| 2-Wire Meters FAQ's |
| For more information see Ap Note 17 |
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| Question: Do digital 2-wire meters still function when their inputs go to zero? |
No. All digital instruments require a minimal amount of power to drive their internal electronics. However, the input level at which 2-wire instruments stop functioning is normally well below the level at which the system being monitored has stopped functioning.
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| Question: What happens when the input goes below the minimum specified level? |
The answer to this question is model dependent. LCD display models will normally continue to operate well below their specified minimum input levels, but the display's contrast will gradually diminish, and more importantly, the readout accuracy is no longer guaranteed.
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For LED display models, the intensity of the display will diminish, but the readout will remain fairly accurate down to the level at which the display becomes so dim that it becomes unreadable. In applications in which the display is required to be totally off at times, be sure to drive the meter with components which bring the input signal all the way to zero.
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| Question: Why aren't absolute maximum input ratings specified? |
Absolute maximum input levels are specified on our data sheets as the "Input Voltage Range" or simply "Input Range". This rating is the meter's maximum continuous operating level at the highest-rated operating temperature. In practical applications, momentarily exceeding the input range by 10% will not harm any of our 2-wire meters.
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| Question: What happens when a 2-wire meter's input signal polarity is reversed? |
Absolutely nothing - the display will not operate. Except for ac-mains powered devices whose inputs are not polarity sensitive, all DATEL 2-wire meters are fully protected against reversed-polarity inputs (i.e., "input signals that are hooked up backwards").
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| Display FAQ's |
| For more information see Ap Note 19 |
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| Question: What exactly is a "half-digit"? |
When digital panel meters first appeared on the market the convention of using a full-scale display range of "000" to "1999" for 3½ digit meters and "0000" to "19999" for 4½ digit meters became an industry standard. And, for reasons still unknown to this writer, the leading left-most "1" (also referred to as the most significant digit, or MSD) became known as the infamous "half-digit". This half-digit represents the one-thousands digit on a 3½ digit meter and the ten-thousands digit on a 4½ digit meter.
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| Question: How is the half-digit displayed? |
The half-digit, or left-most '1', is always OFF (not visible) with display readings between "000" and "999" on 3½ digit meters and between "0000" and "9999" on 4½ digit meters. The half-digit is turned ON with display readings greater than or equal to "1000" on 3½ digit meters and with display readings greater than or equal to "10000" on 4½ digit meters. An enabled half-digit in combination with all other digits OFF is used to indicate an input overrange condition which occurs when the applied input exceeds the meter's measurement capability.
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| Question: What is a "count," and how is it related to resolution? |
| First, let's define the terms 'display resolution' and 'counts'. A 3½ digit meter can resolve an input signal into 1,999 parts, while a 4 ½ digit meter can resolve an input signal into 19,999 parts. A change of 1 count is defined as the right-hand most digit going up or down by one (±1). For example, a display which is continuously alternating between "1899" , "1900" , and "1901" is said to be indicating "1900" ±1 count (or ±1 digit). |
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| A 3½ digit meter with a full-scale input range of ±2Vdc resolves a 1.999V input into 1999 counts, or 1999mV, with one count representing 1mV (0.001V resolution). If the first decimal point (DP1) is enabled, 1999mV can be displayed as "1.999". If the third decimal point is enabled (DP3), the same 1.999V input can also represent "199.9". The 0.001V resolution of the meter hasn't changed, but the decimal point has made the 1999mV look like "199.9". |
The decimal points do not change the meter's basic resolution or sensitivity, they are simply place holders used to suit the user's engineering scaling requirement. The "199.9" reading in the preceding paragraph could represent 199.9 pounds (or, for the rest of the world, 199.9 Kilograms), 199.9 °C, etc.
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| Question: How is input polarity displayed? |
DMS Series meters indicate negative input signals by turning ON a minus sign display-segment ("-") located to immediate left of the half- digit. Positive inputs are indicated by turning OFF the minus sign segment. For example, "-1234" is displayed for negative inputs, and "1234" for positive inputs. The minus sign is ON when IN HI (pin 11) is slightly more negative than IN LO (pin 12). Operation of the minus sign is automatic and can not be externally disabled by the user.
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| Question: How do the decimal points function? |
| This is the second most frequently asked display-related question. The meter's decimal points are totally user selectable, that is, the meter's analog-to-digital converter (ADC) has no control over which decimal point is ON or which decimal point is OFF. This feature is due to the fact that most fixed-input range digital panel meters are not intended to directly readout in volts, but instead are intended to display some other physical parameter such as pounds, degrees, amperes, etc. |
| One of the few exceptions to the above are DATEL's self-powered 2-wire meters. These meters are designed to measure only one electrical parameter (ac volts, dc volts, etc.) and, depending on the display resolution, a decimal point is pre-wired ON and cannot be disabled by the user. |
Both the individual product data sheets and DMS Application Note 13 contain detailed information on how to enable the meter's decimal points.
