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GP-HR mA Application Notes
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Any combination of the three analogue inputs of the GP-HR Logger can be configured to read milliAmps. A load resister is required between the input line and the common line. The value of load resister chosen will determine the current range of the input. The channels must then be setup to milliAmp mode using the OmniLog Software. 1, 2 and 3 input 0 to 20mA adapters are supplies as standard probe sets. These have the correct load resisters fitted inside the adapter socket. mA1-PS  mA2-PS  mA3-PS   

The Load Resister

Typically for 4 to 20mA sensors a 68ohm 0.1% 25ppm (Farnell part number 434-887 or 343-935) load resistor is used. This gives the input range of 0 to 30.1mA

If you use the factory default reset to configure a GP-HR as a 1, 2 or 3 channel milliAmp Logger, the offset and gain for the milliAmp channels will be set assuming a 68ohm load resistor. This will give an accuracy of ± 3%.

Using manual calibration, an accuracy of ± 0.1% can be obtained at any given temperature. For operation over the loggers temperature range of -30° C to 70° C an accuracy of ± 0.3% can be obtained.   

To calculate the required resistance for any given current range use the following formula:

Required resistance in ohms = 2.048 / max current required in amps

Where low thermal coefficients are required use high precision resisters. (We recommend at least 0.1% 25ppm )

Example:

Circuit for GP-HR with 3 current inputs

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Setting up mA channels using the OmniLog Software

Connect the GP-HR logger to a computer and Run the OmniLog software

From the Logger Control Screen select Channel and Probe Setup

Select the Channel that you wish to setup (Ch 1, 2 or 3)

From the Probe box select the milliAmp option as shown

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Notice that after the milliAmp probe has been chosen the Units in the Unit and Scaling box has been set to mA and the Gain will be set to a default value.

Now give the Channel a meaningful Long and Short name in the Logger Channel Name box as shown below

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Click on the Write Unit, Scaling and Calibration values to the Logger button to send the new settings to the logger.

If you wish to calibrate the mA channel at this point, follow the calibration procedure in the OmniLog Help User Manual. You can see this procedure by clicking on the Calibration Notes button in the Calibration box.

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