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What is the difference between an Air Core, Mechanical and Digital Stepper Motor gauge?

In the Auto Meter gauge line, you’ll find three very different meter movements used in the construction of our instruments. These are Short Sweep Electric (SSE), Full Sweep Mechanical (FSM) and Digital Stepper Motor (DSM). Here we’ll touch upon the key differences between them and help to clarify which works best for each type of application and why.

Air-Core (Short Sweep Electric)
The first type we will discuss, and probably most popular, are our Short Sweep Electric gauges. At the heart of these gauges is Auto Meter’s tried and true air core meter movement. An air core, at its essence, is a small magnetic core with copper windings around it that will move in relationship to voltage it sees. This is regulated by an electronic sending unit, which will change its resistance to ground based on the reading. As a temperature or pressure changes, the resistance the gauge measures and thereby the voltage the air core sees varies dictating that the indicator move to a calibrated position on the dial. These gauges are physically differentiated from the other two by their use of a smaller percentage of the dial (90º of rotation), hence “short sweep”. Needle movement will most often start around the 9 o’clock position and move to the 3 o’clock position at full scale. Like all Auto Meter instruments, the SSE gauges are designed to be incredibly robust. Our air cores, custom wound in house in Sycamore, are designed to deal with incredibly high levels of continuous vibration and huge levels of incidental shock. It represents a durable, affordable solution that is ideal for most street and mild race applications.

Of note on the air core gauges. There is no return spring or direct physical connection to the zero point on the instrument. As such, when power is removed (ie the vehicle is turned off), it will not always return to zero. This is consistent throughout all of our electronic gauges and allows for an incredibly responsive and fast pointer movement (air core tachs are capable of moving from 0 to 10k rpm in 333 milliseconds). It is not uncommon to see a new gauge in the box where the indicator is not zeroed out. This is totally normal and not indicative of any failure. Because these types of gauges rely heavily on accurate resistance measurements of the sending unit, a good ground is imperative for the best possible results during installation. The gauge will only be as accurate as its ground is good. We recommend an engine block ground point above all else. A SSE gauge with an inadequate gauge ground will oftentimes read higher than it should, particularly with temperature gauges as they operate over a very wide resistance scale. The sending units SSE gauges utilize are “self grounding”, meaning they find their path to ground through the threads or the body of the sending unit. This design does not play well with Teflon tape on the threads as it will impede the path to ground and will often manifest as a sluggish response or a gauge that reads lower than it should. A small dab of liquid Teflon will provide insurance against leaks without the potential for erroneous readings. The sending units do utilize a tapered pipe thread, so leaks are not typically a problem, even without any thread compound whatsoever.

A properly installed SSE gauge will return accuracies levels no worse than +/- 5% at full deflection. While this is often much better, because of the shorter scale and slightly decreased dial resolution in relationship to our full sweep offerings, the pointer width is the limiting factor in how accurately the gauge displays its reading.

Digital Stepper Motor (Full Sweep Electric)
The next type of instrument we’ll touch on are our Digital Stepper Motor gauges. These highly advanced instruments represent the pinnacle of individual gauge technology and provide the highest possible levels of accuracy, precision, response and durability. Each instrument is controlled by a microprocessor on an circuit board printed in house, which allows for incredibly quick calculations as well as a number of other features not seen in the other gauge types. These gauges utilize laboratory grade sending units for the ultimate in accuracy, no more than +/- 1%. These solid state sending units are the same ones used in our high end data acquisition solutions. In fact, many of them can simultaneously be used to provide data streams to a logger or standalone engine management system. To further increase accuracy, every time the gauge is powered up it will zero calibrate to assure that even as it wears over a lifetime, it will always remain as accurate as it was the day it was built. These gauges utilize 270º of the dial, making them very easy to read, even at small levels of change. Included with each DSM gauge is a wiring harness specific to the instrument and included sender. These harnesses also provide a ground for the sending unit, so there aren’t any concerns about the sending unit ground through the threads affecting instrument accuracy. We still prefer liquid Teflon to the tape, but these also have the bonus of being able to be easily installed even in composite and other non-grounded materials.

