Wireless Water Meter

8 Mar 2013

A few months ago I talked to a local environmental activist opposed to the installation of wireless water meters. The root of the opposition is that some new technologies, even technologies that seem completely safe, will have surprising unforeseen risks, so these activists say avoid all new technologies unless they’re desperately needed.

In response to the opposition, my city has a page discussing the safety or RF technology. I have mixed feelings about their putting up this page because it legitimates the claims of risks of athermal RF.

This does not affect me because even if there are some bizarre unknown risks, I get much more exposure at a typical day at work than I’ll get in a year of sleeping next to a water meter transmitter.

This week I got my new water meter installed. The technician (photographed in my basement) removed the head containing the indicator on the existing water meter and replaced it with one that has a cable that connects to a transmitter. The cable contains three wires. The transmitter contains several chips and a 900MHz meandering trace antenna.

Check out the inside of the new meter. Note the meandering trace antenna toward the top and the A-size lithium cells to the left.

Some Details on My New Meter:

Manufacturer: Itron
Distributor: Badger Meter
Installation: Corix
Max TX output power 27.13 dBm (516 mW)
Max Antenna Gain: 3.22 dBi
Sensitivity: -108dBm
FCC: 15.247(a) - This one allows up to a watt as long as the signals hops or has a large bandwidth. This meter passed by hopping.
# of hopping channels: 50 to 120
Modulation: FSK, 37.5 kbps
Battery Type: 2 A-size Lithium, approximately 2Ah capacity
Battery Life: 20 years
Ave power consumption: 12Wh/(20 years * 365 days/year *24 hours/day) = 69uW.
Message Duty Cycle: 1%, depending on mode.
Max instantaneous input power during transmit: 1W - 2W (estimate)

The length of the transmission depends on the mode the meter is used in. Here is a typical burst of data from the meter from FCC testing:

(All of this is from Itron's website and public FCC filings. Itron did respond for this article, but all the info I needed was on their website or public record.)

If each message requires 200ms of transmission time, max number of transmissions over 20 years is 12Wh * 3600s/h / 0.2ms/transmission = 216,000 transmissions, or about one transmission per hour.

Meter Safety

Over the course of 20 years, the meter can transmit a maximum of 12 hours. The meter critic I interviewed was concerned about more power in a failure mode. Now I know the transmitter is battery powered. The worst that could happen is it could transmit its full 0.5W output power, that’s about 1/8th of a CB radio, constantly during a 12h period before exhausting its battery. A CB radio transmits as long as you hold the key down, while this meter sends messages mostly under 200ms long. It's hard to imagine anyone who has been around a CB radio being concerned about these RF emissions.

My meter is mounted in the basement, about 5 meters from my bed. Let’s calculate the signal strength in my bed, neglected loss from passing through the floor:

27dBm - [20*log(5 meters) + 20*log(915*10^6Hz) - 147.55] = 27dBm - 46dB = -19dBm

Now let’s calculate the signal strength of the 100kW TV station located 2km south of my house:

80dBm - [20*log(2000 meters) + 20*log(63*10^6Hz) - 147.55] = 80dBm - 74dB = 6dB.

So even if for some reason only instantaneous power matters, I’m still getting more power from a TV station.

The pulses are so infrequent and they follow pseudorandom sequence. It was hard to catch them with my portable spectrum analyzer (VIA Echo 2500). After monitoring for a day on a 915MHz duck, I caught several in the -25dBm range near my bed. It's mind-boggling that anyone should care about these signals.

Environmental Issues

Within months, residents of my city who have the wireless meters will be able to view hourly water useage information on line. That means they can see if there's a slow leak at night of if someone is using most of the water during a single activity. This should be something environmentally-minded people can ge behind.

Privacy Issues

Perhaps the real concern of the meters critics is privacy. This is true of a lot of wireless technology. The US Supreme Court struggled with this issue a year ago with regard to the question of police planting a transmitter on someone without a warrant. The justices ruled it was a search because the police came on the property to do it. Some of the justices said using wireless technology to get the same result without going on someone’s property would still be an illegal search. The argument they made was that even though years ago the police could have had someone driving by and reading an external water meter every day, it would have been impractical. These justices would say searching through wireless water meter updates would be practical today but still not allowed without a warrant.

Regardless of what you think of the Constitutional issues, it seems absurd to use old water metering technology just to avoid those issues.