless you will need an analog input for your PLC that is compatible with your transmitter signal, as Keith says. Some PLCs have an analog input built in, others require a separate module. You don't give much info on your PLC - that would help.

 

[beng]

thanks for all your replies.. i do appreciate the fact that you guys are so willing to help me with this one.. these friday we will be installing our thesis in school.. the only thing that will not be included yet is the temp sensors.. as of the moment.. im thinking that we will just use thermocouple coz its a lot cheaper compared to other.. the plc that will be use a plc siemens 1224 rc.. that is a school property anyway..