Using pH Sensors with DAQ devices

Updated Jan 26, 2018

Reported In

Hardware

  • NI-9215
  • cDAQ-9174

Other

Extech 6012B: pH sensor

Issue Details

I want to use a pH sensor, but I can't accurately measure the pH from a voltage input going into my DAQ system. Using a pH voltmeter, I get the correct reading but not on a regular voltmeter. I suspect it's an impedance mismatch, how should I proceed?
 

Solution

pH sensors and certain other chemical sensors have one electrode made of glass. This causes the transducer to have a very high output impedance, which can cause current leakage problems when connecting to a DAQ device.

Resolve this issue by
1) Buying a sensor with a built-in pre-amplifier, which significantly reduces the effective output impedance of the sensor.

2) If your sensor does not have a pre-amplifier, adding a unity gain buffer (an operation amplifier with a gain of 1) in line with the output will achieve the same result (just as pH meters do).  Ensure that your buffer has a high input impedance relative to the sensor, and a low output impedance relative to the DAQ device.
  • Third party manufacturers of operation amplifiers include Analog Devices, National Semiconductor, Harris Semiconductor, and others.

3)  Find the input impedance of the DAQ device by looking in the Specifications section of its respective product manual. For example, NI C Series 9205 and 9206 have input impedance up to 10GOhms which may work for the application depending on the input impedance of the sensor

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