Hello, I am attempting to collect temperature data by measuring the voltage across a temperature input terminal of a piece of industrial equipment. A thermistor is currently wired to the terminal, and my USB-6212 analog input terminal is connected in parallel. The equipment needs to remain functional, so the thermistor cannot be removed. The problem is, the USB-6212 significantly disturbs the measurement - for example, the equipment's temperature reading spikes from 45degF to 60degF whenever the USB-6212 is connected. Using a Fluke DMM does not cause the same problem. I tried measuring the analog input impedance for the USB-6212 with a Fluke 87-III. I get out of range (>50M Ohm) with the 6212 turned on and ~12M Ohm when off. This seems to meet specifications and should be high enough to not disturb the measurement. I don't know the rating of the thermistor, but estimate it is around 20K Ohm based on DMM voltage measurements with and without parallel resistors. Can anyone suggest a way to fix this problem, or an alternative way of making the measurement? The primary constraint is that the thermistor must remain in the temperature terminal. Thanks, Leo

Leo, This is only a guess, although a somewhat educated guess. Two clues: Measurement with a DMM does not distuirb the result. Measurement with the USB-6212 disturbs the result. The DMM is most likely battery powered and not connected to any ground reference. The USB device is definitely connected to the USB ground which in turn is connected to the computer ground. My guess: A ground loop between the DAQ/USB/computer and the industrial equipment. Unless the industrial equipment specifies that one end of the thermistor is grounded and the equipment and computer grounds are connected to the same ground point, it will be difficult to make a measurement as you have described. Things to try: 1. Using the DMM measure the voltage (both AC and DC) between each end of the thermistor and the equipment ground. 2. With the computer/DAQ disconnected from the equipment measure (DMM) the voltages between the computer or DAQ ground and the equipment ground. 3. If all the voltages measured in 1 and 2 are within the common mode specification for input voltages to the DAQ device, then try a differential connection with suitable bias resistors to ground/ Do you have a technical manual or schematics for the industrail equipment? Lynn

Multifunction DAQ

Voltage Measurement Across Externally Excited Thermistor

Solved!

Well if the HVAC Chiller uses the Rs485 , it's quite easy to listen to the protocol without disturbing 🙂 just capture the temp readings that are requested. Less risc than clamp on the existing temp sensor. However I would still just clamp on another thermistor to the water outlet... that's the only clean solution.

Greetings from Germany
Henrik

LV since v3.1

“ground” is a convenient fantasy

'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'