Price differences reflect mainly options. Temperature controls may include an automatic preheat phase as either standard or optional. Digital encoders are available to measure piston travel for Method B with much higher precision than the older optical technology using photocells. The latter can take 12 data points in 1 in. of piston travel, but a digital encoder can take up to 40 data points per inch. Motorized lifting and lowering the weight onto the piston is another newer option.

Some suppliers, like Tinius Olsen, take a modular or building-block approach, which permits adding capabilities to a basic unit after purchase. Besides option packages, price differences among melt indexers also have to do with the way the units are manufactured and calibrated. While technical specs are prescribed by ASTM, some suppliers just meet them, while others exceed them. A key point when buying a melt indexer is that temperature control must be absolutely stable. For example, some units have two heating elements around the barrel, while others only have one. Also, some have smaller ovens (the cylindrical block that surrounds the barrel), which means insulation and heat retention are not as good. A bigger oven around the barrel will allow temperature to stabilize more quickly.

Suppliers also say calibration is critical to proper use of a melt indexer, and they recommend that buyers make sure their vendor offers calibration services.Calibration requires checking the temperature inside the barrel at a specific point above the die, and also confirming the dimensions of the barrel, piston, and die.

Finally, the die, piston, and barrel must be easily cleanable. Old polymer left in a barrel or die will degrade and cause large variations in MFRs. Some units have been designed for easier access, with simple swing-away systems built into their automatic weight lifting system.