Hello All,
I just thought I'd let everyone know that I do plan to continue blogging, but a number of orders came in over the last couple of weeks leaving no time for writing. Please stay tuned, I might be able to post next week, if not the week after.
Friday, July 2, 2010
Friday, June 18, 2010
High Altitude Altimeter
I thought I would use this week's blog to announce the informal release of a new product available from Bemco! A high altitude altimeter capable of indicating altitude from 500 feet below sea level to 200,000 feet above sea level. I say informal release because I have not yet assigned it a model number, but I am designing one to be integrated into an altitude chamber. So when and if someone asks for a stand alone unit, 90% of the design work will be complete.
We currently offer the AI100, which reads from -500 feet to 99,999 feet with an accuracy of +/- 3000 feet at 99,999 feet, and +/- 30 feet at sea level. The new high altitude model will be accurate within +/- 1,500 feet from 100,000 to 200,000 feet, and even more accurate at lower altitudes (+/- 12 feet at sea level).
For more information on the AI100 click here: http://www.bemcoinc.com/AI.htm
And if you're interested in the new high altitude model, you'll have to call me directly at 805-583-4970.
We currently offer the AI100, which reads from -500 feet to 99,999 feet with an accuracy of +/- 3000 feet at 99,999 feet, and +/- 30 feet at sea level. The new high altitude model will be accurate within +/- 1,500 feet from 100,000 to 200,000 feet, and even more accurate at lower altitudes (+/- 12 feet at sea level).
For more information on the AI100 click here: http://www.bemcoinc.com/AI.htm
And if you're interested in the new high altitude model, you'll have to call me directly at 805-583-4970.
Friday, June 11, 2010
PID Tuning
Just about all of our products here at Bemco involve a PID control loop somewhere. Whether you're controlling temperature, humidity, vacuum or flow odds are a PID loop is behind it somewhere. For those of you who aren't familiar, a PID control loop is a method of calculating an output (throttle), based on an input (process value) and a set point. An example that most are familiar with is cruise control on a car, the input is the speedometer, the output is the gas pedal, and the set point is whatever you set your speed at. Your car's electronics use a PID algorithm to calculate the gas pedal's position based on your speed and the set point.
PID stands for proportional, integral and derivative, these are the three parameters that are used to make the calculation. In simple terms, the proportional band looks at where the system is currently, the integral band looks at where it has been, and the derivative looks at where it's going. Let's apply this to the cruise control example. Say your speed is set at 50mph and you're currently traveling at 30mph. With a proportional setting of 10mph, your car will floor the gas pedal until it gets within the proportional band (40mph to 50mph). Once it reaches the proportional band it will decrease the throttle proportionally, so if you're at 45mph, it will throttle at 50% because you're half way through the proportional band. The problem with proportional control alone is that the car will stabilize at some speed slower than your set point (maybe 46mph). At 50 mph, proportional control will call for 0% throttle, and we know from experience that to maintain 50 mph we need to have some throttle (proportional control alone does work in cases where drag / friction doesn't apply). This is where integral comes in. This parameter looks back in time by a settable value (fixed in car electronics). If that value is 30 seconds, the controller will calculate how are off the car has been from its setting over the last 30 seconds, and increase the throttle to compensate for it. The derivative band is also set with a value of time, but looks into the future that far. It calculates how fast the car will be going based on its current rate of acceleration, and will decrease the throttle if it will overshoot the setting of 50 mph.
The hardest part is coming up with PID settings that lead to stable control of a system. Every chamber or chiller that we build here at Bemco is so different from the last that they all require different settings. I've come up with my own version of the Ziegler Nichols tuning method that seems to work for most of our products:
Start with I & D off (off = 0) and a large proportional band (maybe 25F for a temperature application). Enter a set point and observe the system's response. Gradually decrease the proportional band in small increments, and each time enter a new different set point (different by several times the proportional band, in our case maybe 70F) to observe the system's response. Eventually, when the proportional band is small enough, the system will oscillate around the set point in a continuous sine curve shape. For instance if your set point is 100F and the chamber bounces from 107F down to 93F and back to 107F repeatedly. Once this is noticed, record the proportional value that caused this as well as the cycle time (peak-to-peak) of the oscillation.
