Shared Knowledge The Value of Dynisco Instrumentation Dynisco sensors and instrumentation are primarily used as safety lockout system, which will prevent over pressure conditions in extrusion and injection molding processes, by interrupting the power source to the equipment. Learn More Diaphragm Material Selection The selection of diaphragm material used on the Dynisco pressure sensor, depends on the application where the sensor will be used. We must first ask the question, does the sensor come in contact with a pure material, or a blend of materials? Learn More Coping with Transducer-Crippling Pressure Spikes After investigating these spikes, we developed a laboratory method for determining the magnitude of the actual pressure spike that caused a specific field failure.
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Melt Pressure Measurement: Environmental Effects The Need For Pressure Measurement in Extrusion In order to maintain the dimensional stability necessary to produce extruded products that meet todays precise quality and tolerance specifications, it is necessary to keep both the output rate and the melt condition constant 1,2.
Although it is not possible to measure these quantities continuously, closely related variables such as melt temperature, and constant melt pressure at the die, the output rate can be considered constant.
Some of the more important ones are discussed in terms of the transducer itself as well as the overall measuring system. This information is user-oriented and serves as a practical guide in the selection and application of strain gage pressure transducers.
The Importance of Monitoring Extrusion Stability It has long been known that pressure instability for polymer melt entering the die usually results directly in ouput variation. Instrumentation used in these early studies was too delicate and expensive to be practical for routine commercial use.
An excellent article written by B. With this type of process, can I use a standard melt pressure transducer?
Do you want an accurate signal from the transducer during the vacuum part of the process? If so, the answer to the original question is "no"!
If the real question was, "Will the transducer be harmed by a vacuum? Remember a full vacuum is 0 psia or psig. Compared to the full scale range of a transducer of psig and probably higher, 15 psi is almost nothing. The transducer will barely notice anything different. The construction of the tip and diaphragm is such that a vacuum causes only a tiny fraction of stresses that it is designed to withstand.
It will probably work. We have performed some testing here, on units from our stock, that indicates a fairly accurate negative output corresponding to increasing vacuum, i. But this depends on a perfectly filled capillary system. In an ideal world, I would have no qualms about recommending melt pressure transducers for these applications. Since reality is often not ideal and the transducer may not be perfect, a fact we do not like to admit, the output in vacuum may not have the same accuracy as it would for a pressure measurement.
Nevertheless, most transducers should give a negative signal in vaccum. Two caveats - Do not try this with an amplified model such as a 2-wire transmitter.
The output cannot go below 4 mA enough to be meaningful. Secondly, if there are significant temperature variations in the process the change in tip temperature could cause a much greater change in output than the vacuum. What does this mean? Perhaps it is time for a little lesson on screw threads! The standard further defines the dimensions, shape height, depth, angles, tolerances, etc. These series there are also "C" for "Coarse-Thread Series" and "EF" for "Extra-Fine-Thread Series" are particular combinations of diameter and threads per inch that are selected based on the application.
The Fine-Thread Series are found in most other applications where the Coarse threads are not appropriate. The "2A" or "2B" calls out the thread class.
Without going into a long dissertation, this defines the tolerances and allowances on all the parameters of the thread. Now, I hope you see why the transducer is specified with the "-2A" designation and the mounting well is "-2B".
The Class 2 fit is used on most high quality commercial products. Conversely, Class 3 is found on exceptionally high precision products. Anyone, with an insatiable curiosity, can consult any mechanical engineering handbook for pages of tabulations of all threads, their dimensions and tolerances.
I am indebted to the Dynisco Drafting Department for their assistance in providing me with more information than I ever wanted to know about threads. On your pressure transducers many times you list the accuracy as 0. Does this mean that the real accuracy of a unit will be worse than 0. If you have a lot of time, this could lead to a lengthy discussion about the meaning of accuracy, what it includes and how it is calculated, and where does your lap go when you stand up.
The practical and succinct answer to the question is "No". But you have to understand the meaning of accuracy as usually used by Dynisco and others in the pressure transducer industry. Accuracy or Combined Error, as it is correctly called includes the error due to nonlinearity, hysteresis, and repeatability.
