. Author manuscript; available in PMC: 2020 Aug 26.

Published in final edited form as: J Test Eval. 2019;47(6):https://doi.org/10.1520/jte20180652.

TABLE 4.

Historical review of the guard gap width in ASTM C177

Version	Description of Guard Gap Width, w, or Guard Gap Area
ASTM C177–42T	“Heating units having metallic surfaces shall have a definite separation of air gap not greater than 1/8 in. [3.2 mm] between the measuring area of the central surface plate and the guard surface plates.”
ASTM C177–63	Metal-Surfaced Hot Plate	Refractory Hot Plate
	4(c)…“Heating units shall have a definite separation or gap not greater than 1/8 in. [3.2 mm] between the central surface plates and the guard surface plates.”	5(c)…“In a refractory plate having a gap between the central and guard areas, the gap shall not exceed 0.08 in. [2.0 mm] (see Note 5).”
	“Note 5 - It is recommended that the area of the gap in the plane of the surface plate be not more than 6 per cent of the metering section area on that side.”
ASTM C177–71 (SI units)	Metal-Surfaced Hot Plate	High-Temperature GHP
	4.3…“Heating units shall have a definite separation or gap not greater than 3 mm between the central surface plates and the guard surface plates (Note 5).”	5.3… “In a high-temperature plate having a gap between the central and guard areas, the gap shall not exceed 2 mm (Note 5).”
	“Note 5 - It is recommended that the area of the gap in the plane of the surface plate be not more than 6 percent of the metering section area on that side.”
ASTM C177–76	Metal-surfaced hot plate apparatus	High-temperature GHP apparatus
ASTM C177–76	5.3…“The heating unit shall have a definite separation or gap not greater than 4 mm between the surface plates of the metering area and the guard. In addition, the area of the gap in the plane of the surface plate shall be not more than 8 % of the metering section area.”	6.3…“…the gap at the surfaces shall not exceed 2 mm. In addition, the area of the surface shall not be more than 8 % of the metering section area.”
ASTM C177–85	“6.2.2 Gap - The gap between the concentric meter/guard heaters shall be uniform, and its width should be optimized to minimize gap heat flow while simultaneously minimizing temperature distortion effects within the specimen near the gap… Most past experience is based on gap areas that are about 5 % of the metered area.”
ASTM C177–13	“6.4 The Gap - The metered section and the primary guard shall be physically separated by a gap.. .The area of the gap in the plane of the surface plates shall not be more than 5 % of the metered section area.”