|   Voltage conversions
This window lets you specify how raw input voltages are converted into appropriate units (such as speed, temperature, flow rate, gas concentration, etc.). The standard window looks like this (but note that you can produce customized buttons using the custom conversions option in the EDIT menu):
Occasionally it is necessary to pre-process instrument outputs before they are read by LabHelper. Common examples are amplification of low-level signals, applying an offset voltage (sometimes called "zero suppression"), and using a voltage divider to reduce a high voltage to one within the limits of the A-D converter. The Measurement Guide pages at the Sable Systems website has helpful information about signal manipulation.
Voltage conversions take several forms:
If the exponent = 1.0, it is better to use the faster two-order polynomial option (below).
Polynomials have several advantages over power functions. They handle negative voltages easily, and compute much faster than power functions.
Exponentials accept negative voltages but do not compute as fast as polynomials.
where V is a scaled input voltage:
The default polynomial scaling is conversion of input volts into millivolts (mV, = volts X 1000). However, you can enter any appropriate conversion factor for input volts. This is particularly important if you are using a voltage divider, and you are using an instrument with a pre-defined polynomial conversion factor. You have to account for the effect of the in order to properly use the polynomial. Accordingly, the input voltage scaling factor should incorporate the voltage reduction (and any other needed scaling) so as to yield the correct output of the instrument for the polynomial. In this example, the instrument's high output voltage has been reduced by a factor of 10 by means of a voltage divider and is being multiplied by 10,000 to obtain millivolts.
A 'breakpoint' conversion is a complex set of calculations in which different equations are applied to input voltages depending on where they lie in relation to a specified 'breakpoint' voltage. For example, you might apply a linear conversion to all data of <2 volts, but a polynomial conversion for values >= 2 volts. This window lets you set the breakpoint conversion conditions:
Each breakpoint conversion operation involves up to three different equations, each of which can be linear, exponential, 2-order polynomial, or power. The initial conversion is (optionally) applied to all data regardless of voltage. The other two 'secondary' conversions are (optionally) applied to data above or below breakpoint, respectively. The secondary conversion can either be added to or multiplied by the results of the initial conversion (this is selected with the 'operator' buttons) and can use either raw voltages or the values from the initial conversion (selected with the 'apply to' buttons). Note that the breakpoint itself is always specified as a raw input voltage.
The window shows a summary table of the three equations at the bottom. You enter the coeffients in the three edit fields at top center, and then click the appropriate button to enter them into the initial or secondary conversions. The 'get x' and 'store as x' buttons let you save and retrieve up to 6 complete breakpoint conversions in memory. To save them more permanently in a preferences file, click the 'store values' button or use the 'store preferences' option in the FILE menu. If you save them in a file called "LabHelper prefs" and it is located in the same folder as the LabHelper program, it will be loaded automatically at startup.
This lets you apply secondary conversions to your data; most of these options are specialized for aerial respirometry. You can compute VO2 VCO2, or VH2O, using several basic conversion algorithms (more comprehensive options for respirometry are available in the LabAnalyst analysis program). Here is an example showing calcualtion of oxygen consumption, with flow rate obtained from another channel:
This example also shows use of the response correction (Z-transformation). You have the option of showing conversions during acquisition but saving the raw data only, or of saving the converted data.
The window shows the entered points, the regression line and regression coefficient, and the conversion equation. At least three points must be entered before a valid conversion is generated. For each point, sent the instrument in question to a particular value, enter that value in the edit field, and click the 'get voltage' button (or hit return). Repeat as necessary with different instrument values. Note that negative inputs (values or voltages) are not accepted when using power functions. A display of the regression residuals is shown when the residuals button is clicked. Here is an example (a 2-order polynomial conversion is selected).