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USB-2408 incorrect time



I have USB-2408 running with DAQami. 3 Channels, 2 Voltage measurements @ 2.5S/s, 1 TC @ 10S/s. Taking a data point every 2 minutes (0.008333333333/s) for 5000 datapoints until it stops recording.

My first few tests using USB-2408 and DAQami have come back with weird results. I've only run tests that I've previously completed using a battery cycle test machine. But all the capacity calculations [Current * d(time)] are coming back significantly lower than when tested using the cycle test machine despite using the same test specimen and same start/end voltages. This only makes sense if the current is higher than measured. Or if the amount of time that's passed is actually higher than what's being shown. The current measurement is very close to the DC load's reading.

I believe the timer in the USB-2408 is incorrect for the following reasons:

1. One test I ran with a 0.001333333333333s data log rate (1 sample every 5 minutes). I manually stopped the test after a week. The date shown in the test data was Monday at 3:25pm. But I stopped the test Tuesday morning. The time I started the test shown in the exported file is correct.  I turned on a DC load 15 minutes after starting the recording. The DC load has a timer as well. It showed 101.7 hrs of time actively discharging. The data file, from when current begins flowing to when it stopped, showed 91.5hrs. 1774 data points were taken. 10 hours is missing.

2. I ran a test replacing the DC load with a battery cycle tester which has capability to charge/discharge/monitor voltages and collect data. So I'm logging Time, Voltage, temperature, current redundantly on both the cycle tester and USB-2408. after about 2 hours I stopped the test. The current values measured by each are within microamps of each other. But the USB-2408 data is 5 minutes shorter than the cycle tester data.

3. After running instacal on the USB-2408. I set up a new test. I started a timer on my phone at the same time as hitting record on DAQami. 6 minutes later I enabled the DC load. Left it overnight and now my phone has shown 18:44 since the test began, the DC load shows 18:38 since discharge began. But USB-2408/DAQami is showing 17:58 of data. it's missing 46 minutes.


Is this an issue with USB-2408? with DAQami? What can be done to fix this?


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Sample rates above 1 Hz use the device timing circuitry, which uses a crystal oscillator for accuracy. Sample rates below 1 Hz use the Windows system timer with an accuracy of 15.7 ms. You will find that increasing the rate to one sample every 20 seconds (0.05 Hz) will produce better results.

If you need one sample every 5 minutes (0.0033 Hz) or longer, our DASYLab software has features that let you reduce the data by averaging or skipping values. 

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