Many of our DSM gauges, such as the Elite and SpekPro lines, also include a number of other great features. Because they are microprocessor controlled, these gauges can recall the highest value measured over a period of time (exactly like you’d see in a “tattle tale” tachometer). They will also have user configurable warnings, wherein the instrument will change its lighting at different points on the dial to better communicate certain conditions to the driver. These particular gauges have a high and/or low warning point (depending on series and parameter monitored) in addition to their normal operating color. When either of these warning points are exceeded by a configurable percentage, the gauge will begin to flash to further call attention to itself. You can see these gauges in action in every televised NASCAR race. The instruments used professionally by the teams are off the shelf units with a couple of the features disabled in order to comply with the sanctioning body’s rules. Taking the feature set a step further, these instruments also feature Auto Meter’s Pro Control outputs. This allows the gauge to control external devices based on the reading. For example, a water temperature gauge could be used to control electronic fans. Here’s a cool example of something we did in the shop recently. One of our techs lost an engine due to oil pump failure. Once the new engine was in, we were able to measure 4 psi of oil pressure being made by just the starter motor. We configured the gauge to not allow the computer to come online until the gauge saw 4 psi of oil pressure. This allowed for us to prime the engine reducing wear and tear during startup. Better yet, should he experience this issue again while out on the track, the gauge would automatically shut down the vehicle if it were to read less than 4 psi, saving the cost of a third engine in one season.

While more expensive than the other gauge types, the DSM gauges represent a great value to the professional and semi pro racer. Because they so easily communicate critical information to the driver, offer unparalleled accuracy and in many case automatically trigger key condition based events, these gauges can easily be the difference between a lost race and a lost motor. The driver stays more focused on the task at hand, confident that should a condition outside the norm pop up, they’ll be warned and be able to easily see exactly what that condition is. There is, quite simply, not a more accurate or durable instrument solution available from any manufacturer, at any price.

The final type of gauge movement we’ll touch on are our traditional Full Sweep Mechanical gauges. These gauges make their measurement directly. For example, an oil pressure gauge has a physical oil line running up to the back of the gauge. A temperature gauge will use a long capillary tube, similar to an old style glass thermometer. A bulb on the end of this tube is heated and an ether gas, encapsulated within the tube expands, pushing the meter movement forward and displaying the temperature. The advantage to this is they can be run completely without electricity (only the lights require any power). A vehicle shut off in the pits will still display the current temperature. While best suited for race applications, FSM gauges are not as ideal a solution for street applications because of the fluids being passed through the firewall. The physical connections necessary are often more bulky and difficult to install as well. The most common contradiction we see to this rule is with boost or air pressure gauges, where a simple air line is run to the gauge, rather than a heated fluid or flammable liquid. A FSM fuel pressure gauge will need to be mounted outside of the vehicle for safety or a gauge isolator will need to be used. These gauges do use the entire 270º of the dial like a DSM gauge and are very easy to read. Accuracy is very good, with no more than a +/- 2% variance at full deflection commonly observed. Based on the scale, a SSE gauge returns very similar real world accuracy. These meter movements are also ruggedized for extreme situations and are even available filled with liquid silicone for additional dampening for use in high shock or vibration environments such as desert racing or drag racing applications that experience tire shake. They tend to cost a bit more than their SSE electric counterpoints but do undercut the DSM gauges on price.

Regardless of the choice of movements you make, each Auto Meter gauge is engineered, built and calibrated by our skilled technicians here in Sycamore. We take great pride in offering the finest American instruments in all price points for all applications. With more motorsports wins than all other gauge manufacturers combined, our nearly 60 years of excellence assures you the performance and durability necessary to achieve whatever goals you have for your project.