WorldWide Drilling Resource

33 WorldWide Drilling Resource ® MAY 2015 Simply Elegant Process Control Part 1 by Britt Storkson Owner, P2FlowLLC To properly monitor and control equipment, we often have to monitor environmental factors such as temper- ature, pressure, humidity, etc. To do this, we need to utilize materials with prop- erties that will change relative to changes in the environment. One example: For temperature sensing, we need to use a material which will reliably change “state” with changes in temperature. A ther- mistor does just that. A thermistor changes resistance with changes in temperature. While it is nonlinear, it is repeatable, making it a reliable indicator of temperature. By non- linear, we mean the resistance does not double when the temperature doubles. While this may seem to be a major lim- itation with inexpensive microprocessors and software, it isn’t a problem. The thermistor comes with a look- up table we can program into software. So when we see a resistance of, say, 10,000 ohms, we can “look up” the equiv- alent temperature for that resistance value. One thermistor I use has a tem- perature rating of 77ºF (25ºC) at 10,000 ohms resistance. So if we “see” 10,000 ohms, we know the temperature at that thermistor is 77ºF (25ºC). So while we spend a little more (just pennies) in microprocessor mem- ory, we can spend considerably less on the sensor itself. Overall, this approach is a net gain. It does the job adequately and reliably, with minimal expense. Often, a sensor does not have to be particularly accurate or expensive to be very useful, reliable, and totally adequate for the application. It’s very likely the fuel gauge in your car is no better than +/-20% accurate. Why? Because it doesn’t need to be accu- rate to be adequate for the task, which is to indicate how much fuel you have in your tank. Besides, the accuracy of the fuel gauge isn’t likely to impact your decision whether or not to buy the car. And (by far the biggest issue for the car maker) it’s cheaper to make. All materials have certain physical properties which can be measured. All materials have resistive and/or capaci- tive properties. Low resistance is where electrons readily flow through the materi- al. High resistance is where electrons do not flow readily through the materi- al, but are instead stored on the surface of this material. This “storing” of elec- trons is the property of capacitance. One can test for resistive or capac- itive properties by connecting a known resistance (resistor) to a regulated Direct Current (DC) voltage; then measure the voltage between the resistor and the material to be measured. If the voltage at this point goes down, then the mate- rial has resistive properties. You can calculate the resistance with the formu- la: Vout = R2/(R1 + R2) [Vout is voltage measured at the junction of the two resistors. R1 is the resistor connected to constant voltage source. R2 is the resistance to be measured.] If the voltage does not go down, then the material is an insulator and can be “charged” like a capacitor. We first “discharge” the material (connect it to ground) to “drain off” all of the elec- trons, then “charge” it with a constant current (amperage) source. A constant current source is nothing more than a regulated DC voltage flowing through a resistor. Then we time how long the material takes to “charge” and we can calculate the capacitance with the fol- lowing formula: C = (I [times] t)/V C = Capacitance in Farads. I = Current (amperage) used to “charge” the material. t = Time in seconds. V = Voltage measured at the end of the time period. Next month, we will explore how being able to measure resistance and capacitance can make process control simple and elegant. Britt Britt Storkson may be contacted via e-mail at michele@ worldwidedrillingresource.com WWDR is read by thousands of international drilling professionals each month - in print and online. We have affordable marketing opportunities for any budget. Let us share your products with the World ! (850) 547-0102 +9 &8 ,&6"4/23 *,'*&,% "4&2 &,, /.6&.4*/.", ".% &6&23& *2$5,"4*/. 2*,, *0& 15*0-&.4 +9 &8 ,&6"4/23 2& 3&% /2,%7*%& ! 2&8 2&8-$'"%%&. $/- ,&6"4/23 /"% 4&34 $&24*'*$"4&3 "6"*,"#,& * $ "% * $ ( # * $ "% * " # % ! ! $% " ! ! # " ' $ # ) " ! ) " "# ) % # ) ! "# & )"6& &,&6"4/23 /' ",, 3*:&3 490&3 ".% 7&*()43 ".% $". #5*,% 4/ ".9 30&$*", "00,*$"4*/.3 ) ' & # ) $ ( "# ) $" % % !# ) " ## & " % # ) $# ) ' # &7 3&% 15*0-&.4 6"*,"#,& 777 2&8-$'"%%&. $/-