- How Low Can You Go? Seismic!
- How to Check Your Lithium Backup Battery
April 2000
Daniel R. Burk
IST Applications Engineering
mailto: support@isthq.com
Hello from IST Applications Engineering!
It has been a quiet few months since January, and I have missed a few Tech Tip columns. I have, however, kept up with the monthly tech tips mailing list, which I send out each month. The intent of these mailings and the intent of these on-line columns are to provide you with ideas and tips which either make your use of IST equipment easier, or help you find new ways in which you can use your systems.
One of the ways in which our systems have been used is for quick and easy field measurements of ground vibration. Until recently, IST systems have been limited to about 1 milli-g of resolution. Anything smaller was difficult to resolve using the current set of available piezoelectric transducers. This is due to the fact that our systems, which are fully self-contained, use battery power. Conventional p.e. transducers provide a zero-bias voltage of 8 to 18 volts, and require 2 to 4 milliamps of current per axis. These types of power requirements make them impractical for use with a six-volt, 10 milliamp device like the EDR3. Current Battery technology doesn't have enough power density to yield a decent recording time with these types of power requirements. Thus, we have special circuitry designed into the PCB piezoelectric transducers which we use, that require only half a milliamp of current, and a zero bias voltage of three volts. Just this month, we received our first 10,000 millivolt per g transducer from PCB, which meets our power and voltage requirements. This high-sensitivity accelerometer offers us the promise of low-level measurements of acceleration which are a full magnitude lower than previously measured.
How low can you go? How about seismic level!
Recently, we took delivery on a PCB model 393M65 ICP accelerometer, which sports a 10,000 millivolt per g sensitivity. This 50 milli-g full-scale accelerometer means that an EDR3/EDR3C series recorder can measure a minimum resolution of 100 micro-g. It is the kind of vibration you get from ambient automobile traffic about a city block away. This acceleration cannot be felt by the human being. It is, however, sufficient to cause ghosting in magnetic resonance imaging (MRI) equipment, as well as limit the feature size that can be produced in silicon photolithography equipment. Thus, when considering the installation of this equipment, it becomes important to first characterize the environment before attempting the installation. Specialized dampers can be installed which limit the vibration energy at certain frequencies, and reduce the problems of this ambient vibration. The new transducer, when used with either the EDR3C, or the EDR4 system, enables you to now measure and characterize the environment with relative ease, down to 100 micro-g in the EDR3, and 30 micro-g in the EDR4.
The EDR4 has a variable gain amplifier on the input stage of the accelerometer channels which enables you to extend the operating range over the EDR3. The EDR3 is fixed to a single-ended operating range of +-500 millivolts full-scale, whereas the EDR4 can vary the full-scale from +-1.5 volts to as low as about +-50 millivolts full-scale. Noise floor may limit the practicality of this low of range, however. My investigation in the case of the seismic sensor indicated that the noise floor was about 10 micro-g. Thus, the highest practical gain that could be used with the transducer was about 250 millivolts full-scale. This yields a least-significant bit measurement that represents 12 micro-g. Here at IST, Low-level measurements of the concrete floor show a background of 30 micro-g peak-to-peak at night. The primary frequency seems to vary between 10 - 15 Hz. Further research may enable us to push the noise floor lower in future tests.
Use of the EDR4 and the seismic accelerometer require special handling and setup procedures. If you wish to read more on how to interface the EDR3 or EDR4 with a seismic sensor, give us a call.
Technical note: How to check your lithium backup battery
The EDR3 and EDR3C series data recorders have a small lithium backup battery sealed inside the data recorder. This battery is used only when you remove the main batteries from your system. If you store your recorder with fresh alkaline batteries in it, your lithium backup battery may last the life of the system. If, however, the main batteries are allowed to discharge below 3.6 volts, there is a possibility that charge has been used from the backup battery. This battery is used to keep the memory refreshed in the system, and your data safe. Unfortunately, when the backup battery fails, there is a danger of losing data. EDR3 systems, when turned off, will lose the contents of memory. If your system is more than three years old, or you have stored it without any batteries, you should consider checking the status of the backup battery. In order to check your battery status, you must remove the alkaline batteries, and remove the little LED cover that covers the LEDs and micro fuses. Place a volt meter across the lithium battery terminals and read the voltage. If the voltage is less than 3.3 volts, you may wish to consider sending the system to us for a replacement of the lithium battery. This is NOT a user serviceable item. It involves the complete disassembly of the recorder. Here is a link to an illustration that shows where to measure the battery voltage. Make sure you remove main battery power first.
Please be careful, as we cannot warranty any damages attributable to actions that happen (such as shorting out or blowing the pico fuses) during the time that the LED cover is removed. Lithium illustration diagram