Use a proportional setting of 2.5 times the value recorded above, and use an integral cycle of about 80% of the cycle time recorded above (remember integral is typically entered as cycles / minute in most controllers). Derivative is typically not needed in most of our equipment, however some can be added to minimize overshooting if desired.
We do our best to tune our controller's here at the Bemco factory, however often times we are unable to simulate the exact dynamics of the customer's application. For instance if your testing a massive part with a large thermal mass. In other cases the customer's application changes, and so must the PID parameters. For this reason I thought I would share this technique. For more information visit the Bemco website at www.bemcoinc.com
PID stands for proportional, integral and derivative, these are the three parameters that are used to make the calculation. In simple terms, the proportional band looks at where the system is currently, the integral band looks at where it has been, and the derivative looks at where it's going. Let's apply this to the cruise control example. Say your speed is set at 50mph and you're currently traveling at 30mph. With a proportional setting of 10mph, your car will floor the gas pedal until it gets within the proportional band (40mph to 50mph). Once it reaches the proportional band it will decrease the throttle proportionally, so if you're at 45mph, it will throttle at 50% because you're half way through the proportional band. The problem with proportional control alone is that the car will stabilize at some speed slower than your set point (maybe 46mph). At 50 mph, proportional control will call for 0% throttle, and we know from experience that to maintain 50 mph we need to have some throttle (proportional control alone does work in cases where drag / friction doesn't apply). This is where integral comes in. This parameter looks back in time by a settable value (fixed in car electronics). If that value is 30 seconds, the controller will calculate how are off the car has been from its setting over the last 30 seconds, and increase the throttle to compensate for it. The derivative band is also set with a value of time, but looks into the future that far. It calculates how fast the car will be going based on its current rate of acceleration, and will decrease the throttle if it will overshoot the setting of 50 mph.
The hardest part is coming up with PID settings that lead to stable control of a system. Every chamber or chiller that we build here at Bemco is so different from the last that they all require different settings. I've come up with my own version of the Ziegler Nichols tuning method that seems to work for most of our products:
Start with I & D off (off = 0) and a large proportional band (maybe 25F for a temperature application). Enter a set point and observe the system's response. Gradually decrease the proportional band in small increments, and each time enter a new different set point (different by several times the proportional band, in our case maybe 70F) to observe the system's response. Eventually, when the proportional band is small enough, the system will oscillate around the set point in a continuous sine curve shape. For instance if your set point is 100F and the chamber bounces from 107F down to 93F and back to 107F repeatedly. Once this is noticed, record the proportional value that caused this as well as the cycle time (peak-to-peak) of the oscillation.
Use a proportional setting of 2.5 times the value recorded above, and use an integral cycle of about 80% of the cycle time recorded above (remember integral is typically entered as cycles / minute in most controllers). Derivative is typically not needed in most of our equipment, however some can be added to minimize overshooting if desired.
We do our best to tune our controller's here at the Bemco factory, however often times we are unable to simulate the exact dynamics of the customer's application. For instance if your testing a massive part with a large thermal mass. In other cases the customer's application changes, and so must the PID parameters. For this reason I thought I would share this technique. For more information visit the Bemco website at www.bemcoinc.com
Friday, June 4, 2010
Flow Meters in Wide Temperature Ranges
As some of you may be aware, in addition to temperature, humidity and vacuum chambers Bemco also designs and manufactures fluid conditioning systems for a variety of different applications, including radar / electronics cooling and leak checking. Often times our customers need these fluid conditioners or chillers to operate in wide temperature ranges, just last year we built a system that conditioned fluid from -100F to +300F. Additionally, in most cases the customer needs to closely monitor the fluid flow rate, and this is where things get interesting...
Temperature changes of several hundred degrees typically (depending on the fluid) have a drastic effect on the fluid viscosity. With the popular dielectric oil known as polyalphaolefin (PAO) this means the difference of honey (at cold temps) versus water (at hot temps). This massive change in viscosity has a major effect on many types of flow meters, most notably turbine style flow meters. The higher viscosity produces more drag through the turbine (at the same flow rate), which exerts a force on the bearing(s). This increased force on the bearing(s), increases friction, which slows the turbine down, indicating a slower flow rate. For this reason Bemco strongly prefers the use of gear type flow meters.
