by Keith Yoerg, Reaction Research Society Secretary
The RRS Mojave Test Area (MTA) hosted a launch event and work party on Sunday, September 26th. The USC Rocket Propulsion Lab (USCRPL) had arrived a few days earlier to prepare for a static firing of their 8″ diameter solid rocket motor named “Earthshaker II” which took place on the 26th. Several RRS members also answered the “Yoerg Challenge” to launch model rockets, and Dimitri was out with his water rockets. On the work side of things the Dosa building was re-organized, a security camera was installed, and a discussion began on how best to replace the aging roof on the blockhouse.
USCRPL 8″ SOLID ROCKET STATIC TEST
USCRPL had their setup ready for a static test of their 8″ solid rocket motor in the late afternoon, which was secured below the vertical test stand. Unfortunately, shortly after coming up to full power the motor exploded. All personnel were at a safe distance in the bunker and no one was injured. RRS President and Pyrotechnic Operator in charge Osvaldo approached the site once it was safe and extinguished the resulting flames.
Still shot from a video of the USCRPL motor explosionPyrotechnic Operator Osvaldo bringing a fire extinguisher to the lingering fires
All requests to use the RRS MTA must be made to the RRS president and reviewed by the executive council. For any questions about this test series or any future test series, please contact the RRS president.
president@rrs.org
YOERG CHALLENGE MODEL ROCKET LAUNCHES
Many RRS members had model rockets on hand to answer the “Yoerg Challenge” and launch at the MTA site. Dimitri and his son Max launched a “Helios” and “Dazzler” on C6-3 motors. Keith launched a “Baby Bertha” on a B6-4 and a “Big Bertha” on a B6-2. Dave Nordling launched a “Baby Bertha” on an A motor. Bill Inman & Jon Wells also launched model rocket kits, and John Krell launched a model kit on a G motor. (I will endeavor to do a better job of recording the rockets & motors that everyone uses at these launches for more specific reports in the future).
Keith Yoerg, Bill Inman (on the launch box), Waldo Stakes and Diana Castillo wait as the countdown progresses.
We did not have the new wireless Cobra firing system at the MTA site during this event, so we used the 4-pad controller that Dimitri built earlier this year. The controller split its time between this low-power launch pad and the water rockets which Dimitri had set up on the underground blockhouse.
Several of the model rockets ready to launch on the PVC launch pad built by Keith Yoerg
I will also mention that prior to these launches, we enjoyed a nice potluck BBQ of brats, (homegrown) potato salad, chips, beans, and corn. Several members contributed food which was expertly prepared by Becky. We’ve been doing this more often and seem to keep getting better at it every time!
WORK PARTY TASKS
In addition to the more exciting “fiery” aspects of the day, RRS members also completed a lot of routine maintenance at the MTA site. We completed several general organization tasks in the Dosa Building and the storage containers, and a security camera was installed on the Dosa Building. There was also a lengthy and robust conversation about methods to replace the aging blockhouse roof, which has been high on the the society’s list of desired site improvements for several years.
Keith Yoerg and Jon Wells discuss options for repairing the old blockhouse roof.Security camera installed on the Dosa Building
originally published 9/30/2013, reprinted with permission
It seems that at least one 4-20 milliamp (mA) measurement is required by our typical customer, and the way to do it is a constant source of confusion for many. So I thought I’d zero in on the various 4-20 mA current loop configurations and elaborate on the specifics you need to know to make a successful measurement. The following is ordered from the most to least common configuration, and I hope to cover all those that I encountered in customer applications. If yours isn’t included, please contact DATAQ technical support.
dataq.com
4-20 mA Current Loop Basics
Sensors or other devices with a 4-20 mA current loop output are extremely common in industrial measurement and control applications. They are easy to deploy, have wide power supply requirements, generate a low noise output, and can be transmitted without loss over great distances. We encounter them all the time in both process control and basic measurement data logger and data acquisition applications.
The idea behind 4-20 mA current loop operation is that the sensor draws current from its power source in direct proportion to the mechanical property it measures. Take the example of a 100 psi sensor with a current loop output. With 0 psi applied, the sensor draws 4 mA from its power source. With 100 psi applied the sensor draws 20 mA. At 50 psi the sensor draws 12 mA and so on. The relationship of mechanical property measurement to current output is almost always linear, allowing the resulting current loop data to be scaled with a simple mx+b formula to reveal more useful measurements scaled into engineering units.
