Vernier Tech Info Library TIL #1020

Question

What are the CO2 Gas Sensor power issues? Can I use two CO2 Gas Sensors at the same time?

Answer

When two CO2 Gas Sensors are connected at the same time, the amount of current drawn exceeds the maximum current output of the interface. Due to this limitation, only one CO2 Gas Sensor can be used at a time. This limitation only applies to the CO2 Gas Sensor. All other sensors can be connected to the LabPro in multiples without concern of drawing too much current.

The CO2 Gas Sensor draws about 160 mA each time it flashes. This is a significant amount that can cause power problems when it is powered by batteries or used in conjunction with some other sensors. Some of the implications include:

Using the CO2 Gas Sensor with certain sensors with Logger Pro or DataMate - When certain sensors and a CO2 Gas Sensor are used together, it is necessary to zero both sensors at the same time to get good results. That is, if you zero just the one probe individually, the readings from that sensor taken after the zeroing will not be correct. It seems that this situation depends on the kind of sensor you are using (see list below). Many sensors are fine. The problem has to do with the large current draw of the CO2 Gas Sensor when it is collecting data (when the light source is on). The supply voltage of the LabPro is actually drawn down a bit and then affects the voltage output from some sensors. On newer sensors we have regulators that take care of this situation. This problem occurs when using either Logger Pro or DataMate.

Sensors that may have problems when used with the CO2 Gas Sensor in this way:
Colorimeter (COL-BTA or COL-DIN)
Conductivity (CON-BTA or CON-DIN)
Current and Voltage System (CV-BTA or CV-DIN)
Dissolved Oxygen Sensor (DO-BTA or DO-DIN)
EKG (EKG-BTA or EKG-DIN)
Magnetic Field Sensor (MG-BTA or MG-DIN)
Relative Humidity (RH-BTA or RH-DIN)
Student Force Sensor (SFS-BTA or SFS-DIN)

Using the CO2 Gas Sensor with another sensor in ChemBio - When a power-hungry sensor such as a CO2 Gas Sensor is set up with another sensor in ChemBio, the live graph on the calculator screen is turned off. When there is no live graph, power to the port is delivered intermittently. Anytime power to the CO2 Gas Sensor is interrupted, the sensor begins its 90 second warm-up phase and the reading that is taken at the time power is turned on t the port will be a bogus reading.
The solution to this problem is to use an External Power Adapter for the CBL to provide continuous power to the port. Order code (CBL-EPA) Another alternative is to use a LabPro or CBL2.

Note: the CO2 Gas Sensor does not need to be calibrated. You can check it by reading its signal in fresh air. It should be about 0.2 V. If you blow on it and max it out, the reading should go to 2.5 V. The procedure for recalibration is in the manual.

Created by: gstahmer on October 31 2001
Last updated by: jhopkins on February 04 2004