OMB-DAQ-2416 measurement issues

[Lenloc]

Greetings,

I'm having an issue with this DAQ unit and properly reading measurements.  

I have 5 thermocouples connected to a hot plate, they are set correctly in instacal.  The thermocouples are embedded at different depths of the plate to measure a gradient as the hotplate dries samples.  These thermocouples are shielded except for the tip.  The tip is in direct contact with the metal.  When power is applied to ceramic cartridge heaters the thermocouple readings are bad...very bad.  

This isn't the first weird occurrence when using this DAQ unit.  I have two other sensors that also read very wrong when they are in DIFFERENTIAL and not single ended.  They measure fine in single ended (this is a setup for a different equipment set up, but mentioning it in case they're related).  

I have an older PDAQ-55 and I don't see any of these issues.  It's stable and has amazing readings.  

This is an issue that we need to sort out as we plan on getting the extender for the 2416 and connecting a lot of sensors to it.  And these need to be run in differential mode so we connect all of the sensors. 

It sounds like some sort of grounding issue.   

Any help would be greatly appreciated. 

 

[JRys]

Hello,

To debug the system, disconnect everything except the 5 thermocouples (TCs) connected to ground (hot plate). Set the Data Rate property for each channel to 60 Hz to reject 60 Hz noise and produce a more accurate value. This is an InstaCal setting unless you're using the DAQami software. The PersonalDaq/55 has this same type of setting too, however it is called Measurement Duration.  Set the sample or scan rate to 1 Hz and collect data while the hotplate warms up from cold to hot. It should return acceptable data. The key takeaway here is that the hotplate establishes your system ground. This is because internally, the USB-2416 engages a reference resistor from the low side input to ground.  In other words, the exposed tip of the TC is connected to ground through a resistor.

If you need to also connect a couple of other sensors, use differential mode and physically add a reference resistor from the low side input to ground (GND). Use a value that's between 47k and 100k ohms. The lower the resistance the stronger the reference to ground, but it also diminishes the benefits of the differential input. The reason the sensor didn't previously work in differential mode could have been because it used an isolated power source. The reference resistor reestablishes the ground reference. 

Best regards,
John

 

[Lenloc]

Hi there,  we went and tried out your suggestions.  We still see the issue when the cartridges are engaged.  I've attached a graph and instacal settings.  

 

[JRys]

Hello,

I'm guessing you're using the DASYLab software. An often overlooked fact is that it reads the InstaCal setup once when it is started - not when the worksheet loads or starts. Any changes to InstaCal will not be recognized unless you it is closed and restart DASYLab.  

Also, I can see that you still have two single ended channels. Are the results with only the TCs attached and everything else physically removed?

Last, what can you tell me about the hotplate? Is it something I could buy at Walmart or is it an industrial unit? I ask because if it's an industrial unit, it may have sophisticated switching electronics to control the heat which is generating non 60 Hz noise. 

You may want to see if you can get one TC to work. If you cannot then it could be the switching elements inside the hotplate causing non 60 Hz noise, which cannot be removed by the USB-2416 digital filter. 

Best regards,

John

[Lenloc]

On 8/16/2022 at 10:39 AM, JRys said:

I'm guessing you're using the DASYLab software. An often overlooked fact is that it reads the InstaCal setup once when it is started - not when the worksheet loads or starts. Any changes to InstaCal will not be recognized unless you it is closed and restart DASYLab.

We're using LabView....but this was the fix.  Setting that to 60Hz in Instacal and then restarting LabView.  We saw good values while it was running (at least from what we could tell).  

Can you explain why "Data Rate" is a noise filter?  Does it not say to the unit "Collect this fast"?  Because the plan is to buy the extender and then connect around 30 sensors to this unit.  Will that in any way prevent us from collecting at around 5hz (each channel) data collection?  Can we set different DataRates for different channels (Thermocouples to 60 S/s and Voltage based sensors to probably 1000 S/s) and enables us to collect data faster. 

Edited August 17, 2022 by Lenloc

[JRys]

The IOtech PersonalDaq/55 used a better name for the same type of setting, it used Measurement Duration. So Data Rate is really the time the analog-to-digital converter measures the input value and it's not the sample rate, which is sample to sample time. Although choosing a low setting will limit the maximum sample rate. Page 16 in the following user manual has the formula that explains the relationship. https://www.mccdaq.com/pdfs/manuals/USB-2416.pdf

The analog-to-digital converter is a sigma-delta converter. These can run very fast but when doing so they produce inaccurate data. So the technique is to slow them down to achieve higher accuracy. A side effect of this technique is it creates a low pass filter capable of reducing signal noise too. 

For thermocouple measurements, I found the 60 Hz setting works well. Long story short, when you see the term data rate, think measurement duration. 

Best regards,

John