How you actually measure the 4-20 mA current loop signal is a function of the sensor’s architecture and the capabilities of the instrument you’ll use for the measurement.
Terminology
So that my discussion translates well across the various kinds of 4-20 mA current loop configurations, I’ve opted to standardize the terminology I use to describe each. Here’s an overview:
“E” (DC excitation)
Most configurations that follow will show a DC voltage excitation source that I denote as “E”. Many who use current loop sensors for the first time are surprised to learn that they need to supply this excitation source. Nonetheless, unless the sensor is self-powered (i.e. AC line powered) an external dc source is required. The good news is that this can sometimes be supplied by the instrument, and the range of acceptable supplied voltage values is usually very wide, typically 10 to 24 VDC.
“R” (shunt resistor)
Here’s a bit of trivia for you:
No instruments measure current directly.
They all do it indirectly by measuring the voltage dropped across a resistor of known value, and then they use Ohm’s Law to calculate actual current. The resistor is referred to as a “shunt”, is absolutely required to make a current measurement, and is either supplied externally to, or built into the measuring instrument. For clarity, I assume that it’s supplied externally.
“i” (current loop value ranging from 4 to 20 mA)
This is the 4 to 20 mA current signal generated by the sensor. Note that some sensors may valium draw 0 to 20 mA and even other values, but the vast majority of them use the 4 to 20 mA convention.
“v” (shunt voltage that’s proportional to current)
This is the voltage drop across the shunt that is actually measured by the instrument. Since our industry has standardized on a shunt value of 250 Ohms, “v” will range between 1 and 5 volts for a 4-20 mA current loop signal.
Note that shunt resistor value is arbitrary as long as it’s a known (fixed) value. You also need to ensure that it doesn’t burden the loop, so lower values are better than gabapentin higher.
Yes, I mean lower.
Remember that we’re working with current, not voltage, so the rules are inverted. Just as infinitely-high resistor loads work well for a voltage source, you can take the load all the way to zero Ohms for a current source without consequence.
Self-Powered Sensors (Most Common)
I promised to order these configurations from most to least common, and the self-powered sensor just noses out the first runner up. Self-powered sensors are those that, well, power themselves. The sensor may have an integral ac power supply, thereby negating the need for an external DC power source. https://madronadermatology.com/accutane-online/
Or it may not be a sensor at all. It could be an output from a Programmable Logic Controller (PLC) or other source that is internally powered.
2-wire Sensors (Low-side Shunt)
Okay, this can get confusing for first-time 4-20 mA current loop users.
Yes, it is possible to both power the sensor and measure the current it draws over the same two wires. In the 2-wire examples shown here, only two wires connect the sensor to its power supply, and the sensor draws current from it in direct proportion to the mechanical property that it measures. As current changes, the voltage developed across resistor “R” will change, thus providing a signal that’s suitable to connect to a measuring instrument like a data logger or data acquisition antabuse system.
In most situations, care should be taken to place the resistor in the low-side of the loop as shown here, as opposed to the high-side. Doing so will allow non-isolated instruments to make the measurement. In the next section, I’ll deal with a high-side shunt placement and discuss these cautions in more detail.
2-wire Sensors (High-side Shunt)
This configuration is almost exactly like the low-side, 2-wire approach, but it places the shunt resistor in the high-side of the loop. Note that while the voltage across the resistor is proportional to the current drawn by the sensor (just like the low-side approach), there is also a common mode voltage (CMV) present on either side to ground. On one side to ground the CMV is equal to the supply voltage. On the other side to ground it’s equal to the supply voltage, less the voltage dropped by the resistor (v).
The presence of the CMVs places conditions on the instrument that you use to measure v. Specially, the instrument needs to have an isolated front end so it can float to the level of the CMV and still successfully make the measurement. Try this with a non-isolated, single-ended instrument and you will short-circuit the sensor to ground. A non-isolated differential instrument will either saturate or provide erroneous results.
3-wire Sensors
Three-wire sensors with a process current output have a separate wire for ground, signal (4-20 mA), and the power supply. This configuration is the easiest for current loop beginners to grasp, one input for power and a second for the current loop with a common ground. The primary advantage of a 3-wire sensor over its 2-wire counterpart is its ability to drive higher resistive loads. Resistors drop voltage for any given current in direct proportion to their resistance value. Holding current constant, higher resistances drop more voltage. Turning back to the 2-wire sensor and holding current constant, as the shunt resistance increases the voltage drop across it also increases. You might reach a point where the voltage dropped by the shunt lowers the voltage drop across the sensor below the minimum required for it to operate properly.
