TABLE 1.

Historical review of metered section area in ASTM C177

Version	Description of Metered Section Area, A
ASTM C177–42T,aASTM C177–45	A = actual area normal to the path of heat flow (flat surface)
ASTM C177–63 through ASTM C177–71	Metal-Surfaced Hot Plate	Refractory Hot Plate
	4(c)…“The dimensions of the test area shall be established by measurements to the centers of the separations that surround this area.”	5(c)…“The effective metering area of the refractory plate is determined by the positions of the potential taps used to evaluate the power input to the metering area winding.”
ASTM C177–76	Metal-Surfaced Hot Plate	High-Temperature GHP
	5.3…“The dimensions of the test area shall be established by measurements to the centers of the separations that surround this area unless calculations or tests are used to define the area more precisely.”	6.3…“The effective metering area of the refractory plate shall include one-half the area of this gap unless calculations or tests are used to define the areas more precisely. In designs that do not have definite separation, the effective metering area shall be determined either by the positions of the potential taps used to evaluate this power input to the metering area section of the plate or by analytical means.”
ASTM C177–85 through ASTM C177–13 (9.3)	“8.3 Metered Area - The metered area, A, is obtained from the area, Am, of the metered region plate and the gap area, Ag. If there is no discontinuity in specimen characteristics in the gap region, it is calculated as follows:
	$A=A_{\text{m}}+A_{\text{g}}/2$(2)
	If there is a discontinuity between the specimen in the metered region and the guard region, this equation is modified slightly to include the effects of heat flux distortion in the gap region:
	$A=A_{\text{m}}+\frac{A_{\text{g}}{\lambda }_{\text{g}}}{2\lambda }$(3)
	where:
	λg = thermal conductivity of the material in the guard region; and,
	λ = thermal conductivity of the specimen in the metered region.”

Note:

^aAccepted by the Society⁴⁰ as a tentative method and adopted as standard by letter ballot vote⁴¹ in 1945.