Appendix 4 to ETSO-C69c –
Calorimeter Specification and Calibration Procedure

1. Scope. This procedure shall be used to calibrate all circular foil heat flux transducers (Gardon gauges). Calibration establishes the value of this product to the user.

2. Terminology. For definitions of general terms, refer to ASTM Standard C168. Definitions specific to this procedure are:

a. Sensor Scale Factor – the ratio between the incident heat flux and the transducer output signal produced at the heat flux, expressed in W/cm²/mV or BTU/ft²-sec/mV.

b. Sensor Sensitivity – the ratio between the transducer output signal and the incident heat flux, expressed in mV/W/cm² or mV/BTU/ft²-sec.

c. Calibrated Heat Flux Level – the maximum heat flux reached during the calibration cycle.

d. Calibration Standard Scale Factor – the sensor scale factor for the reference standard transducer used in calibration, expressed in W/cm²/mV or BTU/ft²-sec/mV.

e. Emissivity – the ratio between total radiant energy absorbed by a plane surface and total radiant energy incident on that surface, expressed as a value between 0.0 and 1.0.

3. Calorimeter Specifications.

a. One inch diameter, cylindrical, water-cooled, Gardon gauge.

b. Calibration range approximately 0 - 5 watts.

c. Foil diameter shall be 0.25 in. +/- 0.005.

d. Foil thickness shall be 0.0005 in. +/- 0.0001.

e. Foil material shall be thermocouple grade constantan.

f. Temperature measurement shall be a copper-constantan thermocouple.

g. Copper center wire diameter of 0.0005 in. +/- 0.0001.

h. The entire face of the calorimeter shall be lightly coated with high temperature paint having an emissivity of 0.94 or higher.

4. Equipment and Supplies. Calibrations shall be performed using a 50 kW flat plate heater furnace. The furnace shall be equipped with transducer mountings, water cooling lines and an exhaust system of sufficient capacity to remove the heat of calibrations at 50 kW input power to the plate. The heater shall be a graphite plate 0.125”±0.0625 thick, and 1.50” minimum width by 1.50” minimum length. Electric current shall be conducted through the longest dimension of the plate. An X-Y recorder with adjustable gains that has been calibrated against NIST standards shall be mounted on a shelf on the front of the furnace. A reference transducer of approximately the same full scale range as the test transducer shall be prepared and calibrated against a NIST calibrated transducer. The coating on the surface of the reference transducer shall be high temperature paint, applied and cured according to the standard coating procedure for an emissivity of 0.94 or higher.

5. Preparations for Calibration. The flat plate heater shall be prepared for calibrations as follows:

a. pyrolytic graphite transitions shall be attached to both ends of the flat plate heater with rubber cement and the heater shall be centered and clamped evenly in the furnace;

b. the exhaust fan shall be turned on;

c. the heater shall be gradually heated electrically until all rubber cement burns away and a good bond is achieved at both ends;

d. the heater shall be energized with sufficient electric current to produce a dull red heat; and

e. uniformity of the flat plate temperature from side to side and top to bottom shall be observed on both sides.

If the flat plate temperature is non-uniform, clamping forces shall be adjusted and, if necessary the transitions reattached to achieve uniformity. The reference transducer shall be mounted on one side of the flat plate heater, centered over the face of the heater and 1/8” to 3/8” from the heater surface. The distance shall be set with a thickness gauge. Water cooling lines shall be connected. The test transducer shall be coated with high temperature paint cured for an emissivity of 0.94 or higher. It shall be mounted on the other side of the flat plate heater, centered over the face of the heater and an equal distance from the heater surface. The distance shall be verified with the same thickness gauge. Water cooling lines shall be connected. The water supply system shall be leak tested and reconnected if necessary. The output signal of the reference transducer shall be connected to the Y-axis input of the X-Y recorder. The output signal of the test transducer shall be connected to the X-axis input. Recorder gains shall both be set to 1.00.

6. Calibration Procedure.

a. A fresh sheet of graph paper shall be mounted in the X-Y recorder. X and Y zeros shall be adjusted. The cooling water pump shall be turned on, but not the exhaust fan.

b. If the full scale range of the test transducer is 50 BTU/ft²-sec or less, the potentiometer control of the furnace shall be adjusted to approximately 150 percent of the full scale heat flux value for the test transducer. After a few seconds to stabilize temperatures, the pen shall be dropped on the recorder, and the heat flux gradually reduced to zero. A straight line trace from the upper right quadrant of the graph to the zero for both X and Y should be recorded.

c. If the full scale range of the test transducer is greater than 50 BTU/ft2-sec the recorder pen shall be dropped and then the control of the furnace shall be slowly adjusted to approximately 150 percent of the full scale value for the test transducer. After this the pen shall be lifted and the furnace control quickly reduced to zero. A straight line trace from the zero for both X and Y to the upper right quadrant should be recorded.

7. Analysis.

a. The analysis will relate the sensitivity and scale factor of the test transducer to those of the reference transducer, based on the slope of the graph recorded in step 6.

b. A straight line shall be drawn from X=0, Y=0 on the graph to fit the recorded trace. If this line intercepts the right-hand edge of the graph, the test transducer scale factor shall be calculated by multiplying the intercept by the reference transducer scale factor. If the line intercepts the top edge of the graph, the test transducer scale factor shall be calculated by multiplying the reference transducer scale factor by 7 (the graph is 7 units high) and dividing that result by the intercept.

c. The test transducer sensitivity shall be obtained by inverting the test transducer scale factor.

d. The maximum heat flux level used in calibration shall be calculated from the X-axis excursion on the graph. The scale factor of the reference transducer shall also be recorded. The reference transducer measures incident heat flux, so the calibration will always be in terms of incident heat flux.