Accurate RV tank gauges? This could be the real answer!


By Russ and Tiña De Maris
Ask anyone who has an RV, “Are your holding tank gauges accurate?” The nearly universal reply, “Are you kidding?” Accurate RV tank gauges are the stuff that dreams are made of. The typical OEM holding tank level measuring system is based on that ages-old idea of running electrical current through probes in the tank, and where the fluid in the tank touches the probe. There you’ll get a reading that flashes up on an indicator panel. They work great, for what – “five days or 50 miles, whichever comes first.” After that, the most common complaint is that crud in the tank fouls the probes, and you’ll be presented with a “full” indicator for your black water the moment you dump the tank.

For what seems ages, the question in RVers’ minds is this: Why can’t they make accurate RV tank gauges? Perhaps it’s not high enough on the priority list for those geniuses back in Indiana. But give the problem to somebody who’s directly affected and, you know, sooner or later they’ll come up with an answer. Enter John Van Vander Horst, who, by his count, is now on his fourth fifth-wheel. John was more than dismayed when he, too, fell victim to inaccurate holding tank gauges. Rather than looking at it from the standpoint of electrical probes, wires, and indicator lights – any of which can crap out – John thought about the problem from his point of view. An engineer, John’s background is in fluid hydraulics. How, wondered John, can we skip the electrical and go simple?

Scientific principle to the rescue

Accurate RV tank gaugesJohn’s “light bulb” centered around a simple scientific principle called hydrostatic equilibrium. Our friends at Wikipedia tell us this boils down to: The pressure in any point in a fluid at rest (inside your holding tank, for example) is due to the weight of the overlying fluid.” If you dive down to the bottom of a deep swimming pool, you’ll soon feel the effects of hydrostatic equilibrium. The pressure, due to the weight of the water above you, increases. John applied this matter of hydrostatic equilibrium to RV holding tanks. The more water (or waste) in a holding tank, the greater the weight at the bottom of the tank. But how to use this information in a practical way?

Imagine you have a water glass, six inches tall. Fill the water glass to a height of four inches. Now put a drinking straw in the water. Water will fill the straw right up to the height in the glass – four inches. Now put your mouth on the straw and gently blow. Air from your breath displaces the water in the straw. Blow just until you get a bubble coming out of the straw. If you could then measure the pressure required to displace all the water from the straw, you could calculate how many inches of water are in the glass.

So how does it work?

Accurate RV tank gaugesThis idea is the genius of the Horst Miracle Gauge – making for truly accurate RV tank gauges. From a small control panel, you press an actuator that forces air down a plastic line to your holding tank. The line is run through the sidewall of the tank, and forces air into the tank. Back at your control panel, a needle inside a gauge runs up, then settles as hydrostatic equilibrium is attained. Once settled, the gauge indicates to within 1/8″ how many inches of fluid are in the tank. Say your black water tank is eight inches tall, if the needle settles on 7 3/4”, you know you’ll soon need to dump that tank. Turn the handle of the actuator to a different position, repeat the process, and you’ll know how much water is left in your fresh tank, or how much waste in your gray tank.

Since there are no “probes” in the system, and no electrical wires to disconnect or break, the erratic guesswork of “how much” is eliminated. Even when the typical probe-and-indicator system does “work,” there’s still guesswork involved. Say you press the button and the indicator shows that at least two-thirds of your black water tank is full. For a 40-gallon tank that means you’ve got at least 26.6 gallons in it. Or it could mean you’ve nearly reached the full mark, but the “full” probe just hasn’t been hit. Do you move your RV to dump tanks now? You might still have 13 gallons of free space left in the tank. What a bother!

How do you install it?

accurate RV tank gaugesHow does one install the Horst Miracle Gauge? First you’ll need to find a suitable spot to mount the control panel. John Vander Horst recommends putting it near where the tanks are located, typically in the bathroom. A couple of holes are bored into a wall. The plastic lines from the tanks will come up from the basement (or floor) and into the control box. One plastic line runs to each of the holding tanks – three lines. You’ll need to access the tanks, and carefully drill a hole near the bottom of the tank. The free end of the plastic line is then fed through a rubber grommet, and the grommet inserted into the hole you’ve bored into the tank. The plastic line is routed across the top of the tank, and from there to the control panel. As an alternative to drilling a new hole, you can also remove the existing ground-wire probe for the OEM tank gauge probe and install the grommet through that probe hole.