I do not have the time or space to give a detailed treatise on the meaning of these terms and the methods of calculating them Dynisco has other published material which does address these points but suffice it to say that our accuracy specification means that the worst of these errors will not exceed that published value.
Basically, if you plot straight line on a graph representing output vs. Just for the record, the zero balance is the actual output of a transducer when the pressure is zero and there is similar tolerance on the full scale sensitivity which is output at full scale pressure ignoring the zero balance offset.
Having a tolerance in the specification allows us to economically produce a pressure transducer in a reasonable amount of time. We can forget these errors because they are eliminated when you "calibrate" your readout instrument with the transducer. The accuracy error remains so it is the true uncertainty in your pressure measurement system!
I sent a pressure transducer back to the factory for repair and it was returned to me with the notation that it was "Calibrated" and no further repair was necessary. The transducer looks unchanged, what happened to it during its visit to Dynisco? Our efficient repair department checked our your transducer and found it to be functioning, so we then performed a "calibration".
This confirmed that your unit was not only working, but was operating within published specifications. We assess a small fee to cover the time to do this and prepare the documentary evidence of our test. To many people, the word "calibration" implies that something was adjusted in a device. Our office dictionary confirms that "to calibrate" is "to fix, check, or correct the graduations on a measuring instrument".
When we do a "calibration" nothing is adjusted or changed. The document you received with your transducer is a tabulation of a very accurate known pressure, applied to the unit, vs. In addition, there is a calculation of the error, at each point, expressed as a percentage of full-scale output.
You will note on the top of this paper, that, in an attempt to reduce confusion, we avoid the use of the word "calibration" and call this a "Data Validation". Aside: If the unit has an amplified output and accessible zero and span adjustments, it is possible to set zero and full scale output, but it is not possible to linearize, or improve the accuracy of, the output over the whole range. This adjustment, technically, is not a "calibration". What is that?
It seems only fair that we discuss the pressure fitting of the general purpose transducers, after a dissertation on the threads of the standard melt pressure models. This should satisfy the equal time requirement. Providing a good seal in plumbing connections, where system pressures exceed 10, psig, presents a significant challenge.
O-rings and gaskets may leak. Pipe threads cannot withstand the forces generated by the high pressures. Please remember that we must test the units to at least 1. This cone and thread style was further refined by Autoclave Engineers and others, but most people in our industry refer to the numbering system used by AE. By subjecting the pressure to a limited area, the forces that must be held by the threads is reduced to acceptable levels What is the frequency response or response time to a step pressure change of a pressure transducer?
To give the most precise answer, I will need to know your exact model. There are many variables that influence the transducer response, e. However, you can use the following guidelines for the general types of Dynisco Pressure Transducers.
We calculate response time from the natural frequency of the pressure sensing element or assembly as determined from shock tube testing or computer simulation. This approach would halve the estimated response time. This does not take into account any dampening in the system due to the plumbing to the transducer.
Again, these numbers are approximations. Call with your particular model and pressure range, for more accurate information. Related Items.
Am I in trouble? Dynisco traditionally recommends a well regulated 10 VDC power supply as excitation for most unamplified models of pressure transducers. We use 10 VDC excitation for our in-process and final testing and in most of the instrumentation that we sell. However a lower voltage source is acceptable and, in fact, one could make the case that it is better for the transducer as there would be less self-heating of the strain gages. It should be noted that using a different excitation voltage may result in a slightly different zero output than that recorded on the final calibration data, but there is no deleterious effect on performance. Remember that the full scale sensitivity of the transducer is a function of the input as well as pressure - 3. That means that with 10 VDC input the output, at full scale pressure, will be 33 mV, but when you use your 5 VDC excitation, the full scale output will be
Dynisco Pressure Transmitters
Remote Access Dynisco Pressure Transmitters Dynisco is an industry leader in robust and reliable pressure measurement for the extrusion processing market. These pressure measurement devices must withstand not only high temperatures but also very abrasive media. Dynisco has been setting the standard in the extrusion market by introducing some of the most innovative and dynamic products to assist you in your day to day operations. These innovations allow for more closely monitoring and control of the extrusion processes to produce repeatable results. In controlling the process customers are able to benefit by having higher yields and quality.
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