Hourly temp spikes on thermocouple DAQ (NI 9217)

[dodanimal]

We have a DAQ 9174 inside a foam box, with a NI9217 4-channel thermocouple reader. ABout every 55 minutes or so, at precisely regular intervals, we see a false temp spike of about 0.2C on all channels. 

its about 88 fahrenheit inside the foam box with the DAQ, and 70F in the room. 

We observe a similar false temp spike on all channels if the box is opened. No doubt this is due to the cold junction compensation, and temperature gradients created by allowing a burst of cold air into the box. 

The temp spikes are definitely not real, because identical temp spikes are observed on different Tc sensors in differentl locations, in objects with very stable temperatures. 

Blowing hot air into the box produces downward temp spikes, as expected. This is due to creating an opposite temp gradient that overwhelms the cold junction compensation. 

The mystery is what is creating the temp spikes when the box remains closed and sealed. There is no machinery, fans or anything that could be creating temp changes in the box or outside the box, at regular intervals. All the machinery near the box is operating in steady-state. No changes are observed in nearby equipment when the temp spikes occur! The same temp spikes are observed after replacing the NI9217 module, and after covering the box with additional layers of insulation.

So it looks like the DAQ system or the 9217 module is creating heat pulses (or temp gradients) at 55 minute intervals. Is this possible?

Questions:

Is this temp spike part of a cold junction compensation calibration cycle?

Does the 9217 have an internal heater or something that turns on once per hour or once per 55 minutes or so? If so, this could create temp gradients that produce these false temperature spikes. 

Any other suggestions about what could be causing this would be greatly appreciated.

[mikerophone]

dodanimal,

The NI 9217 is actually an RTD Analog Input Module, not a thermocouple module.  Consequently, there is no CJC in the device since the principles of operation for an RTD are different than a thermocouple; RTDs do not require CJC.  There can be several reasons why you are seeing unexpected voltage readings.  Have you considered placing your DAQ device outside of the system where the measurements are taking place (the foam box)?  This might prevent any noise that is originating from the device itself.

[dodanimal]

Yes I know. that was a typo. the module is actually 9211. 

[dodanimal]

module is actually 9211. 

period of spikes is 0:57:38 exactly. 

[mikerophone]

To answer your question, there is no heater or calibration cycle that initiates every hour with the DAQ device.  That kind of behavior is not typical.  I have a list of questions:

1) Does the spike occur 57 minutes after the start of each test or 57 minutes after the beginning of each hour of the day (i.e. 4:47, 5:57, etc.)?

2) What is the output voltage range of your thermocouple?

3) What is your sample rate?

4) Have you tried taking measurements of the internal environment of the foam box with the module harnessed outside of it?

5) Can you attach a screenshot of your data during one of these voltage spikes?

[dodanimal]

spikes occur at 0:00:000 and each 0:57:38 after the program starts. 

not sure about output range. The spikes occur even if the Tc input is shorted (whether Tc is connected or not). Also the spikes occur even if the module is changed and replaced with an almost brand-new module. 

Spikes occur in MAX, so I am told by my programmer that its not software problem. But it doesnt seem like a hardware problem either, since it occurs for both modules. One thing I have not yet tried is changing the DAQ slot. The DAQ might have a bad slot or something. Going to try that soon. 

 something that might be very important that I had not noticed before: For about 45-50 seconds before the spike occurs, the entire system freezes! NONE of the other values (oven, cal, pressure etc) changes at all. They are static. Then, when the system resumes operation, the temp spike occurs. The pressure value in our system changes with every measurement. But it stops changing about 45 seconds before the spike occurs. Then, once the spike starts, the pressure reading becomes live again. 

This freezing behavior has me very concerned that there is much more going on here than merely these spikes. 

sample rate is 2 seconds. 

Yes tried taking internal temp measurements of foam box. No temp fluctuations observed. Holding a fan or heat gun near the box does not affect measurements. Also, hitting the cables with a heat gun does not affect measurements. 

Screen shot attached. The very large (neg and pos) spikes are caused by me. The troublesome spikes are small and ccur at regular intervals of 0:57:38 (57 minutes, 38 seconds, like clockwork). 

You can see that there are temp offsets that occur as well. Notive that the temp offset changes after most (but not all spikes) and that the offsets are the same on both channels. 

The Tc sensors are in very different locations. One is in a temp controlled oven at 293C, the other is at room temp, and has a 270C programmed offset so that I can watch both signals on the same graph. 

[johnsold]

"...about 45-50 seconds before the spike occurs, the entire system freezes!..."

Please give us a brief overview of the entire system. It seems very likely that something other than your temperature measurement process is causing the spikes. Ground loops are one possibility.

Since you know the timing, take a careful look at everything happening starting about 1 minute before the next spike. My guess is that you will find something which happens in some other part of the system around that time.

Lynn