Clock for Analog Outputs using NI-6008

[Zeejet]

Hi all,

So I am aware that the 6008 does not have a hardware clock onboard and is unable to output analog signals on a timed basis. I have two mass flow controllers that need to change flowrates simultaneously at specified times and I get a SampleClock error (see attached) when I use a VI that was adapted from another DAQ (NI-6229) with hardware clock.

Is there any way to use my PC's clock for this? I am told that this requires a lot of work and is not very accurate, but I do not need very accurate timing (just simultaneous for two channels). 

I have no experience with LabView and this question is so a more knowldgeable associate can help me figure this out. I was also wondering if I could find a way to "plug in" a clock VI from the tutorials or examples database on this website into my VI and use it as a clock.

[Dennis_Knutson]

The outputs aren't truly simultaneous but what is important is how fast you need to change them and what sort of tolerance you can live with. What kind of ' specified times' are you talking about? There is no way for the 6008 to use any sort of external clock. The limitation has nothing to do with LabVIEW. It is the way the board was designed. The use of a wait function or a timed loop both depend on the pc clock but neither guarantees that windows won't step in and cause a lot of jitter.

[Zeejet]

Well, I would need to maintain a constant pressure (I'm mixing gases for calibrating some electrochemical sensors that have pressure sensitive membranes) when changing flows (same total flowrate. For example, going from 100-0 to 50-50 ml/min on my 2 MFCs.

A more specific example would be having the 2 MFCs going at 100-0 for 5 minutes, then switching to 75-25 for 5 minutes, then switching to 50-50 and so forth at given time points. I could these manually (On Demand) but the VI I have is not written for that kind of operation.

It sounds like the lag may be very large if I use a timed loop on my PC, which may cause problems for my sensors. How bad can the lag be?

[Dennis_Knutson]

Hard to say how bad. In the seconds range certainly.

[Zeejet]

Yikes! That would likely be unacceptable for my applications. Is its seconds rnage lag in terms of simultaneous switching or seconds in the time specified for the change? I don't mind lag in the timing, as long as the two switch in relative uniso.

I shall look elsewhere in if thats the case. Thanks for your input!

[Dennis_Knutson]

It is the lag in the time specified for the switch.

[Zeejet]

In this case, I would be OK with it as long as the two MFCs make the switch together in unison. Could I take a clock VI from the dataase and plug it into my block diagram? Or would I need to fashion the timed loop myself?

[Dennis_Knutson]

I don't understand your question. The timed loop is a function on the Structures palette and you have several functions on the timing palette (Wait (ms), Elapsed Time, etc. that use the pc clock). It sounds like you need a basic tutorial on LabVIEW.