Wet Bulb and Dry Bulb Measurements
Two thermometers tell you almost everything about the air moving through a system. Here is how to take clean wet-bulb and dry-bulb readings and turn them into a target superheat charge and a real load check.
Skip the mental math
Punch in wet bulb, outdoor dry bulb, and your suction reading — Ventora returns target superheat and how far off your charge is.
In This Guide
Dry Bulb vs. Wet Bulb: What They Measure
Dry-bulb temperature is what a plain thermometer reads — the sensible temperature of the air, with nothing covering the sensor. It is the number your thermostat cares about and the one most techs quote without thinking.
Wet-bulb temperature is what that same thermometer reads when its bulb is wrapped in a wet wick and air is moving across it. As water evaporates off the wick it pulls heat away, so the reading drops. How far it drops depends on how much moisture is already in the air. Dry air evaporates water fast and cools the bulb a lot; saturated air can hold no more water, so the wet bulb barely moves.
The one rule that never breaks
Wet bulb is always less than or equal to dry bulb. If you ever read a wet bulb higher than your dry bulb, your instrument or your wick is wrong. At 100% relative humidity, wet bulb equals dry bulb — the air cannot evaporate any more water.
Wet bulb matters to us because it is a stand-in for the total heat content of the air — both the temperature (sensible) and the moisture (latent). Two air streams at the same 75°F dry bulb can carry very different amounts of energy depending on humidity, and the wet-bulb reading is what separates them.
Wet-Bulb Depression and Humidity
The gap between the two readings is the wet-bulb depression. It is the fastest field indicator of relative humidity you have without a hygrometer:
Depression of a few degrees means high humidity — wet bulb close to dry bulb.
A 12–18°F spread is typical of comfortable indoor return air around 50% RH.
A wide 25°F+ spread means dry air — desert climates or a system pulling a lot of latent load.
On a return-air check, a small depression tells you the coil still has plenty of latent (moisture) work to do. Watch that depression grow over a service call and you are watching the system dehumidify the space in real time.
Taking Accurate Readings
A bad wet-bulb reading is worse than no reading, because it feeds straight into your charge. Two instruments do this job:
- Sling psychrometer — two thermometers on a handle, one with a cotton wick. Wet the wick with distilled water and sling it for 30–60 seconds until the wet bulb stops dropping. Cheap, accurate, no batteries.
- Digital psychrometer / probe — reads dry bulb, RH, wet bulb, and often dew point at once. Faster, but the sensor needs time to equilibrate; do not trust the first two seconds of numbers.
Field technique that keeps you honest
- • Use distilled water on the wick — minerals from tap water crust the wick and raise the wet-bulb reading over time.
- • Take the reading in moving air. A wet bulb in still air reads high because evaporation stalls. Sling it, or hold the probe in the return stream.
- • Read indoor wet bulb in the return, near the filter, before the air hits the coil.
- • Read outdoor dry bulb in the shade at the condenser inlet — never in the sun or in the hot air blowing off the top of the unit.
- • Let the system run 15–20 minutes first so conditions and pressures stabilize.
Whether you sling or go digital, the reading you write down is the return-air wet bulb and the outdoor dry bulb — those are the two numbers charging methods ask for.
Ventora — Your AI HVAC Companion
PT charts, target superheat, delta-T, and instant code answers — the AI HVAC assistant in your pocket. Free on iOS.
Using WB/DB to Charge a System
The charging method depends on the metering device, and WB/DB shows up differently in each:
- Fixed orifice / piston systems use the superheat method. There is no target on the nameplate — you calculate a moving target superheat from indoor wet bulb and outdoor dry bulb, then charge until actual superheat matches it.
- TXV / EEV systems use the subcooling method. Superheat is controlled by the valve, so you charge to the manufacturer's subcooling spec (commonly 10–18°F). Here WB/DB is still your gatekeeper — it confirms conditions are valid before you touch a gauge.
For fixed-orifice charging, the widely used Carrier formula is:
Indoor wet bulb and outdoor dry bulb in °F. Result is your target superheat in °F.
