I found these Amtrol instructions.
Pre-Installation
1 Ensure expansion tank is sized properly for application (Figure 1).
2. Remove plastic cap from air valve.
3. With tank empty of water, adjust air precharge to match cold water supply pressure.
4. Replace and tighten plastic cap on air valve.
5. ST-5, ST-8, ST-12 only: Insert Turbulator™ into connector. FAILURE TO PROPERLY SEAL VALVE CAP WILL RESULT IN LOSS OF PRECHARGE CAUSING PRODUCT TO FAIL.
Installation
1. Install the expansion tank on the cold water supply line to the water heater at a point between the water heater and backflow preventer or other one-way device (Figure 2). Mount vertically in downward position only. Ensure that tank is supported properly to hold the entire weight of the tank when full of water. Models ST-25V through ST-210V are floor standing and may not be hung from the piping.
2. Once the expansion tank is installed, check the cold water supply line for any leakage. Make repairs if necessary.
3. Before the initial firing of the water heater, open any hot water fixture and draw water until all air is removed from the system. Turn the water heater temperature control to desired ending temperature (see water heater instructions). Do not set temperature higher than the expansion tank maximum.
4. To relieve initial thermal expansion, momentarily open a hot water faucet. Continue until water heater aquastat temperature is satisfied. Once heater is at its operating range, no further bleeding of expanded water is required.
5. The water heater and expansion tank will now be operational. The expansion tank will control pressure increases caused by thermal expansion to a level below the water heater relief valve setting.
