WorldWide Drilling Resource

Close Enough for Horseshoes and Hand Grenades by Britt Storkson Owner, P2FlowLLC While accuracy is a virtue, there are times we don’t need a lot of accuracy to adequately satisfy our need for information. When you want to know about how much milk you have you have left and it’s a paper milk carton so you can’t see the fluid level...do you pour the milk out into a measuring cup, check the level, then pour it back into the carton again? Most of us simply pick up the milk carton and “weigh” it by noting how much force it exerts against our hand/arm. We know from experience (those of us who make computers would call this “memory”) how much a full carton of that size of milk weighs compared to an empty or near-empty carton of milk. In many cases, an approximation like this is more than adequate for what we need to know. Similarly, “floatless” tank fill controls work the same way, by measuring the weight of the fluid at the bottom of a tank. Since it’s the column of fluid which gives us the pressure, not the volume of fluid, we know the higher the pressure, the higher the fluid level above wherever the sensor is located. If the fluid is water, we know 1 psi at the sensor means there’s 2.309 feet of water above the sensor. The advantage of this measurement method is it’s inexpensive, reliable, and all of the needed equipment can be installed in the pump house out of the weather. Nothing is needed in or near the tank. The disadvantage is it’s not very accurate. We’re doing well to achieve accuracies of +/-1 foot or so, but often the user doesn’t need this level of accuracy. They just want to keep the tank more or less full without overflowing, and this will do the job. There are other more accurate ways to automatically keep a tank full, but they are more complex, more costly, less reliable, and require equipment be installed in or near the tank, which may or may not be readily accessible. Also, an electric power source may be needed at the tank to power this equipment, and may be a costly addition. Anytime we increase complexity, we decrease reliability. While accuracy is desirable, repeatability under all conditions is often equally, or more, important. By repeatability, we mean getting the same measurement result every time and under widely varying conditions, like temperature or humidity ex- tremes or things like vibration in automotive applications. High accuracy is meaningless if the results vary with the temperature or some other external event. If a temperature sensor is, let’s say, 10ºF “low”...meaning if the temperature “reads” 40ºF when the actual temperature is 50ºF, it is important the temperature reading be off by that amount every time...not off by 10º one day and 5º another day. We can correct for consistent variation in the software, but we cannot correct a moving target. Salespeople like to sell accuracy mainly because higher accuracy almost always means higher cost. By all means, if one needs a high level of accuracy, spend the money and buy it. Most of the gauges or warning lights in your car or truck are no better than +/-20% accu- rate, but then they don’t need to be. With the temperature gauge in your car, while you don’t need to know the exact tem- perature of the car’s cooling system, you do need to know if it is in danger of over- heating. Good engineering practice dictates we build to the level of expected per- formance and no more. There are a lot of reasons for this; chief among them being cost. The more you add to the project, the more it will cost. Also, sim- plicity often results in better durability and reliability, and that’s what we want, isn’t it? Britt Britt Storkson may be contacted via e-mail to michele@ worldwidedrillingresource.com 54 JULY 2016 WorldWide Drilling Resource ®