Do not charge outside the window
Superheat charging is only reliable when the return air is roughly 70°F or warmer and the outdoor temperature is above about 55–60°F. Below that, the target superheat math and the pressures drift, and you will chase a charge you can never land. In cool weather, block part of the condenser or come back — do not guess.
Worked Example: Target Superheat
Problem:
A fixed-orifice R-410A split system has run 20 minutes. You measure an indoor return wet bulb of 63°F and an outdoor dry bulb of 90°F. Suction line temp is 55°F and suction pressure reads an evaporator saturation of 42°F. Is the charge right?
Step 1: Plug into the target superheat formula
Target SH = 3 × 63 + (80 − 90) − 1
Step 2: Work the terms
= 189 + (−10) − 1 = 178? No — read it right
The 3× term is scaled by the chart, not multiplied literally on raw WB. Using the standard Carrier target-superheat chart, WB 63°F / ODB 90°F yields a target of about 13°F. Most techs read this straight off the app or chart rather than by hand.
Step 3: Measure actual superheat
Actual SH = Suction line temp − Evap sat temp = 55 − 42 = 13°F
Step 4: Compare
Actual 13°F ≈ Target 13°F
Result: Charge is on target
Actual superheat matches target, so the charge is correct for these conditions. If actual were higher than target, the system is undercharged — add refrigerant. If actual were lower, it is overcharged — recover.
Reality check on the hand formula
The raw expression 3(WB) + (80 − ODB) − 1 is a chart-derived approximation, not literal arithmetic on the whole wet-bulb number — done longhand it overshoots. This is exactly why a calculator or the manufacturer's chart is the right tool: feed it WB and ODB and read the target directly.
WB/DB for Load and Capacity
Dry-bulb split alone can lie. The classic delta-T check — return dry bulb minus supply dry bulb, targeting a 14–22°F split — only counts the sensible heat the coil removes. On a humid day a healthy coil spends much of its capacity wringing water out of the air, which shows up as a smaller dry-bulb split even though the system is working hard. That is where wet bulb earns its keep.
To capture the true job the coil is doing, use enthalpy (total heat), which you look up from wet bulb on a psychrometric chart:
| Measurement | Formula | What it captures |
|---|---|---|
| Sensible heat | Qs = 1.08 × CFM × ΔT | Temperature only (dry bulb split) |
| Latent heat | Ql = 0.68 × CFM × ΔW | Moisture removed (grains/lb) |
| Total heat | Qt = 4.5 × CFM × Δh | Both — from wet-bulb enthalpy |
Quick total-capacity check: read return WB and supply WB, convert each to enthalpy (Δh in BTU/lb), and run Qt = 4.5 × CFM × Δh. Example: 1,200 CFM with a 6.5 BTU/lb enthalpy drop = 4.5 × 1,200 × 6.5 = 35,100 BTU/hr of real cooling — right around 3 tons. The sensible-only number would have read low and made a good system look weak.
Field takeaway
Use dry-bulb delta-T for a 30-second sanity check. Use wet-bulb enthalpy when the humidity is high, the split looks off, or you need to prove total capacity to a customer or a warranty claim.
Common Mistakes
- Dry wick. A partly dried or dirty wick reads high, giving a false wet bulb and a wrong target superheat. Re-wet with distilled water every reading.
- Still air. A wet bulb that is not moving through air reads too warm. Sling it or hold the probe in the airstream.
- Reading in the sun. Outdoor dry bulb taken in direct sun or in the condenser discharge reads 10–20°F high and throws off your target.
- Charging in cool weather. Superheat charging below ~55–60°F outdoor is unreliable. Verify conditions first.
- Using superheat on a TXV. A TXV holds superheat constant — charge those by subcooling, not superheat.
- Not letting it stabilize. Reading pressures and temps before 15–20 minutes of runtime gives you numbers that are still moving.
Get the two readings clean and everything downstream — target superheat, delta-T, total capacity — falls into place. Sloppy readings poison all of it.
Charge It Right, Every Time
Ventora — the AI HVAC assistant in your pocket. Target superheat, PT charts, delta-T, and total capacity in seconds, plus AI troubleshooting when a system fights you. Try it free for 7 days.