Daqarta
Data AcQuisition And Real-Time Analysis
Scope - Spectrum - Spectrogram - Signal Generator
Software for Windows
Science with your Sound Card!
The following is from the Daqarta Help system:

# Fcal Low

Controls: Options >> Frequency Counter >> Fcal >> Low
Macros: FcalLoRef, FcalLoSet, FcalLoRaw

The Fcal Low Set and Raw values are used to set the lower calibration point. For temperature measurement, this is typically 0 degrees C, established by placing the sensor into an ice-water bath. See the Fcal High topic for a discussion of the complete procedure.

The "Low" label is actually a button. If a Calibration Table has been loaded, pressing this button changes the label to "Reference". This is for use with thermocouples that are using a reference (cold junction) temperature other than 0 degrees Celsius. (See Thermocouple Tables for a discussion of thermocouples.)

Standard thermocouple tables such as those supplied with Daqarta assume that the cold junction (formed where the thermocouple wires connect to the input of the measuring system) is at 0 C. This is rarely the actual case, so there are two different ways to compensate. The best approach is to use a special circuit called a "cold junction compensator" that includes a non-thermocouple temperature sensor to apply a correction to the measured thermocouple voltage. If you have such a system (or if the cold junction really is at 0 C), leave the Low button up.

Otherwise, if the cold junction is at some stable temperature other than 0 C, select Reference. You'd also do this if you had a cold junction compensator that gave the proper change proportional to cold junction temperature, but added an extraneous (possibly unknown) constant. That's what happens in the Thermocouple To Frequency circuit, so you should use the Reference mode with that. (Note: If you use that circuit, the Calibration section there takes precedence over the information below.)

Place the thermocouple probe and an independent temperature sensor (thermometer, etc) on the cold junction; since both "cold" and "hot" junctions are now at the same temperature, the temperature difference is 0 C and the thermocouple system has an output of 0 mV, which the V-F translates to some offset frequency. Enter the independent temperature measurement as Low Set, and conclude with CTRL+Enter to capture the current frequency.

Note that this assumes that the uncompensated cold junction temperature is constant over time, or that the cold junction compensation presents an apparent constant temperature (as it does in the Thermocouple to Frequency circuit), even if it is not actually 0 Celsius.

With no compensation at all, this assumption may still be a reasonable approximation if the hot junction is very much hotter (furnace or kiln interior, say) and you don't need high accuracy. That might be the case if, say, you just want to know if a heated specimen is getting near its melting point.

However, watch out for situations where the kiln is near the measurement system and radiates heat to the cold junction. As the kiln heats up, the cold junction temperature rises and the difference temperature seen by the system is reduced, resulting in readings that are too low. The error is worse at higher kiln temperatures, because the radiated heat warms the cold junction more.

An error of a few degrees at 1000 C may be significant in some situations. In ceramics work, for example, at slow kiln heating rates (15 C per hour) pyrometric cone number 1 is 1109 C, while cone 2 is 1112 C and cone 3 is 1115 C... only 3 degrees apart. Such situations may require cold junction compensation.

Macro Notes:

The Fcal dialog does not need to be open to make changes to the Fcal Low controls. However, since separate Fcal values are maintained for each Trigger Source channel and valid Frequency Counter mode (Hertz, RPM, and msec), you must make sure the desired source and mode are set in order to change the relevant Fcal Low settings.

FcalLoSet=20 sets Fcal Low Set to 20 units (degree C, etc). FcalLoSet=>1 increments it, and FcalLoSet=>-1 decrements it.

Putting an A. prefix ahead of the FcalLoSet command causes it to automatically update the Low Raw value with the current frequency, just as if a manual entry had been concluded with CTRL+Enter. For example, A.FcalLoSet=20 sets Low Set to 20 and updates Low Raw.

FcalLoRaw is used just like FcalLoSet, except that an A. prefix does nothing.

FcalLoRef=1 sets the Low button to Reference, FcalLoRef=0 sets it back to Low, and FcalLoRef=x toggles the current state. Note that Reference may only be set if a Calibration Table has been loaded.