More information or get one

The Horst Miracle Gauge system comes with the control panel (with actuator and gauge installed), 37 feet of plastic line, and grommets. The whole works sells for $165, with free shipping. The company has an interesting website with a series of videos to explain how the system works and installation information. Here’s a link to the company website. And yes, we know some of you have more than one gray water tank. John Vander Horst is already working on a system that will accommodate up to four tanks.

How well does the system really work? From the questions we put to John Vander Horst, and by the information provided on his website, we’re pretty excited that this is the answer to truly accurate RV gauges. We’ll be installing a Horst Miracle Gauge system in our rig, and we’ll report back with just how it went, and how well this new idea tests out. Stay tuned.

Related reading

How accurate are the holding tank gauges on your RV? (poll)


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6 months ago

If you need absolute accuracy a clear sight tube is the only answer.
Best way to improve on what most of us have currently, 33%, 66%, 100%, is to install a float sensor such as those in a vehicle gas tank. (with the addition of some way to keep the float assembly clear of debris.
It almost seems that any system is destined to fail.

6 months ago

Interesting concept! Thanks!

6 months ago

I used the previous Horst tank gauges which “guaranteed” accuracy and failed within weeks so there is no way I am buying another product from them. I have been in a motorhome since 1973 so have a bit of experience. See Level II is the answer.

Tom H
6 months ago

I’ve used the SeaLevel system on 4 different rigs; they are the first upgrade I make. We’ve been quite happy with them. The sensors read the tank level as a percent of the height of the sensor strip at the point where they are attached, so the numbers do depend on the shape of the tank and how well the sensor strip is adjusted up and down to sense either or both empty and full. After a couple of tank uses you learn what the percentages mean for that particular tank — which is true for any system that isn’t designed for a particular tank size and shape.
This year I finally “calibrated” my fresh tank with an inexpensive flow meter attached to the fill hose: add so-many gallons to an empty tank then read the gage to see what percentage matches that number of gallons.
These gages make for great peace of mind — especially after 14 days in the boonies.

Last edited 6 months ago by Tom H
6 months ago

I don’t know…I empty my black tank when it “burps” or “bubbles” and my gray tank starts to creak when it’s getting full. Don’t use my fresh water tank. Easy peasy!

6 months ago

“The pressure in any point in a fluid at rest (inside your holding tank, for example) is due to the Weight Of The Overlying Fluid.” So why can you not just use the weight of the tank to figure out how much is in it?

6 months ago
Reply to  Rick

How would you determine that?

6 months ago

It is an air tube system. When you switch from one tank and tube to the other there is a pressure release from one tube and then a pressure build up into another. What are the safety measures in place to prevent cross contamination from tube to tube???

dave mackler
6 months ago

The only practical way for this system is a factory install, no way are most of us going to tear up the covered underbelly

Gene Bjerke
6 months ago

The monitor lights for our waste tanks are notoriously inaccurate, but the ones for the fresh water tank are reliable. I noticed that the combined capacity of our holding tanks is slightly more than the capacity of the fresh water tank. Since that the fresh water is gradually moved to the waste tanks (some of it through the occupants), I know that my waste tanks need to be dumped when my fresh tank is almost empty. So I dump my tanks and refill my potable tank and I’m ready to go again. This system works because I rarely hook up to CG water except when I dump.

Ridge Gardner
6 months ago

Richard West
6 months ago

I find the continued search for indicator solutions to be quite entertaining. We’ve been full time in the same motor home for 15 yrs. Within weeks I found the lights were unreliable. I quickly became “Attuned” to the use of water and the filling of tanks during our daily routine. I rarely bother with the Lying Lights. I just know what’s going on around (Under) me.

