WorldWide Drilling Resource

27 WorldWide Drilling Resource ® JUNE 2015 Simply Elegant Process Control Part 2 by Britt Storkson Owner, P2FlowLLC Last month, we introduced two very useful equations. To determine the resist- ance of a material, we connect a resistor to a voltage-regulated DC (Direct Current) voltage. If the voltage goes down, it’s a resistor, because electrons flow readily through the material. If the voltage does not go down, then the material is capaci- tive and can be “charged” like a capacitor. To charge this capacitor, we first dis- charge the material (connect it to ground) to drain off all of the electrons (0 volts). Then we “charge” it with a constant current (amperage) source. A constant current source is nothing more than a voltage- regulated direct current flowing through a resistor. Simply time how long the materi- al takes to charge and we can calculate the capacitance. The “simple and elegant” part of all of this is almost all process control parame- ters - temperature, pressure, flow, light intensity, pH, strain, etc. - can be deter- mined by only three basic measurement functions: 1. Voltage measurements to deter- mine resistance. 2. Voltage measurements to deter- mine capacitance. 3. Differential voltage - measuring the difference between two voltages. Even simpler is the fact we often don’t need to know the absolute values of re- sistance and capacitance for this to be useful. All we need to know is the relative values. When we say “relative” we mean significant changes in resistance or capac- itance from one state to another - not the actual numerical value. For example: We want to determine whether the water in a tank is at a certain level. Simply use two probes (a fancy word for wires) and mount them in such a way they don’t touch. Connect one end of a resistor measuring about 20,000 ohms or greater (roughly the resistance of the water) to a DC power source (the positive or + terminal) to one probe and connect the other probe to ground (the negative or - terminal). When both probes are touch- ing the water, the voltage will go down at the resistor end touching the water. Howmuch the voltage goes down de- pends on the mineral content of the water and how far apart the probes are. The more minerals in the water, the lower the voltage will be because there is more con- ductive material present. Simple electronic circuits can detect this voltage change and provide a “deadband” so the circuit does not switch on and off rapidly. We’re talking about just a few dollars to do this. With the water tank level example, we’re not concerned about the absolute value of the resistance measurement, but only interested in the significant change in resistance. Also, if we’re using a metal tank, we need to have only one probe as the ground (or - terminal) connection can be connected to the tank itself. Make sure the probes are insulated and don’t touch anything else or you will get erroneous readings. What about measuring the presence or absence of oil which is nonconductive? The solution is nearly as simple. Again, using two probes, first discharge (connect to ground) the material. Then charge the material and time how long it takes to charge. Likewise, we’re not concerned about the absolute capacitive value of the material...only the relative change that would indicate whether the oil is touching the probes or not. This can easily be done with an inexpensive microprocessor. For the third process control parame- ter - a differential of two voltages - sev- eral process control components, such as pressure and temperature sensors out- put two voltages and the information out- put, is determined by the difference of these two voltages. An integrated circuit called an “instrumentation amplifier” (In- Amp) is typically used to decode and con- dition these voltages, and the output voltage of the In-Amp is read to obtain the correct information. Again, we’re talking about only a few dollars here, not huge sums of money. Albert Einstein once said, “If you can’t explain it simply, you don’t understand it well enough.” There are all sorts of com- panies in business to sell the most com- plicated process control equipment at the +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 ) ' & # ) $ ( "# ) $" % % !# ) " ## & " % # ) $# ) ' # &7 3&% 15*0-&.4 6"*,"#,& 777 2&8-$'"%%&. $/- 3"- 0)'! +#0* /'! /,,). '+ ./,!( '4#. 6 /, 6 +(. ,1* + 13 -"#+ ))#3 5 2 - 0!&*$% !,* 5 - 0!&*$% $-,+/'#-+#/ !,* ,"#) '4#. /#+/ + " #+"'+% Storkson cont’d on page 28.