Custom Probes

Users can setup Custom Probe Formula for the general purpose channels on SafeT, GP-HR and GP-MC loggers. This allows users to take the raw AtoD converter counts from the logger and translate them using custom formula.

There are eight custom probe types available in the system.

• Custom1 and 2 are ratiometric with a positive correlation
• Custom3 and 4 are ratiometric with a negative correlation
• Custom5 and 6 are absolute with a positive correlation
• Custom7 and 8 are absolute with a negative correlation

Lets define these terms

A ratiometric probe produces an output voltage that is proportional to the input voltage. The quantity being measured, is measured as a proportion of the total input voltage.

An absolute probe produces an exact voltage or current for any given measured quantity. The quantity being measured always produces the same voltage or current for any given reading.

Some probes produce an output that is inversely proportional to the quantity being measured. For example a temperature sensor might have a high output at low temperature and produce less voltage as the temperature increases. We refere to this as having a Negative Correlation. Most probes have a output that increases as the measured quanitity increases and are said to have a Positive Correlation.

If you are going to setup a Custom Probe Formula, first you have to determine if you probe is absolute or ratiometric and weather it has a positive or negitive correlation and then select the correct Costom Formula

• Custom1 and 2 are ratiometric with a positive correlation
• Custom3 and 4 are ratiometric with a negative correlation
• Custom5 and 6 are absolute with a positive correlation
• Custom7 and 8 are absolute with a negative correlation

The user enters the custom formula's into a text file. This text file is read by Omnilog when it starts up and the formulars are then available to the Omnilog system. The text file must be called custom.txt

The formulas can be entered in the text file in any order. The formula must start with the word CUSTOM and then the formula number, an equals sign and then the formula, so for example custom formula 5 would look like this:

CUSTOM5=((x/34.5)*(x/0.23)

where ((x/34.5)*(x/0.23) is the required formula

Any lines that do not start with CUSTOM will be ignored (so these can be used as comment lines if required.

The following Variables, Operators Constants and Functions can be used within the formulas:

Variables

• x is the AtoD converter count passed into the equation by Omnilog
• cg is the Calibration Gain for the Channel
• co is the Calibration Offset for the Channel
• a is the converted value from the first compensating channel
• b is the converted value from the secong compensating channel
• asi is the averaging sub interval
• ap is the averageing period for the fast pulse input
• lp is the logging period

Operators

• + add
• - subtract
• * multiply
• / divide
• ^ power

Functions

• log() returns the natural logarithm (base e)
• log10() returns the common logarithm (base 10)
• sqrt() returns the square root
• exp(x) returns the value of e^x where x is a specified numeric expression
• sin() returns the sine of an angle in radians
• cos() returns the cosine of an angle in radians
• tan() returns the tangent of an angle
• rtod() turns radians to degrees
• int() returns the integer value
• mod(nDividend, nDivisor) Divides one numeric expression by another numeric expression and returns the remainder
• iif(e,f,g) is an inline if function such that, if e, the function returns f, otherwise it returns g

Constants

• -999999999 is used by Omnilog to represent a reading that is offscale high or offscale low. It will be displayed as "No Data" in the realtime screen and Grayed out in views of datasets

Equation Examples

Raise the incomming AtoD value to the power of 3.141 then devide the result by 347
(x^3.141)/347

If the incomming AtoD is less than 850 return "No Data" otherwise return x devided by 655.35
iif(x<850,-999999999,x/655.35)

Multiply the incomming AtoD value by the calibration gain, subtract the calibration offset, convert to radians and return one minus the cosine of this value
1-cos(dtor((x*cg)-co))