Gear type flow meters don't really "care" what the fluid viscosity is. Each gear cavity has a fixed volume, and it will require that volume of fluid to turn the gear a fixed amount regardless of what the fluid viscosity is. However, there is one limitation with gear type meters that can easily be overlooked...
In order for the gears to rotate freely, the gear meter is manufactured with clearances. Conveniently, when using most oils, the viscous / cohesive forces are enough to seal these gaps of just a few thousandths of an inch. If you've ever had trouble lifting a coaster off of a table top because of water in between the two surfaces, then you can understand how liquids can seal gaps. However if the fluid is too thin, it will "blow by" the gears through these clearances and cause lower flow rate readings than what's actually flowing through the meter.
For this reason all gear flow meters have a minimum allowable viscosity for accurate flow rate indication, but many manufacturers fail to publish this information. Above this minimum there is no viscosity affect on accuracy, but you need to be above the minimum. Be sure you call the manufacturer and find out what this minimum value is before you order it.
For more information about Bemco fluid conditioners (PCL Series) visit the Bemco website at www.bemcoinc.com
Temperature changes of several hundred degrees typically (depending on the fluid) have a drastic effect on the fluid viscosity. With the popular dielectric oil known as polyalphaolefin (PAO) this means the difference of honey (at cold temps) versus water (at hot temps). This massive change in viscosity has a major effect on many types of flow meters, most notably turbine style flow meters. The higher viscosity produces more drag through the turbine (at the same flow rate), which exerts a force on the bearing(s). This increased force on the bearing(s), increases friction, which slows the turbine down, indicating a slower flow rate. For this reason Bemco strongly prefers the use of gear type flow meters.
Gear type flow meters don't really "care" what the fluid viscosity is. Each gear cavity has a fixed volume, and it will require that volume of fluid to turn the gear a fixed amount regardless of what the fluid viscosity is. However, there is one limitation with gear type meters that can easily be overlooked...
In order for the gears to rotate freely, the gear meter is manufactured with clearances. Conveniently, when using most oils, the viscous / cohesive forces are enough to seal these gaps of just a few thousandths of an inch. If you've ever had trouble lifting a coaster off of a table top because of water in between the two surfaces, then you can understand how liquids can seal gaps. However if the fluid is too thin, it will "blow by" the gears through these clearances and cause lower flow rate readings than what's actually flowing through the meter.
For this reason all gear flow meters have a minimum allowable viscosity for accurate flow rate indication, but many manufacturers fail to publish this information. Above this minimum there is no viscosity affect on accuracy, but you need to be above the minimum. Be sure you call the manufacturer and find out what this minimum value is before you order it.
For more information about Bemco fluid conditioners (PCL Series) visit the Bemco website at www.bemcoinc.com
Friday, May 28, 2010
Complete High Vacuum Time Table
Hi Everyone,
I did not really want to do two high vacuum topics in a row, but I had the time to do some extended testing on a high vacuum chamber last week, that I've always wanted to do, so I thought I would share...
First, a very brief intro to cryo-pumping as some may not be familiar. A mechanical (rotary-vane style) vacuum pump is limited to a vacuum level of between 0.5 mTorr and 50 mTorr. The oil and other molecules given off by the moving parts of the pump actually "back stream" into the chamber preventing it from reaching vacuums higher (lower pressure) than this range. To get to higher vacuums, a different technology must be used. A cryo-pump is nothing more than an extremely cold surface (just 15C above absolute zero) that will liquefy just about anything that comes in contact with it. When molecules liquefy on this surface, they stick to it, effectively removing it from the chamber volume, reducing the chamber pressure, taking it to higher vacuum.
In the graph above (click on it for a larger version), the chamber and cryo-pump pressures are shown on a logarithmic scale in milliTorr (on the left), and the cryo-pump temperature is shown in degrees Kelvin on a linear scale (on the right). It may seem confusing, but it is the only way to see everything all on the same graph.
