A dynamometer card plots the load on a rod pump against the position of the stroke, and there are two versions of it that every rod-lift operator needs to keep straight: the surface card and the downhole card. The surface card is what you actually measure at the polished rod, but it is muddied by the behavior of the long rod string between the surface and the pump. The downhole card is a computed picture of what is happening at the pump itself, cleaned of the rod-string effects, and it is the one used to diagnose pump condition. This guide contrasts the two cards, explains how the downhole card is computed by solving the rod-string wave equation, and shows why the downhole card is the one SCADA reads to judge fillage, leaks, and gas.
Surface vs Downhole Dynamometer Card in one line: The surface dynamometer card is the load-versus-position plot measured directly at the polished rod at the surface, while the downhole (pump) card is a computed plot of load and position at the downhole pump. The downhole card is calculated from the surface card by solving the rod-string wave equation, which removes the stretch, dynamics, and wave effects the long rod string adds. Because it represents conditions at the pump itself rather than at the surface, the downhole card is the one used to read pump condition - fillage, valve leaks, and gas interference - in diagnostics and SCADA.
The surface card is the measurement you can take directly. A load cell or transducer at the polished rod reads the load the rod string is carrying, and a position sensor tracks where the polished rod is in its stroke, and plotting one against the other over a complete stroke gives the surface card. Because it is measured at accessible surface equipment, it is straightforward to acquire and is the raw data every rod-pump diagnostic starts from.
The catch is that the surface card reflects far more than the pump. Between the polished rod and the pump lies a rod string that may be thousands of feet long, and that string stretches, rebounds, and carries stress waves as it accelerates and decelerates through the stroke. All of those rod-string effects are superimposed on the pump's own behavior in the surface measurement. So the surface card shows the pump's action distorted by the dynamics of the long, springy rod string hanging beneath the measurement point.
That distortion means the surface card is hard to read directly for pump condition. The same pump problem can look different on the surface card depending on well depth, rod design, and pumping speed, because the rod-string dynamics between the pump and the surface change with those factors. The surface card is essential as the input, but on its own it mixes the message from the pump with the noise from the rods, which is why it is usually converted before it is interpreted.
The downhole card is what you would measure if you could put a load cell and position sensor at the pump itself, and since you cannot, it is computed. The computation works by modeling the rod string as an elastic medium that carries waves and solving the rod-string wave equation, which relates the load and motion at the surface to the load and motion at the pump through the mechanics of the rod string. Feeding the measured surface card into that solution effectively subtracts out the stretch, rebound, and wave effects the rods add, leaving the load-versus-position behavior at the pump.
The result is a much cleaner picture. Where the surface card is smeared by rod dynamics, the downhole card shows the pump's action in a form that is largely independent of how deep the well is or how the rods are strung, because those effects have been removed in the calculation. A full pump gives a full, roughly rectangular downhole card; an under-filled pump, a leaking valve, or gas interference each bends that shape in its own characteristic way. The downhole card is therefore the interpretable one - it renders pump problems as recognizable signatures rather than depth-dependent distortions.
This conversion is exactly what a pump-off controller or SCADA system does internally, continuously, on every stroke. It takes the surface load and position it measures, solves the wave equation to build the downhole card, and reads that card to judge what the pump is doing. The whole point of the calculation is to turn an easily measured but hard-to-read surface signal into a computed downhole signal that maps directly to pump condition.
Nearly every automated rod-pump diagnostic works from the downhole card because that is where pump condition is legible. Fillage is measured from where the load drops off on the downhole card's upstroke, fluid pound shows as a sharp load drop on the downstroke, gas interference shows as a rounded, cushioned transfer, and valve leaks and worn pumps show as their own distortions of the downhole shape. All of these are read from the downhole card, not the surface card, because the surface card would blur them with rod-string effects that vary from well to well.
Standardizing on the downhole card is what lets a SCADA system apply the same diagnostic logic across many different wells. Since the downhole card strips out the depth- and rod-dependent distortions, a fillage or pound signature looks essentially the same on a shallow well and a deep one, so a controller can judge them by a common rule. That consistency is what makes it practical to monitor and control a whole field of rod-lift wells automatically rather than tuning the interpretation well by well.
On a cloud platform such as Merobix, this means the downhole card - and the fillage, pound, and gas indications derived from it - can be trended and compared across every well from one dashboard. An operator sees each well's pump condition in the same, rod-string-corrected terms, so a pump that has started leaking, pounding, or fighting gas stands out against its own history and against the field. Both cards have their place, but it is the computed downhole card that turns raw polished-rod measurements into the pump diagnostics that drive real decisions.
The surface card is the load-versus-position plot measured directly at the polished rod, while the downhole card is a computed plot of load and position at the pump itself. The surface card is easy to measure but is distorted by the stretch and dynamics of the long rod string, whereas the downhole card removes those effects to show what is happening at the pump. The downhole card is the one used to diagnose pump condition.
It is calculated by solving the rod-string wave equation, which models the rod string as an elastic medium carrying stress waves and relates load and motion at the surface to load and motion at the pump. Feeding the measured surface card into that solution subtracts out the rod-string stretch, rebound, and wave effects, leaving the load-versus-position behavior at the pump. A pump-off controller or SCADA system performs this conversion continuously on every stroke.
Because the downhole card shows the pump's action cleaned of the rod-string dynamics that smear the surface card. On the downhole card, fillage, fluid pound, gas interference, and valve leaks appear as recognizable signatures that look essentially the same regardless of well depth or rod design. That consistency lets a SCADA system apply the same diagnostic rules across an entire field of wells, which the depth-dependent surface card would not allow.
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