Final answer:
The instrument that most OEMs recommend for checking the degree of antifreeze protection in heavy-duty diesel engine coolants is a refractometer. A refractometer measures the refractive index of the coolant to determine the percentage of antifreeze and calculate the degree of antifreeze protection.
Step-by-step explanation:
The instrument that most OEMs (Original Equipment Manufacturers) recommend for checking the degree of antifreeze protection in heavy-duty diesel engine coolants is the refractometer.
A refractometer measures the refractive index of a liquid, which is affected by the concentration of antifreeze in the coolant. By comparing the refractive index of the coolant to a reference scale, the refractometer can determine the percentage of antifreeze in the coolant and measure the degree of antifreeze protection.
Using a refractometer is a quick and accurate way to ensure that the coolant in a heavy-duty diesel engine has the correct level of antifreeze protection to prevent freezing and maximize engine performance.