At the start of the test, we see that both the chamber and cryo-pump are at atmospheric pressure (760 Torr), and the cryo-pump is at room temp (300K). First we need to "rough out" the cryo-pump, with our mechanical rotary-vane pump. If we started cooling the cryo-pump down at atmosphere, it would become over loaded with liquid water and air very quickly. The cryo-pump cavity is small and takes only 6 minutes to reach 16 mTorr. At this point we can begin to "rough out" the chamber while we start cooling the cryo-pump down.
The chamber itself is considerably larger than the cryo-pump cavity and takes about 30 minutes to reach 16 mTorr. At this time our cryo-pump is only at 223K (-58F), and we'll have to wait about an 1-1/2 hr for it to reach 15K, which is when we can open the gate valve isolating the cryo-pump from the chamber.
At about an hour into the test, we see that the cryo-pump temperature of 181K (-134F) is beginning to take the cryo-pump to vacuums beyond the capability of the mechanical roughing pump (below 16mTorr). Ten minutes later we are beyond the range of the convectron gauge and can no longer measure the vacuum level in the pump.
We finally reach 15K at 2 hours and 14 minutes into the test and we can finally expose the chamber to the cryo-pump. Starting almost immediately after the gate valve is opened, there is a period of a few minutes that we are out of range of both the convectron and ionization gauges. The ionization gauge starts reading the chamber pressure at 6 x 10-6 Torr, and the chamber pressure continues to decay to the -7 scale in approximately 40 minutes.
Unless otherwise requested by our customer, the majority of Bemco's high vacuum chambers will behave in this fashion. Although the overall time scale will depend on the chamber's size and the cryo-pumping speeds. For more information on our high vacuum systems visit our website at www.bemcoinc.com
Thursday, May 20, 2010
Understanding High Vacuum
High vacuums can be tricky. We're dealing with kinetic theory on a molecular level. One of the more striking facts to me has always been that even at high vacuums there are still millions upon millions of molecules present in just a few cubic feet of volume (in interplanetary space there are about 10 molecules per cubic centimeter). It is a reminder of just how complex our world is.
The process of achieving high vacuums can be difficult for someone new to the subject to grasp. When I first started working with vacuum systems someone gave me this helpful analogy:
Pulling a vacuum is like waiting for mice to find their way out of a maze. When you begin, imagine the maze is packed full of many mice (lets say 100 mice) with no empty space between them. When the exit of the maze is opened, within the first minute the majority (say 70) of the mice have already found their way out simply due to how many mice were in there to begin with. These mice were statistically bound to find the exit. Within the second minute the mice that remain are working harder to find the exit, and only 15 of them find it. The 15 mice that remain after the second minute take between one and three more minutes to find the exit, and some of them may never find it.
The mice in this analogy are obviously gas molecules or atoms. A mouse that is stuck in a corner and takes a long time to find the exit (or doesn't) may represent an oil molecule embedded in an o-ring seal, or a residue on the chamber wall from someone's hand (wear your gloves).
Feel free to share this with any vacuum industry newbies. For more information vacuum chambers as well as a helpful altitude versus pressure table (up to 250,000 ft) visit the Bemco website at www.bemcoinc.com
The process of achieving high vacuums can be difficult for someone new to the subject to grasp. When I first started working with vacuum systems someone gave me this helpful analogy:
Pulling a vacuum is like waiting for mice to find their way out of a maze. When you begin, imagine the maze is packed full of many mice (lets say 100 mice) with no empty space between them. When the exit of the maze is opened, within the first minute the majority (say 70) of the mice have already found their way out simply due to how many mice were in there to begin with. These mice were statistically bound to find the exit. Within the second minute the mice that remain are working harder to find the exit, and only 15 of them find it. The 15 mice that remain after the second minute take between one and three more minutes to find the exit, and some of them may never find it.
The mice in this analogy are obviously gas molecules or atoms. A mouse that is stuck in a corner and takes a long time to find the exit (or doesn't) may represent an oil molecule embedded in an o-ring seal, or a residue on the chamber wall from someone's hand (wear your gloves).
Feel free to share this with any vacuum industry newbies. For more information vacuum chambers as well as a helpful altitude versus pressure table (up to 250,000 ft) visit the Bemco website at www.bemcoinc.com
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