ASTM D3321 Standard Test Method for Use of the Refractometer for Field Test Determination of the Freezing Point of Aqueous Engine Coolants
6. Apparatus
6.1 The hand-held critical angle refractometer is a rugged die-cast portable instrument that is covered with a high-impact plastic to minimize damage to the eyepiece lens if dropped. A polished glass prism is opposite the viewing end. A hinged plastic cover is moved over the prism (sampling end) to allow for even sample distribution and prevent liquid sample spillage during the test. No eyepiece or prism adjustments are required for sample testing.

6.2 The telescopic recessed eyepiece is located at one end and the graduated, translucent prism on the opposite end (see Fig. 1).

7. Calibration
7.1 Calibration of these coolant testers should periodically be verified by testing a water sample in accordance with the procedure outlined in Section 8.

7.2 If the sample tested deviates from 0°C (+32°F) the coolant tester is out of calibration and should be recalibrated.

7.3 This calibration test is best performed with the coolant tester and water sample at room temperature. If the instrument used is designed to be automatically temperature compensated, work within the stated temperature-compensated range.