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Some loggers have been designed to work with a specific probe (sensor) on a specific channel. (Temperature Probe on Channel 1, Humidity on Channel 2 for example.) Other loggers allow a range of probes to be used on a number of channels.
Some loggers allow the user to specify Long and Short names for each channel on the logger or weather station. These names are used on the Values, Statistics and Graph Views of the Dataset and in Exported files. Other loggers have preset names assigned to the channels that can not be changed by the user.
All loggers allow the user to calibrate the available probes in some way. This may involve manual entry of offset and gain values, single point or two point calibration.
Some channels use the readings from other channels on the same logger to improve their accuracy. This is known as compensation. The readings from a humidity probe will become inaccurate with changing temperature however if the temperature is also being logged, this value can be used to correct or temperature compensate the value read from the humidity probe.
After download, all probe values are converted to SI units. These SI units can then be scaled to other measurement systems, for example, empirical units. (10°C becomes 50°F) On some loggers the user can manually enter scaling factors and unit names to give custom units. For example, a methane concentration probe may have an output of 4 to 20mA and a range of 0 parts per million to 500 parts per million. A 4 to 20mA logger channel can be scaled and the units renamed so that the 4 to 20mA output of the probe will be displayed as 0ppm to 500ppm.
The Channel and Probe Setup section of the Logger Control Window is used to:
For loggers that have a variety of standard probe sets (such as the GP-HR) there is a corresponding variety of factory defaults available to set up the loggers.
There can be some confusion between the function of the Calibration Offset & Gain and the Units & Scaling Offset & Gain.
Calibration is used to correct readings to their true values. If a Temperature sensor is reading 0.3°C low then calibration is used to take account of this and display the correct reading.
Scaling is used to change correct readings to other Units. If the logger is reading in °C we can use the Scaling Offset and Gain to convert the readings to °F.
These are used to convert the A2D converter and counter readings to SI Units
As much as possible we try to convert everything to SI units then Scale the readings to whatever the user requires.
There are five exceptions we have made because the SI units are of inconvenient scale
These are:
1... Temperature we use °C not °K
2... Distance we use millimeters not meters
3... Pressure we use hectaPascals not Pascals
4... For a milliVolt logger we use milliVolts not Volts
5... For a 20mA channel we use milliAmps not amps
There are tables of equations in the Omnilog software that convert the readings from the logger to SI Units (or preferred units). There can be small errors in these readings because of small inconsistencies in the sensors and electronic components used in the loggers. Calibration procedures minimize these small errors even further.
These are used to convert the SI units (or our preferred units as listed above) to the users required units.
So for example:
A 4-20mA channel will be calibrated so that the Calibration Gain and Offset will give the correct result in mA
The Scale Gain and Offset can be used to convert the mA's to any other units the user may want.
Another example:
Temperature Logger - the Calibration Gain and Offset will give a reading in °C - the Scale Offset and Gain can be used to scale this to °F or °K.
This scaling can be done automatically by clicking the Select button in the Scale box and selecting the required units.
In the case of the rain gauge the Calibration offset and gain are used to get the logger reading in mm. 0.5mm per tip therefore 2 tips per mm therefore Gain of 0.5
The Scale Offset and Gain could be used to read in meters or feet or yards of kilometers or miles or even fathoms if the user requires. The Select button will only give the common conversions and as feet and meters of rain are uncommon the only options available are inches and tips.