Friday, October 15, 2021


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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27 days ago

Funny this should pop up again in this newsletter, since I just started research/experimentation on solving this issue once and for all on my new rig which I HOPE to keep for a long time (hence, worth the work). I’m looking into:

1) Horst teflon-shielded replacement probes. Used this on a past rig; better, not perfect.
2) DIY “headunit” that detects resistance teirs rather than set ranges with same probes. Better, not perfect.
3) DIY captive-air pressure sensor (inverted tube with pressure sensor at the top. Water rising up inverted tube raises air pressure. Believe it is still subject to clogging with ICK).
4) DIY capacitive sensor system (like SeeLevel does) — no probes, it “feels” water level through the tank wall. This seems TRIVIAL cost to build, and (being an embedded system programmer) I can calibrate precisely and display all 4 tanks plus DC voltage on one tiny screen. This is looking like the winner by FAR, pending some “Tuckertote testing”… 😀

Mike A Schwab
27 days ago

Use the Geo Method or Happy Camper to liquefy anything on the sensors.

Rock & Tina
3 months ago

In the strictest sense what we all use to measure our tank levels are electrical probes connected to meters, not gauges, and these electrical devices can be fooled by any damp material laying across the probes which completes the electrical circuit. What the article discusses is a mechanical pressure sensing system connected to a mechanical pressure gauge which is simpler, more reliable and not subject to an electrical miscue.

3 months ago

Why can’t we have something simple like a dipstick?
it will be rather accurate for a long time.

Mark Birnbaum
3 months ago

I am just happy if gauges are approximate. When I got my 17 year old Lazy Daze MH, the black tank gauge was useless. Lazy Daze coats the exterior tanks (black and grey) with 2+ inches of protectant/sound insulation. Because of that coating, I didn’t bother with replacement sensors. This year, I read about “Happy Camper” treatment. I had tried other brands and a swivel stick. NOTHING. I tried Happy Camper. It took some driving time, with tank 1/2 full and the mix sloshing around, but dried chunks starting coming out. My sensor now works, very reliably.

They also have “Happy Camper Extreme”. I would guess it is even more potent, but the regular product made me a happy camper.

Ron Lane
3 months ago

While indicating empty, 1/3, 2/3 or full is not really accurate by any means, it does provide a simple measurement that if you keep track of the gauges, it is sufficient to supply enough info to allow you to monitor your tank levels. Of course this requires that the sensors stay free of debris and work as designed. Unfortunately, most rv’ers don’t have a clue and think by dropping in some so-called tank chemicals will do the trick. Tanks (both) need to be routinely washed out with soap and water….not chemicals….and requires lots of water usage, which of course makes boon docking difficult.

3 months ago

It all depends on the shape of the tank. Some Lance trailers have a stepped blackwater tank so the top 3rd has less volume than the rest. [Also makes for problems as the shallow end is a shelf directly under the toilet]

4 months ago

Newmar has invested in a black tank pressure sensor. We suffered for 2 years with black tank gauges not working or partially working until our system was modified to this newer sensor. No problems now,

1 year 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.

1 year ago

Interesting concept! Thanks!

1 year 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
1 year 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 1 year ago by Tom H
1 year 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!

1 year 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?

1 year ago
Reply to  Rick

How would you determine that?

27 days ago
Reply to  FrankB

There are simple stress sensors, but I doubt that method would stay accurate for long.

1 year 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
1 year 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

3 months ago
Reply to  dave mackler

Tearing up the underbelly is my concern too. Besides, the price is a deterrent, and I’m fine with “approximate”. My first trailer didn’t have any gauge at all. You could open the cubbyhole and see how much fresh water was there. If the water level was very low then it was time to dump.

Gene Bjerke
1 year 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
1 year ago

Richard West
1 year 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
1 year 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

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