This sensor was placed on a football helmet by a Purcellville, Va., parent. Parents there defied their county school administration by placing sensors on the back of football helmets of Loudoun Valley players who wanted them. (Richard A. Lipski/For The Washington Post)

For nearly a year, a group of Loudoun County parents has asked the local school system to install impact sensors on the helmets of high school football players to better detect potential concussions. And for nearly a year, the Loudoun school system has told the parents no.

But four high school football programs south of Loudoun have moved ahead and given their players football helmets containing sensors. Three programs in Williamsburg and one in Norfolk are using devices that send an alert to a trainer holding a monitor on the sideline. They are believed to be the first schools in Virginia to offer the equipment.

Dan Gotthardt, an athletic trainer at Lafayette High School in Williamsburg, said he has used it “two or three times” since practice started last week to evaluate players who had taken hard shots.

“I would have no idea whatsoever” that the players had taken hard hits, Gotthardt said of the devices made by Riddell, one of the nation’s leading helmet manufacturers. “Me and the other trainers in the county think this is a great tool for us.”

Loudoun parents said Wednesday that they were hopeful that the use of sensors in other Virginia districts would change their school system’s view on the issue. Parents at Loudoun Valley High School in Purcellville have felt so strongly about the issue that they briefly defied their school administrators and placed sensors on about 30 helmets before a practice in early August — then removed them the same day. Several of them said Wednesday they were hopeful that the use of sensors by other schools would change things.

“I can only hope that now that [Loudoun County Public Schools have] been made aware that the sensors are being used in Virginia public schools, that they will be enlightened enough to allow us to use the football sensors at Loudoun Valley,” said Dee Howard, a parent of a Loudoun Valley player.

Loudoun school officials have said that they opposed the use of the sensors, made by Brain Sentry, because the devices had not been sufficiently tested, were being used to market a new device, and that players wearing sensors might be targeted by other teams.

Another concern for Loudoun was that Riddell, the maker of all helmets in Loudoun, had warned that the safety certification “is void if the helmet or face mask is modified in any way.” But Riddell’s own sensor, the “InSite Impact Response System,” is fully tested and certified to meet national standards, spokeswoman Erin Griffin said. Griffin also said the internal Riddell sensor is designed to monitor and alert for head impact, not helmet impact.

Howard said Loudoun Valley parents are scheduled to meet next month with Debbie Rose, head of the Loudoun County School Board’s health and safety committee.

Loudoun schools spokesman Wayde Byard declined to comment Wednesday on Williamsburg and Norfolk’s actions.

The sensors do not detect concussions. Instead, as “accelerometers,” they measure the force that moved a helmet either back-and-forth or rotationally. With Brain Sentry, a red light on the back of the helmet alerts when a force exceeding 80 g’s is absorbed, and a trainer then uses the school’s concussion protocol to examine the player and either hold him out or send him back in. Riddell says its sensor monitors both linear and rotational force.

Brain Sentry is one of more than a dozen companies vying to establish itself in the helmet sensor market, and it has placed its sensors on all teams in the Arena Football League, and also at DeMatha High School in Hyattsville and the Maret School in Washington. “Most schools that look at this technology conclude that it is a ‘must have,’” said company founder Greg Merril.

But two respected experts in the field of football head trauma said the accuracy of the sensors has not been fully tested. Steven Rowson Jr. of Virginia Tech’s Center for Injury Biomechanics, which tests and rates helmets, said there was “no real independent data to verify whether [sensors] work,” and they planned to start testing them soon.