Chuck B
6 months ago

There is nothing new or a miracle about this system. It has been around for many years and known as Tank Tenders. They are even more simple since they work by pushing a pump button on the panel. We used them for many years. Chuck

6 months ago

Most people figure out after a trip or two, how to judge how full the tanks are and when to dump. Took me two trips in my new 5er to figure it out. Not worth the $165 to me. In the words of Shark Tank, neat concept, but it’s just not for me, Im out.

6 months ago

As another engineer, I view the requirements as accuracy, precision, and reliability. Our black water idiot lights are none of the above, because they get fouled so easily. Grey water light not so bad, fresh water light fairly precise and reliable but not accurate. Ours show full when the tank is at about half capacity, the only way to be sure it is actually full is to fill it until it overflows. When it shows empty, we really are close to empty.

My big complaint is that the grey water tank is smaller than the fresh water tank, so it is easy to overfill if you don’t have a good indicator. Water standing in the shower is not the best way to decide to dump. So, we dump when the light shows the grey water is full, even though we may have a few more gallons. Experience has shown we are close enough, and the black water tank is never more than half full at that point. Therefore, even though the lights are not accurate or precise, we can reliably manage the system.

Sink Jaxon
6 months ago

Ok this is all fine and dandy…as is the SeeLevel product as mentioned here in the comments. But the way my OutdoorsRV TT is built, there’s no access to the tanks, unless I want to cut open the underbelly. Which would be idiotic. It’s just NOT that important!

6 months ago

Let me start a poll….

Who even cares what the “little lights” indicates?

A quick look down the horn, with the water turned off, reveals all…so to speak.

I will say that my wife and I do the Happy Dance and high-five each other anytime we dump the black water and the indicators show “empty”. The lights I guess do give us something to talk about. : )

6 months ago

We’ve had a Tech-Edge brand of tank monitors at, for 12 years. They don’t require any holes drilled & there is nothing inside the tank. They are fairly accurate though I’ve had some problems with accuracy over the years, mostly due to the fact our tanks are sloped to drain better. So the water accumulates at one end & gives a false reading for much of the time it’s filling up. They do have a solution for registering every 10% of the fill process, but the programming to set it up is lengthy & complicated, so I’ve never tried it.
The Horst system would have the same problem with sloped tanks. It would measure the pressure at the lowest point in the sloped tank, but would not give an accurate reading of the overall tank capacity. My 6ft long tanks are sloped, both from each side towards the center line, & from one end to the other which makes measuring fill rate difficult, but they drain great.

6 months ago
Reply to  Fred

I don’t like the idea of drilling a hole in my tanks at all. Thank you for the alternative offered. I think the problem with slope is just calibrating the measurements (i.e. 20% = 28%, 40% = 51%, etc.) by doing a one-time gallon by gallon fill.

6 months ago

Just read the comments. The gauge was marked in inches, the physics of the pressure is always the same. You just need to know the height/depth of that tank. Waste tank was 12” tall, so 12” on gauge means full. Water and fuel were around 30” tall, so 30” on gauge would mean full. It was so simple and accurate. Never had any solids cause a problem in waste tank. Maybe because constant moving of the boat kept the waste stirred up?

6 months ago

I had a similar system on a boat that I lived aboard. It worked good , was very accurate. Had a single gauge and a button for each of 4 tanks, 2 fuel, water, and waste. You press the button for the tank you want, pump the little plunger a couple times to push air through the tiny tubes. It used 1/8” copper tubing, so you could drill the hole on the top corner of a tank and run the tubing straight down. Less chance of a leak that way.
Tanks on a boat are usually taller and not as long/wide, but I think it would work in an RV tank.

6 months ago

So now we have another “better mouse trap” in the Horst Miracle Gage for $165. Any unbiased reviews on the gadget? I wonder how susceptible it is to the tube clogging with crud? Yes, I am cynical but I have been down this road before. Hope it pans out. It would be an improvement on “Honey. it has been 5 days, time to dump the tanks”.