It is important to note the distinguishing features of the two general categories of oilfield dynamometers:

• Horseshoe (or Donut) Load Cell Dynamometer:  These load cells are placed between the Polished Rod Clamp and the Bridle—which requires the rod string first be stacked out on the wellhead to create separation for the dynamometer to be inserted (as shown in the image where the stack-off "suitcase" is resting on the stuffing box to support the rod load). With the Dyno being installed and the well returned back to pumping, the load cell acts like an ordinary bathroom scale and measures the true load on the polished rod.  A Donut Load-Cell operates similarly but it is used for continuous dynamometer acquisition with POC's (Pump Off Controllers) while the Horseshoe is made for temporary installation by a Well Tech. Understandably, a horseshoe dynamometer is more accurate than a PRT because it takes direct measurements of the polished rod load. However, horseshoe dyno's are not used as frequently as the PRT for routine diagnostic analysis due to many issues related with its installation (described below).

• PRT (Polished Rod Transducer) Dyno:  This is Echometer's quick-install Dynamometer and is the most commonly used. The PRT does not directly measure the Polished Rod Load.  Rather it is a clamp-on strain-gauge that is screwed onto the polished rod and it measures the minute changes in diameter of the polished rod along the stroke length.  The rods carry the fluid load on the up-stroke but not on the down-stroke.  The added fluid load adds stress to the rod string and that axial stress causes a proportional radial diameter change.  Using Hooke's Law (which is a relationship between the radial strain caused by an axial stress) the minute changes in polished rod diameter are used to back-calculate the axial load causing the diameter change:  i.e. the fluid load.  Adding the fluid load to the calculated buoyant rod weight, the Surface Dyno is computed.  From this point on, the Wave Equation is used to calculate the Pump Card in the same fashion as the Horseshoe Load Cell.  Comparative studies performed by Echometer analyzing the accuracy of the PRT found it is within 2-3% of the loads recorded by the Horseshoe Load-Cell as long as accurate input data is used and there is no bending of the Polished Rod due to vertical misalignment (effects described below).

So....while Load Cell Dyno's are more accurate, their installation process creates other problems.  Usually Well Techs will only use the clamp-on PRT Dyno unless specific circumstances warrant more accurate information—at which point the Horseshoe Dyno can be installed. 

These reasons illustrate the installation issues associated with the Horseshoe Dyno:

1. In order to install the Horseshoe, the well must have a good functioning brake.  This is needed when stacking the rods out on the wellhead.  

2. It takes much more time to install:  A 2nd (stack-out) Polished Rod Clamp must be installed on the Polished Rod (shown in above image), lubricators or any other non-load bearing devices (if present) on top the stuffing box must be removed, & the stuffing box bolts must be tightened to protect the rubber packing before stacking out the rod weight.  After recording the data, these steps much be back-tracked to restore the well back to its original condition.  

3. Due to stacking out the rod-string and the other steps involved, the liability risks of injury to the operator and/or well equipment are greatly increased.  

4. The well is turned off during the installation process—which disturbs the well's normal run-time cycles causing the fluid level to build up and thus changes the downhole conditions.  This disturbance usually necessitates a Well Tech acquire longer periods of data so the well can re-stabilize (or risk acquiring unrepresentative data).  Also, this disturbance to the pumping cycles interferes with the ability to properly calibrate the time-clock to the well's production. 

5. The disadvantages associated with raising the rod-string.  The rod-string is raised 3" by the addition of the Horseshoe underneath the polished rod clamp and this is disadvantageous for two reasons:

   • It changes the pump spacing.  This reduces the pump's compression ratio and can aggravate gas interference (making the acquired data look worse than it actually is during normal operation).  Or it can hide the fact that the well actually has a 2-3" tag on the pump during normal operation (which will be removed when recording the dyno).

   • Raising the rods 3" higher equally raises the plunger 3" higher relative to the pump barrel.  At the top of the upstroke, this will cause the plunger to move into an unswept area of the barrel—which can be problematic if the pump has been downhole a while and built-up scale deposits exist here. 

In contrast, the installation of the PRT Dyno is extremely easy and can even be performed while the well is pumping (which is necessary if there is no working break and unit is counter-weight heavy to where the horse-head always settles up when the unit is turned off).  

Now, lets evaluate the downsides of the PRT Dynamometer:

1. PRT Data Accuracy:  The PRT data is not as accurate as the Horseshoe and is (slightly) more qualitative in nature when compared to the Horseshoe.  However, the PRT is sufficiently accurate for 95% of most work unless more precise measurements are needed for a particular situation, for example:  when performing a scientific analysis on the acquired data or when greater clarity in the numbers are needed for a well that is difficult to accurately diagnose.

2. Polished Rod Bending:  if the polished rod is out of vertical alignment with the wellhead (or hits the horse-head at the top of the up-stroke), bending stresses will result in the polished rod.  These stresses will cause additional changes in the rod diameter—which the PRT naturally interprets as being "real" load changes occurring downhole.  If the polished rod is out of vertical alignment—the effect will cause the Pump Cards to slightly slant.  If the polished rod hits the horse head at the top of the up-stroke—there will be some form of anomalous load change during that side load (a good operator must recognize this while at the well site).  The distortions to the pump card are often subtle, but not always.  Thankfully, however, they usually do not prevent the accurate diagnosis of the downhole pump performance.  Sometimes these distortions can even be removed by repositioning the PRT in relation to the angle of bending.  

3. Load/Temperature Drift:  The internal strain-gauge in the PRT is extremely precise in measuring minute changes in diameter but temperature changes on the strain gauge itself cause the gauge to expand or contract.  The good gospel news is:  the PRT recalibrates itself to zero each stroke (so the temperature change will cause little disturbance...unless your SPM is really really slow and there is a significant, sudden temperature change).  The problem with Temperature Drift is more pronounced during Valve Checks or performing counter-balance tests (where the zero recalibration process is not occuring as frequently).  Good operator practices can usually minimize this small annoyance.  

Downhole Diagnostic has both types of Dynamometers and can employ either one as needed.