True RMS Measurement Includes DC

Simple Modifications meet Customer's Needs

When measuring AC voltages, DC components or offsets usually should be ignored. Examples include measuring AC ripple from a DC power supply, and data or audio signals superimposed on DC power lines. As shown in the figure, a blocking capacitor removes any DC component.

Therm-O-Link, a wire manufacturer in El Paso, uses rectified but triac-modulated power in their annealing operation. To monitor and control the annealing process they need to know the total voltage, DC plus AC. They also need true-RMS measurement for accurate power monitoring regardless of the waveform.

A simple modification to our true-RMS process transmitter, Model JH6010IR, removed the blocking capacitor for full DC response. Additional changes speeded the transmitter's response time for proper operation in their control system.

For the complete application note, True RMS: AC or DC Response?, click here.

True RMS Measurement Basics

RMS measurements express the power capability of an AC source. Low-cost meters and transmitters often make a simpler average measurement rather than true RMS. The two are not the same, and the relationship between average and RMS measurements varies for different waveform shapes. The figure shows a pure sine wave, a chopped and rectified sine wave and a square wave pulse.

True RMS measurements require more complex and expensive circuitry, generally available in slightly higher priced instruments.

To learn the fundamentals of RMS measurement click for our application note, What is True RMS - and When do I Need It?


JH Technology manufactures:
Plug-In True RMS AC Process Transmitter
DIN Rail Mount True RMS AC Process Transmitter
Field Mound True RMS AC Transmitter with Display
Plug-In AC Alarm Trips with True RMS Response

Our web site: http://www.jhtechnology.com/

Thermocouple Application Note

Unusual Thermocouple Application

Our customer's client had a large spa facility. The controls had been located near the spa, but the client felt they should be moved to a more benign environment, several hundred feet away. The system included four thermocouples. Sending thermocouple signals this distance is expensive, since thermocouple wire (not copper) must be used. Even more basic, sending weak millivolt signals hundreds of feet leaves them subject to interference and signal degradation.

To boost the signals, four standard thermocouple transmitters (our Model JH4130) were added near the spa. Their 4-20mA current signals easily traveled the distance with no degradation. The system designers opted not to change the control system, so it was necessary to convert the current signals back to their original thermocouple voltages. "Reverse" thermocouple transmitters, 4-20mA input, thermocouple output, were needed.

We provided them. We modified our Model JH4300 DC input, DC output transmitters to have the proper millivolt outputs. The most interesting challenge was, the millivolt outputs needed to be connected to thermocouple wires (not copper) for proper connection to the system's thermocouple inputs. This called for the reverse of cold junction compensation (we called it "cold junction un-compensation"). A temperature sensor embedded in the output terminals and some properly-designed circuitry solved the problem

Temperature Transmitters & Signal Conditioners - our web site: http://www.jhtechnology.com/

Application Note

AC to DC Translation Monitors Shaker Motion

Here's an application we did 10 years ago for a major manufacturer of membrane-based filtration equipment. They're still using it.

The need - to monitor and control filter movement. Motorized systems are used to oscillate large filter elements back and forth. Peak-to-peak motion is controlled using motor drives. Motion is continuously monitored using laser-based displacement (position) measurement equipment.

The problem – the laser’s output represents instantaneous position, varying continuously as the filter shakes back and forth. The company needed to convert peak-to-peak displacement to 4-20mAdc input current for the motor drive controllers.

The solution – our Model JH5600 AC Input Transmitter. Because the filters are large and driven by rotating equipment their motion is purely sinusoidal so the relationships between average, RMS and peak values are well known. In this application an input range of 0 to 3.536 volts RMS (odd ranges are not special for us) corresponds to 0 to 10 volts peak-to-peak. We have in the past created special modifications for true peak-to-peak measurement, but this was not required for this application

For those who prefer plug-in style modules, the same function is available in our Model JH6010I.

Click here for a link to a pdf of this application note.

AC Input Transmitters, Signal Conditioners - our web site: http://www.jhtechnology.com/
 
JH5600 data sheet: www.jhtechnology.com/dinrail_tx/jh5600_5610.htm

Application Note

Detect Thermocouple Failure Without Upsetting the Process

One from the archives. Our customer needed to alarm on thermocouple failure without shutting down the process.

Thermocouple instruments commonly include upscale burnout indication. If the thermocouple breaks or fails open-circuit the instrument's output goes high, just as if the temperature had gone out of control. This often is used to insure failsafe failure, shutting the process down and sounding an alarm.

Our customer, a utility, needed an overtemperature shutdown on a monitored water temperature. They also wanted to trip an alarm upon thermocouple failure but without interrupting the process. We were able to suggest a simple dual-trip alarm setup to do this.

Our thermocouple instruments, like most others, offer a downscale burnout option. (Thermcouple failure causes the output to go low instead of high.) Using a dual-trip alarm, we set trip #1 for HI trip and trip #2 for LO. The HI trip, of course, was used to shut the process down on excessive temperature. The LO trip was set below freezing. Its relay would trip when the thermocouple failed but never under normal operation.

For the full application note, click here.

Our web site - Signal Conditioners, Process Transmitters: http://www.jhtechnology.com/

E-mail: jhtek@jhtechnology.com

Signal Isolator Serves as Subtractor

Measurement and control applications sometimes need a difference (A - B) calculation. Examples include:

• Liquid level measurement in a closed vessel. Level is proportional to the bottom liquid pressure minus the top ambient (gas or air) pressure.
• Liquid blending control. Must maintain a zero difference between two measured flow rates.
• Relative speed control. Control for zero difference between two speed measurements.

JH Technology offers a complete range of multiple input add/subtract instruments, but when the inputs are voltages a lower-cost isolator may do the job.
The figure shows two voltages, both grounded. The signals may come from two pressure or flow transmitters, tachometers or other sources. The isolator's input is connected between the two positive voltages; thus, the input is (V1 - V2). Its output may be set to equal the differential input or it may be scaled up or down, offset or converted to a current signal such as 4-20mA.
We offer plug-in, DIN rail and field mount style isolators and transmitters.

Links: Add/Subtract Transmitter Modules
DIN-Rail style add/subtract transmitters
All Process Transmitters and Alarms

Split 4-20mA Loops without a Loop Splitter

Save money! Don't buy a loop splitter if you don't need one.

Users often specify dual-output 4-2omA current loop splitters in their systems. Common reasons include:

• The need for an output boost to drive several loads at once.


• The need to run independent isolated current loops to two different systems.
  

These needs often can be met with a less-expensive single-loop isolator. The diagram shows the basic idea - use the source transmitter to drive the isolator's input plus the first loop, then use the isolator's output to drive the second loop.
















For our complete application note (pdf) click here , or visit http://www.jhtechnology.com/ and click on the Application Notes link.