Is a typical transducer programmed to output its signal in intervals? Say every 10 seconds it would generate a signal via 4-20mA loop? The other 50 seconds it would output 4mA corresponding to stand-by mode.
No. No known 2 wire loop powered transmitter works in that fashion. Too much difficulty synchronizing a field device's 'idle' mode with the Data Acquisition side.
I'm sizing a battery for my telemetry project and am curious if these field devices are continuously outputting a value or can be programmed to output a value in cycles.
Continuous 4-20mA devices consume a lot of battery power over time.
There have four approaches for low power or battery powered 'telemetry' that I'm aware of.
1. Rosemount offers a "low power" pressure transmitter for oil/gas field installation for use with RTU's that is at three or four wire device, it's output is voltage, not current. RTU's tend to be (relatively) low power consumers. I'm not aware of other specific 'low power' field transmitters.
2. Wireless HART and ISA 100 wireless process systems use battery powered field instruments (pressure/temperature, primarily) that are designed to go to sleep, wake up, take a measurement, transmit/shoot the measurement to the base station, then go back to sleep, on scheduled, periodic intervals.
I have hard evidence that Wireless HART battery life is much shorter than ISA100 battery life for a number of network design reasons.
Both these are 'systems', where the wireless devices are designed to work in a networked system; the battery powered field instrument will not operate stand-alone in your system.
3. There are battery powered Banner Engineering node (field) radios that can take an RTD or Thermocouple direct for temperature readings.
Then there is a Banner Engineering battery powered node (field) radio that uses the single lithium D cell that ordinarily powers only the radio to intermittently power a 2 wire, loop powered field transmitter.
One configures the node so that the field transmitter is only intermittently powered up and is turned off the rest of the time. During the power-on time, the radio takes a reading and transmits the value to the gateway radio. One needs to experimentally determine the time period for the field instrument to initialize and make a reading. Dumb pressure transmitters can initialize and make a reading in less than 3-4 seconds, some 'smart' radar level devices require 40 seconds to initialize and make a reading.
There's one Banner node radio model with a 'boost' power feature to get the higher voltage needed to run certain radar or ultrasonic level transmitters (which are power hogs compared to pressure or temperature transmitters).
The update rate for a power hog radar level transmitter can be as long as every 30 minutes in order to get a 3 month battery life. Banner has a battery pack of 6 lithium cells that will last longer at a price closer to Wireless HART batteries ( $300).
The node radio configuration is a pain, calculating wake up and sleep times using some odd ball value like 64mS and converting the number of intervas to hexadecimal value, but it can be done.
4. Solar powered rechargeable batteries powering the loop
I'm sure people do this but they're not in our customer base. In fact, we stopped quoting this option because we never made a sale. I just tell people to look elsewhere for someone willing to shop the market for cheaper solar powered options. Sounds like it might be your bailiwick.
FYI, I once ran a test with a typical 'motorcycle' lead acid battery power source (sorry, don't have the amp-hour rating on this computer) running a 4-20mA level transmitter for 14 days continuously at a deliberate continuous 20mA output, and the loop died on the 15th day. No recharge during the test, just a test to see how long a lead acid battery would last.
Dan
