JAYLOR’S TMR CORNER: Is the scale less accurate with more feed in the mixer?
Published on Mon, 01/11/2016 - 10:46am
By Dr. Alan S. Vaage Ph.D.
Q (me): “Why are you putting your corn silage in last?”
A (feeder): I put the minerals, supplement and grains in before so they will be weighed more accurately since there is less feed in the mixer.
Q2(feeder): Isn’t that right?
A2(me): “Not really.”
Modern scale systems should have the same degree of accuracy, as percent of weight applied, across the full capacity of a TMR mixer. Usually there are few concerns expressed when mixers are not run while being loaded. However, when mixers are run during loading, the degree of “scale bounce” increases as the load becomes heavier, which gives the perception of reduced accuracy. Fortunately, most scale indicators have an adjustment that can stabilize the readout to compensate for the effects of mixer movement. This is valuable, since improper ingredient sequencing can create greater problems with mix uniformity and animal performance and health than making adjustments for perceived scale problems can solve.
Weighing system accuracy: A basic weighing system, or scale, consists of two parts, load cells that output an electric signal proportional to the weight applied, and an indicator that is essentially a microcomputer that supplies power to the load cells and converts the return electrical signal into readable units of weight.
For all intents and purposes, there is a constant (linear) relationship between the weight applied to and the signal output from a given load cell over its rated capacity. Though each load cell will also specify a maximum total error value from three potential sources, repeatability, non-linearity, and hysteresis, the combined effect of these sources of error on actual weights, as a percent of the measured weight, will be roughly equivalent across the rated capacity of the load cell. The actual variation, or error, over part of the range, will generally be considerably less than the total.
Jaylor load cells, for example, specify a maximum total error of 0.1%. Thus, a load cell with a rated capacity of 10,000 pounds would be expected to have a maximum total error of 10 pounds over the full range; when weighing 1,000 lbs. the error subsequently might be one pound or less. Ultimately, the number of load cells, the characteristics of their mounts and the overall engineering of the system will have more to do with the sensitivity and accuracy of the system than the individual load cell characteristics, but, in general, the accuracy of the system is expected to remain constant over its rated capacity.
TMR mixer scale system capacity: The scale system on a TMR mixer does not use its full rated capacity for weighing the feed in the mixer. Depending on the design of the system, up to the first third of the capacity will be used to hold, or weigh, the empty mixer tub, or even the entire mixer if axle and tongue load cells are used. There also needs to a considerable amount of load cell rated capacity kept in reserve to absorb shocks delivered by travelling over rocks, rough roads or other uneven surfaces. This means that once a scale system has been tared, or zeroed, it should do most of its weighing in the middle range of its rated capacity (where most people believe it is most accurate) even when full, lest one risk damage to the load cells by over-loading the mixer.
Dealing with mixer movement: Mixer systems are designed to be used, and are most accurate, when the mixer is completely level (in all directions) and stationary. When the mixer is running, the weight on the indicator will “bounce” due to the movement of the mixer and the force(s) that are applied to the load cells. Fortunately, modern scale indicators have circuitry capable of sampling the signal from the load cells as much as ten times per second, as well as adjusting how often the average result is displayed on the indicator to give an accurate reading. These sensitivity settings, as they are often called, can be adjusted to stabilize the display if mixer movement makes reading the scale difficult when the mixer is running. It may subsequently be necessary to be a little more patient while adding the last amount of each ingredient to allow the scale time to equilibrate.
The relative importance of ingredient sequencing: The question above arose during a visit with a customer who was concerned about a lack of mix uniformity (high grain concentration at the start of feeding) and its effect on animal performance. In short, the customer that had been persuaded that adding his grain and supplement in the middle of the loading sequence, and his corn silage last, would make the weighing and use of his more expensive ingredients more accurate. As I have explained before in this column, this resulted in the lighter corn silage tending to float above the denser grain that, in turn, circulated below and failed to mix adequately in the time allotted (5 min), which otherwise should have been adequate. Changing the ingredient order to the recommended sequence of forage first and heavier grains and supplements last, as well as reminding the customer that there was no need, nor benefit to mix rapidly until the last ingredient had been added, solved the problem.
Though it is always important and recommended to regularly check your scale system to ensure it is operating correctly, modern systems should be capable of delivery the same level of accuracy over their entire rated capacity. This is important, because you can then confidently use the ingredient sequence that ensures delivery of an optimal mix for animal health and performance — Because Nutrition Matters.™
Dr. Alan Vaage is a Ruminant Nutritionist with over 30 years of experience in the beef industry, and currently provides technical support for Jaylor, in Orton, Ontario. Dr. Vaage can be contacted by email: firstname.lastname@example.org.