|
WARNING: If you are using an Applied (AES) surface CCL meter unit model CCL-030* (powered by AC mains / generator voltage), take it out of service immediately; it is unsafe and should be modified by a competent service facility. Any surface CCL unit deriving its power from a source other than the truck battery 12 volt DC supply should also be reviewed for safety by competent service personnel. If you have any doubt about the safety of any surface CCL unit, have it looked at promptly.
*It is not known if other AES products containing a CCL meter suffer from the same unsafe design problem.
Below are some random notes on shooting CCL surface amplifier devices, not a comprehensive design guide. Use or reliance on the information and materials contained herein is entirely at your own risk.
Shooting CCLs have no downhole amplifier and therefore place a very small signal on the line. This small CCL signal often must be amplified by the surface equipment to be useful. For decades, the surface CCL amplifier has been a one (sometimes two) transistor affair; beginning in the early 1960's with the old germanium transistors, and soon thereafter replaced with silicon. One might think that the design of a one transistor amplifier would be hard to botch, but a few manufacturers have managed to do just that.
The shooting CCL surface amplifier is designed to drive a meter (usually 100-0-100 or 500-0-500 microampere analog meters), giving a visual indication of collar location. There have been many variations, including some with audio beepers, indicator lamps, and a few other odd types of indicators. The SIE design is incredibly simple using negative feedback; it is perhaps the best basic design in common use. On the other hand, the GO / GOI / MLS design is not so hot. GO / GOI / MLS must have recognized they had a problem since there are no less than six versions of the little circuit floating around, each revision an apparent attempt to improve the design. The last version uses a voltage divider biasing scheme, but still does not perform nearly as well as the SIE design. The inferior GO / GOI / MLS design is found in the CDM 401 Collar Depth Module (see illustration below), and in some other GO shooting panels.
AnaLog Services, Inc. uses a version of the SIE style shooting CCL surface amplifier for new panels, and for retrofits. We often install this circuit in upgraded GO / GOI / MLS CDM 401 modules and the improvement is profound. Our version incorporates improved current limiting for safety purposes, though none of the properly implemented common designs of shooting CCL surface amplifiers could theoretically set off a cap (but see the warning above). See
Shooting CCL Surface Meter / Amplifier Schematics (pdf files) for a look at our simple and effective circuit. Also see Application Note CCL Meter for some information on the use of our little shooting CCL surface meter printed circuit board (PCB) (see illustration below).
Our design also includes Paul Knight's meter protection circuit, which feature can be installed in existing SIE panels as well. Historically, logging engineers sometimes used the slamming of the CCL meter against the mechanical meter stops as an audible collar annunciator. Needless to say, this practice shortens the life of meter movements (though Simpson meters are able to withstand more abuse than most). Paul's meter protection circuit limits the abuse suffered by CCL meters to some extent by clamping the voltage that can be present across the meter.
For safety reasons, shooting CCL surface amplifiers installed in trucks are usually powered with the truck battery 12 volt DC power supply. It is customary safety practice to not start the truck generator (Onan) until the shot is several feet into the well (API recommends 200 feet before power is turned on). Having the shooting CCL surface amplifier running off the truck 12 volt supply simplifies this practice and makes it possible to delay starting the generator for as long as is deemed prudent. In the case of portable shooting panels, or any unit powered by mains / generator AC power, said unit should be reviewed for safety by a competent service facility. There are a few dangerous designs out there (one special case of bad design judgment is discussed below), and it is always better to be safe than sorry.
We have encountered a couple of instances where mains / generator AC power is placed on the same switch as the shooting / logging line in panels not powered with the truck 12 volt DC supply. A couple of manufacturers did this to take the shooting line to ground (in older panels) or to the shunting resistor (in newer panels) whenever the power is off. However, this "safety feature" puts 120 volt AC within a fraction of an inch of the shooting line conductor. The problem is exacerbated in miniature switches with their even closer spacing than standard switches. We know of no accidents attributed to this design flaw, but any such panel should be regarded as potentially unsafe. Any panel with AC mains / generator voltage and the shooting / logging line on the same switch should immediately be taken out of service and modified by a competent service facility. See the top of this page for a special safety alert for an Applied (AES) panel with this design flaw.
See also Shooting Panels and Shooting Continuity Meters. If you need more information please feel free to
contact us.
|
|
