Reusable Hygrometer Probes Vs. Disposable for Concrete Rh Testing

Imagine if every time you sat down for a meal at your dining room table, there was nothing permanent about your tableware.

Regardless of whether it was a special occasion or just another run-of-the-mill dinner, your table's place settings were flimsy paper plates, single-use plasticware, and thin-plastic 12-ounce red cups.

From a convenience standpoint, disposable place settings make cleanup a breeze. However, they're not an economical solution. Nor do they deliver the same level of performance as traditional tableware. Purchase enough replacement packs of each, and eventually the cost of one-time-use items will start to exceed what you would have spent on a reusable set of plates, cutlery, and cups.

With testing moisture (more specifically, humidity) in concrete flooring, the same principle applies to the hygrometer probes used. Though inspectors have their choice between reusable and disposable hygrometer probes, the former option delivers the best value and performance.

In Brief: Measuring Moisture in Concrete

Against other building materials, concrete poses a unique challenge when checking it for moisture. Put simply, it can't have moisture content evaluated the same way.

In modern construction, vapor barriers installed underneath a slab to prevent moisture from the ground from permeating the concrete. The problem with newly installed concrete is the moisture takes a long time to evaporate from the surface top to become dry enough to accept a floor covering that won’t fail from excessive moisture. 

Checking a concrete's moisture level (more on this in a bit) isn't as simple as testing it with a pin-type meter as you would with wood or drywall. To be frank, it's impossible to use a standard moisture meter to accurately assess moisture presence in concrete.

Instead, measuring moisture in concrete is done using a hygrometer with probes and the in situ method, in which probes are inserted directly into a slab and left for a period of time to measure equilibrium relative humidity (Rh).

Which brings us to the matter at hand: which type of probe – reusable or single-use disposable – should an inspector use to not only get the best results but also get the best value from?

Breaking Down Reusable Hygrometer Probes Vs. Disposable for Rh Testing in Concrete 

A comparison of both hygrometer probes isn't just based on how soon after use each ends up in the trash. There are two key measuring sticks to evaluate reusable vs. disposable hygrometer probes:

1. Financial
2. Technical
   
   

1. Financial

At first glance, disposable probes seem like a better way to spend your money. It's hard to argue a lower price point against the investment reusable probes require.

But that's simply not the case.

The dollars and cents of comparing both electronic hygrometer probes is fairly straightforward.

In short, because reusable probes are indeed reusable, there's a one-time investment made during their useful life. Disposable probes, however, require spending money every time – something that quickly adds up.

The numbers tell a simple story: 

If you complete one job a month using 12 test holes each, reusable probes start to pay for themselves in a big way. After four years, you’d save nearly $17,000 than if you had used disposable probes. 

One more point of consideration – the ATSM f-2170. The standard for measuring moisture in concrete, the ATSM F-2170 mandates that three probes be used in the first 1,000 square feet of a concrete slab being tested. For every additional 1,000 square feet, meeting the standard requires using another probe.

From our numbers above, completing one test with disposable probes that are in compliance with ATSM standards means spending at least $90-100 for the first 1,000 feet– an entry fee of sorts you'll pay every time you test a concrete floor.

2. Technical

Again, at first glance, both digital hygrometer probe types generally appear the same (certainly there's aesthetic differences between probe manufacturers).

Beyond the surface, however, is where the probes start to set themselves apart in how they each measure moisture presence in concrete.

Some reusable probes use resistive sensor technology, while all disposable probes use capacitive. Let's dig into both.

• Resistive hygrometer probes determine Rh by measuring changes in electrical resistance caused by changes in moisture content in air. Resistive sensor technology tends to be far more robust, and when exposed to liquid moisture (dew point), they are less prone to drift. Relative humidity reading on industrial-grade concrete slabs are generally between 75 and 100%, this is very high and can often mean your humidity probes are going to get wet as they reach dewpoint. 
  
  
• Capacitive hygrometer probes work by measuring changes in the dielectric constant of a material due to changes in the air. As the amount of moisture changes, the dielectric constant of the probe's polymer changes, which in turn changes the capacitance value of the probe. The change in capacitance is proportional to the change in relative humidity, allowing the probe to accurately measure humidity levels.
  
  While this sensor type is the most common used to measure humidity, it does have its downfall. Despite being a good full-range sensor, it doesn't recover well from getting wet or being exposed to high humidity. What's more, it loses accuracy and tend to drive, giving inaccurate results. 
  
  

Putting the In Situ Method to Work

An upgrade on both accuracy and reliability from the once-popular calcium chloride moisture testing method, in situ testing measures humidity levels inside a concrete slab from the inside.

The preferred method of moisture evaluation in concrete, in situ testing is a seven-step process. To meet ASTM F-2170 standards for in situ testing:

1. After using a pinless meter to locate areas with the highest potential for moisture, drill three holes for the first 1,000 square feet in the area of a concrete slab being investigated to a depth equal to 40% of the slab’s thickness. You’ll need another hole per 1,000 square feet after. 
2. Clear the holes of dust and debris.
3. Visually inspect the bottom of each hole to make sure you haven’t drilled into the heart of a piece of coarse aggregate (e.g. gravel).
4. Set Rh probe sleeves to the correct depth and insert them into each hole. 
5. Insert your reusable humidity sensor probes into their respective sleeves and place a cap to seal each opening. 
6. Wait 24 hours for the probes to equilibrate with their microenvironment’s relative humidity. 
7. After one day’s time, take & record Rh readings using a hygrometer.

Keep in mind: successful in situ tests require a high level of control. Changes in temperature and humidity in the room where the tests are being conducted can affect outcomes.

Hygrometer Probes | Getting the Best Value & Results

While inspectors have a decision to make between reusable and disposable hygrometer probes when testing moisture levels in concrete, the decision is clear: Reusable hygrometer probes is undoubtedly the better choice.

While disposable probes may give the appearance of savings, they are not cost-effective in the long run and may compromise accuracy. Just like investing in a reusable set of tableware, investing in reusable hygrometer probes ensures reliable performance, cost savings, and a more sustainable approach to concrete testing.

Checking Moisture in Concrete? 

Download our testing form to help you record your findings!