How to Understand the Difference Between RH and GPP in Moisture Readings
Restoration
3 min read
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You know the moment. You’re standing on‑site, checking your psychrometer, and the reading says 55 % RH. Is that good? Bad? Should you crank the dehumidifier or is the space on track?
🤔If you're hesitating, you're not alone.
Even seasoned techs confuse Relative Humidity (RH) with Humidity Ratio (GPP). And when that happens, you can make calls that cost time, equipment, and credibility.
Let’s break this down — field‑style — so you can call it right every time.
Relative Humidity (RH)
If Relative Humidity would be a cup, you could understand it like this:
🔹The air = the cup
🔹The temperature = how big the cup is
🔹RH = how full it looks
🥵Hotter air → bigger cup → holds more moisture.
🥶Colder air → smaller cup → looks fuller, even if no more water was added.
So RH is relative to temperature — it moves even when actual moisture doesn’t.
Field Example:
You measure 50 % RH at 70°F. Now the same air cools to 60°F. RH might read 70 %.
Why? Because cold air can’t hold as much, so the cup looks “fuller.”
📌Bottom line: RH tells you how “full” the air feels — not how much moisture is really in it.
Humidity Ratio (GPP) → The Number That Actually Matters
GPP stands for Grains Per Pound — how much actual water is in the air by weight.
This number does not change just because the temp does.
55 grains is 55 grains, whether the room is hot or cold.
It’s the number that tells you if drying is really happening.
👍🏻Use GPP to track your true progress.
😈 RH Tricks You — GPP Tells the Truth 😇
Let’s start with an example: your RH drops from 60 % to 45 %. Looks good, right?
But your GPP hasn’t budged. You’re not drying, you’re just heating. This happens all the time on jobs when teams focus only on RH. You think you're winning, but the moisture's still hanging around.
What to Do on the Job
Here’s how to use both numbers the right way:
Check RH to understand comfort levels, dew point risks, and whether your air is “wet‑looking”.
Track GPP to confirm how much moisture you’re actually removing.
If your GPP isn’t going down, neither is the water load — no matter what RH says.
When dealing with water damage using an app like magicplan can be very helpful. It can help you document the areas with moisture, which makes it easier for you to figure out the best placement for your equipment, such as dehumidifiers and air movers
Checklist: What to Ask When You Check Your Meters
✅ What’s the RH and temp in the wet zone and the dry zone?
✅ Is the GPP dropping — or just the RH?
✅ Are you running heat without adjusting equipment load?
✅ Are you pulling enough pints based on actual moisture, not just “feel”?
Final Note
If you're using tools like magicplan for job documentation, be sure your psychrometric readings and visual records line up. It makes your scopes tighter, your billing clearer, and your team smarter — every job, every time.
📌Wanna discuss this topic further with other professionals? You can easily connect with them in our magicplan Community.
Learn More:
📘 How to Choose the Right Moisture Meter for Restoration Work
