Controlling and acquiring temperatures at the same time

[Ronak2]

Hello all, 

Setup: I'm running an experiment where I'm heating air and measuring its temperature at various points through a system. I'm operating the heaters using a Heater control (OMEGA CN4000 series temperature controller). The thermocouples (K-type) measuring air temperatures are going into the DAQ board (SCB-68). I'm using an analog input step in SignalExpress to measure thermocouple temperatures. I’ve attached my SignalExpress file to this thread.  

Problem: What I want to do is to display the thermocouple temperatures in the Heater control as a processed variable while I change the set point. Simultaneously I want to record the temperatures using SignalExpress. I’ve tried to do this by taking in Voltage from my thermocouple inputs on the board and generating an output voltage through my analog output channels on the board. However, this has’nt worked very well. I see two major problems:-          The program does’nt allow me to acquire Voltage and thermocouple temperatures simultaneously from the same channels.-          Even if I acquire only the Voltage from the board and feed it directly into my Heater Controller, the temperatures displayed are completely incorrect.

I'm completely new to Data Acquisition, so I'll need a lot of help with this.

Help greatly appreciated,

Thanks,

Ronak

Message Edited by Ronak2 on 02-19-2010 10:09 AM

[EricS.]

Hi Ronak,

I have a few more questions about your setup to confirm I'm understanding this correctly.  I believe you have thermocouples that are wired into your SCB-68 analog inputs, and then you ore reading that voltage, and then outputting that same voltage using your DAQ card as well to your CN4000 heater control.

Is that correct?

If so, then it does not surprise me that the temperature now being read on the CN4000 is incorrect.  This is because the DAQ card most likely does not have a very precise analog output and has large incremental voltage steps that correspond to a large temperature change.  The actual accuracy will depend on the specific DAQ you have. The SCB-68 is simply the terminal block for the DAQ card, not the actual card you are using.  You can find out what card you have from the Measurement and Automation Explorer (Start » Programs » National Instruments » Measurement & Automation) under the Devices and Interfaces section.

To get the best accuracy on both systems would be to wire the thermocouple to both your DAQ card and the Heater Control.  You would have to split the thermocouple leads using proper thermocouple wire, and send the them to both of your devices.  Both devices should have their own cold junction compensation as well.

This would result is using the Heater Control for instantaneous feedback and control of your heater, and Signal Express would be used for data logging.