Montana is about as far from the sea as it gets, and has been since at least 60 million years ago. And yet when it comes to measuring the acidity of ocean water, a group from Missoula has proved to be the best at building both a cheap and durable sensor, capable of accurately measuring changes in pH, or degree of acidity/alkalinity, while plumbing depths of 3,000 meters. That's how Sunburst Sensors came to win both of the Wendy Schmidt Ocean Health XPRIZEs, as announced at a gala on July 20.

"It's so much easier to fly by Pluto than to get data out of the oceans," says Wendy Schmidt, who funded the prizes to help develop technologies to solve real-world problems. "But one is our lungs, and one isn't. One gives us food and one doesn't."

Schmidt has previously funded an XPRIZE that developed better methods for cleaning oil spills. But for this set of prizes the goals were an easy-to-use device that could reliably measure ocean pH cheaply and a more robust sensor that could accurately measure pH under extreme conditions, such as the cold temperatures and crushing pressures of the deep sea while still detecting variations of as little as 0.002, which is roughly the annual rate of change presently. Ocean acidification is a "silent threat," Schmidt says. "We need to attach people to the oceans as their life-support system, pantry and playground."

The atmosphere now hosts roughly 400 parts per million of carbon dioxide, thanks to fossil fuel burning and the cutting down of forests, among other human changes to the planet. But roughly a quarter of the excess CO2 released by all this human activity currently dissolves into the waters of the global ocean, creating carbonic acid and rendering seawater more acidic. Based on measurements to date the sea has become 30 percent more acidic, a drop on the logarithmic pH scale from 8.2 to 8.1. "We know nothing of pH at depth, which is a real concern," says biologist Paul Bunje, a senior director for oceans at the XPRIZE Foundation and administrator of the contest.

But with cheap, easy-to-use sensors much more data on ocean acidification can be gathered, including variations that can shift in a matter of few centimeters up or down or side to side. "They have to be inexpensive because they get killed all the time," Schmidt says. "They need to be ubiquitous and disposable."

So 18 teams entered the competition back in 2013, ranging from an outfit that proposed putting pH sensors on surfboard fins known as Smartphin to a group of crowdfunded high school kids out of Carmel, Calif. All the finalists passed through three rounds of tests, including tests in a 27.5-meter tank at the Monterey Bay Aquarium, a monthlong trial run in the coastal waters of Puget Sound and finally a six-day deep-sea test roughly 160 kilometers off the coast of Hawaii. Durable sensors will mean that some could be outfitted on autonomous gliders and perhaps sent under the Arctic sea ice in winter to gather data for six months or a year.

At the end of all the testing, Sunburst Sensors emerged as winner of $1.5 million of XPRIZE money to further develop the devices it has been working on since 1999. Missoula high school acquaintances, mechanical engineer James Beck and ocean chemist Mike DeGrandpre developed an inexpensive submersible autonomous moored instrument, or i-SAMI, that costs less than $1,000 per sensor as well as a titanium version for durability, known as t-SAMI. The i-SAMI is basically built on "all the cheap parts they could find," Bunje explains. Both devices rely on pulling in a seawater sample, injecting dyes and shining a laser on the resulting mix to detect pH level. "It shouldn't take an expert in marine sensing to operate one of these things," Bunje adds. And in the case of Sunburst's i-SAMI, it doesn't, thanks to self-calibration and the like.

Of course, Sunburst was not the only winner. All the finalists continue to develop their sensors, including an offshoot of oil-and-gas technology giant Schlumberger dubbed ANB Sensors, for acid-neutral-base, as well as a collaboration between researchers at Monterey Bay Aquarium Research Institute (MBARI) and the Scripps Institution of Oceanography with companies like Honeywell, known as Team DuraFET for the sensors that are already found on many ARGO floats surveying the global ocean.

The tests didn't all go smoothly, including one of the sensors breaking free of its moorings during the tank test and sinking to the bottom, thanks to greater than expected corrosion of the marine grade steel in more acidic seawater. And in the open-ocean test the ANB sensor, thanks to some faulty plastic, found itself crushed at a depth of 2,000 meters. That didn't prevent that sensor from garnering second prize for affordability and ease of use. "I want the most elegant sensor," Schmidt says. "Who will be the Apple of sensors?"

Even those teams who didn't make the finals, like the Smartphin pH sensors for surfboards (and possibly the keels of sailboats), are moving forward. "They are testing in San Diego this fall and giving them out to surfers," Bunje says. "They want surfers to start having this deeper relationship with the oceans."

Monitoring and knowledge of the pH problems of the ocean are about to get a whole lot better worldwide, whether from citizen scientists on surfboards or from coastal managers, who may find it easier to regulate ocean pH now or for long-term monitoring of coral reefs. "There is this whole emerging field of ocean services," kind of like a weather forecast, Schmidt says. "When shellfish are not available, when the salmon are faltering, when jellyfish fill the oceans, we'll be there."