Heat tape control

[booxcar]

Hello everyone,

I am new to Labview (both hardware and software) and I am having some troubles pertaining to which modules I need.  My goal is to be able to control a heat tape (Voltage: 120V, max temp 760 degrees Celsius) when the internal temperature of a vessel hits a specific temperature.  I understand that I will need to get a thermocouple module in order to translate the temperature to the computer.  What is the best way to control the heat tape (a source module?  a voltage out module along with a relay module?)

Thanks,

Boox

[booxcar]

And I forgot to mention that the controller that is currently being used for the heat tape is a Barnant Company Temperature Controller Model# 68900-01.

[johnsold]

If you have a controller, what exactly are you trying to do with LabVIEW? 

Although the controller you have does not appear to have communication capability, there are many which do (and at a lower cost as well). It is far easier to get good temperature control with an off the shelf controller than to roll your own in LV. Such controllers often have built in protection against broken or disconnected thermocouples and othe safety features. Find a controller with Ethernet or RS-232 communication.  Use that to control the temperature.  Use the LV program to monitor the controller, read temperatures, change setpoints, and save data.

Lynn

[booxcar]

Sorry, I didn't clarify.  We have the controllers but we want to get rid of them in order to control the heating tape solely through Labview.  I was looking at the NI 9481 in order to relay power to the heating tape.

Heating tape specs:

120V

5.20A

624W

Now from my understanding, using the relay to provide 60VDC @ 1A output can power the heating tape but will not be able to provide maximum temperature of the heating tape.  In order to control the temperature of the heating tape, I was planning on using an analog output 4-20mA just like the Barnant controller that we currently have.  Am I on the right track here or totally off?

Thanks!

[johnsold]

I am more confused than before.

1. You want to eliminate the controller.  OK.  That was a management decision, not a technical one.

2. You have a heater which requires more power than the relay device you are considering can handle.

3. What is the 4-20 mA an output from?

4. What are you going to do with the 4-20 mA signal?

Let's go back to the beginning.

How many heat heat tapes will you be controlling?

What is the temperature range for your system?

What is the maximum rated temperature of the heat tapes?

What is the nature of the load? In particular how quickly can its temperature change (other than by the heat tape)?

What kind of temperature profile in time do you need to control?

What power sources do you have available?

What temperature accuracy and resolution do you require?

What safety features are required?

Lynn

[booxcar]

So the 4-20mA output is to control the temperature of the heat tape.  We will be controlling one heat tape at around 300-400 degrees Celsius (maximum temperature of the heating tape is 760 degrees Celsius).  We would like to turn the heat tape on to operate at a specific temperature whenever specific conditions are met (a set internal temperature and pressure).  I understand that I will require NI modules (NI9211 and NI9203 for temp and pressure respectively) to obtain the temperature and pressure readings to Labview.  There is no crucial requirement on temperature accuracy.  My plan was to get cDAQ-9178 (8-slot chassis just in case more modules are added in the future).  The power source to the heating tape is what I am having troubles with.  My thought process was that the heating tape requires both a power source and some type of controller.  I was hoping to use a NI9265 module (Analog out 4-20mA module) in order to control the temperature at which the heating tape will operate.  I figured this would work becase the current controller uses a 4-20mA output to control the heat tape.  As for the power source, I was thinking that I could use a relay module (NI 9481) or a sourcing output (NI 9475).  I'm quite confused as to why the functionality of one should be chosen over the other.  From what I understood, I thought that a relay requires a small input voltage so that if a set input voltage is met, it will output a set voltage (say 60VDC).  Now given the specs of the heating tape, I figured it might still be possible to obtain the 300-400 degrees Celsius with 60VDC @ 1A.  Does this clear things up at all?

[AnalogKid2DigitalMan]

Sounds like the power control based off of a 4-20mA control signal uses a proportional controller to provide a PWM signal to an SSR, something akin to this method:

http://www.omega.com/manuals/manualpdf/M0957.pdf

-AK2DM

[SnowMule]

Honestly, rather than reinventing the wheel, just buy a controller with some comm capabilities and plug it in. They're worth the time/money.

Monitoring and setpoints can be brought into LV, but if the computer or VI crashes the controller will keep on running. 

As far as efficiency goes, a PWM-capable controller is the way to go.  Not sinking current into a dummy load just to regulate temp; only power used is what's going into the heating element. 

[johnsold]

The temperature of a heater is not linearly proportional to the applied voltage.  It is closer to the square of the voltage or linear with power.  At higher temperatures more heat is lost to the environment so the temperature increases less than the power.  My point is that at 60 V you probably would not get 300-400 degrees.  Further, you cannot run that heat tape at 60 V and 1 A. You indicated that its rating was 5.2 A at 120 V.  The resistance of the heat tape probably does not change too much with temperature so it would likely draw 2.6 A at 60 V.  Just because the relays in the 9481 are rated at 1 A, they do not LIMIT the current to that level.

The Barnant controller you mentioned earlier has a 4-20 mA monitoring output available.  The power to the load is provided through switched AC connected via a standard receptacle.  Are you controlling through something like what AK2DM posted or what is your system actually doing now?

Lynn

[booxcar]

Thanks for all your guys' help and patience so far!  Lynn, currently we are using that Barnant controller to power and control the heating tape.  Sorry for not previously mentioning this, but I am going to need an analog output module regardless (4-20mA) to control some gas flow controllers.  Assuming that there is a channel available on the module to control the heat tape as well as the gas flow controllers, would this mean that all I need is a means to power the heat tape?  In such a case, is a power source all that I require?