Hygrometer Probes for Concrete Rh Testing: Evaluating Reusable vs. Disposable

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 consisted of 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.

In concrete moisture testing (specifically, relative humidity testing) for flooring, the same principle applies to the hygrometer probes used. Although inspectors have a choice between reusable and disposable hygrometer probes, the former option offers the best value and performance.

Measuring Moisture in Concrete Slabs

When compared to other building materials, concrete presents a unique challenge in terms of moisture testing. Put simply, it can't have moisture content evaluated the same way.

In modern construction, vapor barriers are installed beneath concrete slabs to prevent moisture from the ground from permeating the concrete. The problem with newly installed concrete is that excess moisture takes a long time to evaporate from the surface, making it dry enough to accept a floor covering without risk of failure. 

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 the presence of moisture in concrete.

Instead, measuring moisture in concrete is done using a hygrometer with probe sensors and the in situ moisture test method, in which probes are inserted directly into a slab and left for a specified period to measure the 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?

Comparing Reusable vs. Disposable Hygromete Probes 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 appear to be a more cost-effective option. It's hard to argue against a lower price point when considering the investment reusable probes require.

However, that is not the case.

The dollars and cents of comparing both electronic hygrometer probes are 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 compared to using disposable probes. 

One more point of consideration – the ASTM F-2170 standard. This is the industry standard for measuring moisture in concrete, and mandates that three probes be used in the first 1,000 square feet of a slab. For every additional 1,000 square feet, meeting the standard requires the use of an additional probe.

From our numbers above, completing one test with disposable probes that comply with ASTM standards means spending at least $90-$ 100 for the first 1,000 feet — an entry fee of sorts that 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 are 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 sensor technology. Let's dig into both. 

Resistive hygrometer probes determine relative humidity (Rh) by measuring changes in electrical resistance caused by changes in the moisture content of air. Resistive sensor technology tends to be far more robust, and when exposed to liquid moisture (such as dew point), it is less prone to drift. Relative humidity readings on industrial-grade concrete slabs typically range from 75% to 100%. This is very high and can often mean your humidity probes are going to get wet as they reach the dew point. 

Capacitive hygrometer probes work by measuring changes in the dielectric constant of a material in response to changes in the surrounding 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 commonly used to measure humidity, it does have its downfall. exposed to high humidity. What's more, it loses accuracy and tends to drive, giving inaccurate results.

Using the In Situ Method for Concrete Moisture Testing

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

The preferred method of in situ moisture evaluation in concrete 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. Add one additional hole per 1,000 square feet beyond that. 
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 the 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 secure each opening with a cap. 
6. Wait 24 hours for the probes to equilibrate with the relative humidity of their microenvironment. 
7. After one day, 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 choice is clear: reusable hygrometer probes are undoubtedly the better option.

While disposable probes may appear to offer savings, they are not cost-effective in the long run and can compromise accuracy. Just as investing in a reusable set of tableware ensures reliable performance, cost savings, and a more sustainable approach to concrete testing, investing in reusable hygrometer probes also provides these benefits.

Checking Moisture in Concrete? 

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