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
John’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?
This 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?
How 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.