USB-2408-2AO Problem with the Sample Rate

[Jonny]

Hello everyone😄  

My name is Jonathan and I'm currently writing my Bachelor Thesis. Therefore, I use the MCC USB-2408-2AO device to get the data from three thermocouples. The channels are configured in the code, using Python. Two of the thermocouples should sample with a rate of 2 Samples per second, while the sample rate for the third thermocouple should be as high as possible during the measurement. During the fast measurement for the third thermocouple, the slow measurements should be turned off. Therefore, I set the BoardInfo.ADDATARATE to 3750, which should return, according to the documentation, about 1100 Samples per Second. If I execute the following code, I get a value of 1087 for the sample rate:  

 

But if I print out the actual values, I get way less updated values per second. (about 100 per second) 

What could be the problem?  

Thanks for your help in advance.  

Greetings, Jonny  

Edited May 27 by Jonny

[DAQman]

When using a thermocouple, the ADDataRate (or just Data Rate) must be set to the power grid frequency, either 50 or 60 Hz. This sets the A/D converter into a mode to reject 50/60 Hz noise. This is crucial because it must be able to resolve voltages below 40 microvolts, which is approximately one degree when using a K-type. So, the ADDataRate is a filter setting whose value influences the sample rate. Please look at page 16 in the user manual to see how it affects the sample rate.

[Jonny]

Hi,
thanks for your quick answer.

So did I get this right? If I want to measure the values of the thermocouples, I can only sample with a rate of 50 or 60Hz, which would give me 57.78 or 48.45 samples per second, respectively? Is there no other way to increase the samples per second for TC measurements? This would be really important, since I have to do some measurements with a short rise time, where I need as much samples as possible...

The device used before was a MCC DT9806. Does this device have a faster sampling rate for thermocouples compared to the USB 2408-2AO?

Thank your for your help.

Greetings

[DAQman]

To answer your questions, yes, and yes, and you can use a faster sample rate when the channel's data rate is set higher. It's possible to read a single thermocouple at 1100; however, the temperature values will be less stable. I tried it with a 36-inch thermocouple and got values ranging from 16.3 to 20.3 C. When I used 60 Hz for the data rate, I got a consistent 18.2 C. 

The DT9806 is a different type of device. It can run much faster than the USB-2408-2AO in thermocouple mode, but I suspect with less accuracy. It's also more expensive.

[Jonny]

Hi JRys,

thank you for your fast response and especially for the tip with the sample rate of 50/60 Hz. 

But I'm still wondering about the 1100 Hz. Based on your answer, I would assume that it is possible to get 1100 samples per second if I'm using one channel. But for me, for the t_in and the a_in method, that didn't work. I only got about 350 values per second. Can you maybe help me with that problem?

Greetings

[DAQman]

use the a_in_scan function. I've attached an example.

usb_2408_AInScan.py