|- Total Capacity Equation -|
A recent addition to the Fieldpiece lineup, the SDP-2 Dual Psychrometer, makes quick work of measuring the properties of air. The key measurement which makes this new method possible is Delta H, or change in Enthalpy. Enthalpy is the heat content of the air, expressed in Btu/lb. When using the standard weight of a cubic foot of air, one can use a simple equation to arrive at Btu/hr, or capacity. Even without measuring airflow, this method is useful using an estimated value based on the setting of the ECM motor, as the measured CFM is within 10% of the set point regardless of static pressure.
Prior to the SDP-2, I would measure the wet-bulb temperature in the Supply and Return Plenums (yes, in the ductwork close to the unit) and convert the readings to Enthalpy using the ASHRAE Chart. Due to rounding, I frequently arrived at numbers that were far from accurate. The fact that this gauge displays to the hundredth of a Btu/lb, and can display the Delta H without any user error, makes this the new way to test refrigerant charge without connecting gauges!
To make this even easier for you, let me share some insight. Over the years of testing, I have found some general rules for Enthalpy change based on the airflow measurement/settings. Take a look at the chart, or print it out if you must. This will save you some additional time with the math. If your Delta H falls in these ranges, don't bother connecting your gauges as the refrigerant charge will likely be close enough! Similar to Delta T: too low may indicate high airflow or undercharged; too high and you have low airflow, etc.