WorldWide Drilling Resource

79 WorldWide Drilling Resource ® DECEMBER 2014 Be Cheap...If You Can Get Away with it by Britt Storkson Owner, P2FlowLLC It’s all right to be cheap...if you can get away with it. For example, don’t try to sell a cheap car for a high price. You probably won’t get away with it, as most people know the approximate value of the car they are looking for and would refuse to pay the high asking price. Electronics are the same way, but there are things you can “go cheap” onwhile not reducing the value or durability of the product...if you know how to do it. One of those things is temperature sensing. There’s no sense in spending a lot of money on a temperature sensor with elaborate decoding circuitry and high accuracy when all you need to know is if the tempera- ture is above or below a certain level. Of course, the vendors want to sell you the most expensive stuff they have because that’s how they make money, but you don’t have to go there. On the other hand, don’t buy inadequate accu- racy when you need something better to do the job. Smart designers and engi- neers don’t “overbuild” the project be- cause it costs more money. Likewise, they don’t “underbuild” it either because it won’t do the job adequately. Smart designers and engineers also test their products to see if they will con- sistently perform to expectations. If the test shows the temperature sensor needs to be more accurate, then they move “up a notch” with accuracy. Again, the goal is to make all parts of the product adequate to service the task at hand without increasing cost. A good example of a cheap tem- perature sensor is a silicon diode. All diodes have what is called a “forward voltage”...that’s where voltage at the anode end of the diode has to rise to a certain point before it “turns on”, meaning current (amperage) flows through the diode. This “forward voltage” decreases with an increase in temperature at about 2.1 mV (thousandths of a volt) per degree centigrade. All that’s needed is a series resistor to limit current through the diode, a DC voltage source, and a way to measure the voltage at the diode anode terminal (the cathode of the diode is connected to ground). What’s the cost for all of this? About five cents. So the price is right. Now let’s see if the application is right. Since diode forward voltage varies from diode to diode (forward voltage is not a guaranteed value), we probably cannot get any better than +/- 20% accuracy unless extraordinary calibration meas- ures are implemented...which, again, raises the price. As a practical matter, if we need better accuracy, we should buy the more expensive temperature sensor. But if we can tolerate +/- 20 degree accuracy with a 0-100 degree temperature span, then it would work fine. Please note, we are only concerned with the fact Storkson cont’d on page 80.