We had a customer whose 2-wire current loop measurements functioned beautifully until loop current reached about 18 mA, at which point everything went haywire. Upon close examination, we determined that the supply voltage she used was too low by at least 0.56 volts. She needed 2 mA more measurement to reach full scale, which translates to 0.56 V with her 250-Ohm resistor. The solution was to use a higher voltage power supply to ensure that the voltage drop across the sensor stayed above the minimum level. She could have also used a 3-wire sensor, which ensures that the voltage applied to the sensor is independent of shunt resistor voltage drop.
Watch Your Grounds (or use an isolated instrument)
Contrary to what many believe (and have been erroneously taught in school), grounds are almost never the same in industrial settings, exactly where most 4-20 mA current loop sensors are used.
Two or more grounds that are the same means that they are at the same potential. If so, a measurement between the grounds of the various field sensors and the instrument using a digital volt meter (DVM) on both its DC and AC settings will show zero volts, or very close to it.
In reality, you’ll measure at least several volts, and I’ve seen as much as 75 Volts. When grounds that are not at the same potential are tied together (which you need to do to make a measurement), current flows through them, creating several possible measurement outcomes for non-isolated instruments:
The measurement is noisy.
The measurement is inaccurate.
You irreparably damage the instrument.
You saturate the instrument (it’s not damaged, but you can’t make a successful measurement, either.)
To remedy these problems requires the following:
Use an isolated instrument for your 4-20 mA current loop measurements. This single decision allows you to ignore all other grounding issues in exchange for successful measurements in any situation. If you don’t have an isolated instrument, read on…
Ensure that the loop power source is isolated. This means that its output ground (the one connected to the sensor) is not tied to its input ground (the one that connects to AC line power.) An isolated power source means that the output ground can be tied to another ground (like a non-isolated instrument) without consequence.
If using self-powered sensors, ensure that the low-side of the loop is isolated from its power source.
If using sensors that require an external dc power source, ensure that the shunt resistor is placed in the low side of the loop (see “2-wire Sensors (Low-side Shunt)” above.)
If you lack control over the power sources and determine that they are not isolated, then your only option is to power ALL devices (power supplies, self-powered sensors, the instrument, and its connected PC) from exactly the same power outlet. Don’t make the mistake of using outlets that are close to each other. If you run out of receptacles on a single outlet, then use a power strip.
Again, it’s worth repeating that all of the cautions associated with proper grounding disappear if an isolated instrument is used to make the measurement.
Sensors with 4-20 mA outputs are encountered in all disciplines and in many configurations.
Contact DATAQ with any questions that arise in your unique situation.
dataq.com
This article has been reprinted with permission from DATAQ Instruments, a manufacturer of quality data acquisition and data logger products used by many professionals and amateur rocketry hobbyists.
The RRS is thankful to DATAQ for their assistance.
The latest meeting of the Reaction Research Society took place Friday, September 10th and had 16 attendees, including several members of the University of Michigan Aeronautical Science Association (MASA) team which conducted a long campaign of testing at our MTA site last month. After brief hellos & a short discussion on updates to this website, we got the meeting rolling with a presentation from the MASA team debriefing us on their August tests.
Screenshot of discussion during the monthly meeting
PRESENTATION ON MASA CAMPAIGN AT THE MTA
4 members of the MASA team who were present for the long campaign of testing at the MTA site were at the meeting and provided a presentation detailing how the nearly 2 weeks of work went for the team. A detailed write-up from RRS member Dave Nordling who was on site assisting for some of that time is available here.
MASA logos – Tangerine Space Machine is named after a craft brewed beer local to the Michigan area
While the end goal of conducting a hot fire test was not accomplished, there was a ton of great work and learning opportunities for both the MASA team and the RRS. The first challenge for the MASA team was in driving their equipment all the way from Michigan to Southern California. Unfortunately, they had some equipment suffer damage during the trip when their smaller “Ground Support Equipment” (GSE) trailer broke through the plywood floor of their larger travel trailer. This required them to stop in Texas and complete repairs to the travel trailer before continuing on to the MTA site.
Once at the site, the MASA team set up a 2-shift schedule (9am – 6pm and 5pm – 12am) effectively working 18-hour days to conduct the activities required for their tests. Many of these tasks took a lot longer than the team had anticipated, in particular with the supply and delivery of the gas & cryogenics needed. The MASA team was originally in contact with Airgas, but their communication was with the office closer to their home in the Midwest and not the Southern California branch. Ultimately sorting through those issues proved to be too difficult to secure a delivery while the team was at the MTA, but fortunately they were able to work with Praxair to secure a supply of the gas and cryo.
MASA presenting a great photo of nighttime work at the MTA to the members at the September meeting
Once the gas and cryogenics were at the site, the team was able to complete pressure tests of the fuel side of the system, and were ultimately able to perform 2 coldflow tests through the entire system near their target pressures. These tests revealed many more areas for design improvements that the team hopes to implement, including reducing fittings, changing the location of vents and drains, and possibly even replacing the LOx and fuel tanks.
Improvements the MASA team hopes to implement
This campaign goes down in the books as the longest to ever take place in RRS history, and proved to be challenging for both the MASA team and the RRS. Several society members graciously volunteered their time to help make this testing effort possible, and the experience revealed many ways that the society could improve our procedures to better support extensive tests like this. Namely: limiting the duration of a test period to no longer than 1 consecutive week, requesting that some members of University staff be on-site when these long campaigns take place, and requiring a longer notice time before approving this sort of test were all brought up by RRS president Osvaldo.
PERMANENT BATHROOM
Progress is continuing on the permanent RRS Bathroom structure. Work on cutting holes for doors and windows has been completed on the 20-foot shipping container and delivery is expected imminently to the new work site at Wilbur’s hangar. The next stages of construction including adding plumbing, fixtures, and the doors and windows, and Osvaldo has already acquired some of these items to install
View from inside the container with the doors and vent windows installed
SEPTEMBER MTA EVENT & WORK PARTY
The USC Rocket Propulsion Lab (USCRPL) plan to be out at the MTA site from Friday, September 24th – Sunday, September 26th to conduct a static fire of an 8″ diameter solid rocket motor. The first few days will consist of prep work and the firing is planned on that Sunday. Several members including Bill Inman and John Krell indicated in the meeting that they have stepped up to the Yoerg Challenge and built model rocket kits to fly at the MTA. This will give us a great excuse to test out the new PVC wire rail launchers as well as the newly purchased Cobra wireless firing system.
In addition, the society has decided to use Sunday, September 26th as the date for our annual “Work Party” to perform maintenance and cleaning tasks at the MTA site. The expected tasks we would like to complete are:
Weld the plate on the vertical test stand
Removal of dry vegetation
Move drum of RP1 (from MASA testing) into one of the lockable containers
Fix the 2 broken latches on the Dosa Building roll door
Prep/measurements in area for new container bathroom
YOUTH ROCKETRY CLASS
RRS Vice President Frank updated the membership on the upcoming youth rocketry class in Boyle Heights with details on the schedule and overall plan. As opposed to 2 alternating classes, we will now be working with a single class of up to 30 students. The classes will run every Friday starting on September 24th, with a launch planned at the MTA site on November 13th. The plan is for each student to build their own Estes Baby Bertha kit and fly it twice on launch day. RRS Secretary Keith is currently working on 3D printing fin alignment jigs for the students, which will help in both teaching the students about that technology as well as properly installing straight & aligned fins on their rocket kits.
Several of the 3D printed fin alignment jigs
MISCELLANEOUS DISCUSSION
The end of the meeting consisted mostly of miscellaneous discussions around the various projects that RRS members are currently working on. Some of the highlights included John Krell’s new, very small (20x80mm) electronics board capable of collecting 7 channels of data onboard a flight, with over half of those channels at 500 Hz! Bill Inman updated members on the progress with his Solar Cat steam rocket, and there was a brief discussion of ham radio operations with Tom Hendricks sharing some of his wealth of knowledge in that subject. Overall it was a fun meeting with a lot of good discussion & participation from the membership.
NEXT MONTHLY MEETING
The next RRS monthly meeting will be held virtually on Friday, October 8th at 7:30 pm pacific time. Current members will receive an invite via e-mail the week of the meeting. Non-members (or members who have not received recent invites) can request an invitation by sending an email to:
secretary@rrs.org
Please check your spam folders and add secretary@rrs.org to your email whitelist to make sure you receive the invitation.