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| Question: I don't want any decimal points enabled, what do I do? |
The answer is simple: absolutely nothing!-leave all the decimal-point control pins open (i.e., not connected to anything). If not specifically enabled by the user, all decimal points on DMS Series panel meters are OFF. Decimal points are ON only when the desired decimal-point input pin is externally connected by the user to the meter's power return (pin 3). However, unless otherwise indicated on the meter's individual data sheet, unused decimal point inputs can be connected to the meter's positive power supply input (pin 1). See the applicable product data sheet and DMS Application Note 13 ("Decimal Point Drivers") for more information.
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| Question: Are low-power red LED meters dimmer than standard-intensity meters? |
| The answer is an emphatic no! DATEL's low-power red LED meters, identified with an '-RL' part number suffix, are actually brighter than their standard-intensity ,'-RS' suffix counterparts! Red low-power LED meters typically consume 10-15 times less power than '-RS' meters. However, this dramatically lower power consumption is achieved with no sacrifice in display intensity or electrical performance. |
DATEL's low-power red LED meters cost approximately 10% more than standard-intensity meters. However, in many applications this extra cost is easily offset by using lower-cost power supplies. Low power LED meters can be used in many applications where LCD display meters were once the only option-including battery powered instrumentation. DMS Application Note 1, titled "The LED versus LCD Decision," provides more information on the pros and cons of LED and LCD display technologies.
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| Question: Do I need a 3½ or a 4½ digit meter? |
| In the course of answering literally thousands of application calls, we encounter many customers who specify 4½ digit meters for applications in which a lower-cost 3½ digit meter would be adequate. For example, monitoring the output of a 30Vdc supply with a 200V full-scale input range 4½ digit meter will result in a display reading of "30.00" with a resolution of 0.01V (1/100 of a volt). A 200V full-scale input range 3½ digit meter will display the same 30V input as "30.1" (1/10 of Volt resolution). |
Most voltage and/or current monitoring applications do not benefit greatly from the extra resolution 4½ digit meters offer, in fact, in many applications, the extra digit may result in unwanted, difficult-to-view, display bouncing of the last digit. In general, use a 4½ digit meter only when values over 1999 must be displayed in their entirety for clarity or calibration purposes. If you are not sure which meter to use in your application, don't hesitate to contact DATEL-we are more than happy to discuss your requirements in greater detail! (800)233-2765(USA).
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| Process Monitor FAQ's |
| For more information see Ap Note 21 |
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| Question: Why does my process monitor's display reading stay fixed at "000" as the 4-20mA signal is varied? |
The most common cause for a process monitor not responding to changes in the loop current occurrs when a single-ended transmitter is driving two single-ended input process monitors. The second process monitor's grounded negative (-) input effectively shunts the transmitter's output current around the first monitor, that is, no loop current passes through the DMS-30PC-4/20s-24RL's input circuit. The solution is to either power the second process monitor from an isolated dc power supply or use one of DATEL's "-I" isolated-power process monitors.
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| Question: How do I hook up a +24V supply to a loop-powered DMS-20LCD-4/20S? |
You don't! The two input terminals on DMS-20LCD/4/20S can only be connected to the modulated 4-20mA transmitter output current. The system power supply's two "+" and "-" output leads should never be connected directly to a loop-powered device's "+" and "-" input terminals; doing so will place the full supply voltage (sometimes in excess of 36Vdc!) across the monitor's input resistors. These input resistors typically measure less than 30 Ohms, and can easily be damaged by the resulting excessive power dissipation.
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| Question: What do the 'S', 'B', 'P' and 'I' 4/20 suffixes in DATEL's 4-20mA process monitor part numbers stand for? |
| The 'S', 'B', 'P', and 'I' suffixes in the 4/20 part number field are used to denote the monitor's display-readings capabilities. For example, the DMS-20PC-4/20S standard unipolar-reading model's gain and offset adjustments are optimized to display "000" with a 4mA input, and up to "1999" with a 20mA input. The DMS-20PC-4/2B bipolar reading model is designed to display a negative number with 4mA, "000" with 12mA, and a positive number with 20mA. The "4/20P" positive-reading model is optimized to display positive numbers with all inputs between 4 and 20mA. |
The inverse-reading '4/20I' model is designed to display inverse readings, that is, a 4mA input will display a full-scale positive number, while a 20mA input will display "000". The '4/20I' model uses the same DIP-switch settings tables as its '4/20S' counterpart, the only difference is its full-scale and zero display-readings are reversed. For more information see product data sheet.
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| Question: How does ambient light affect the selection of a process monitor? |
| Process monitors which must be readable in direct sunlight should use LCD displays, while indoor applications can usually take advantage of an LED display's superior legibility. Outdoor applications that must also be viewable at night, or in dimly lit areas, should use locally-powered process monitors with backlit LCD displays. |
At the present time, high-intensity LED's can not be illuminated adequately with currents much below 60mA. Therefore, if display legibility is an important requirement, locally-powered LED display process monitors are the better choice for indoor applications since their power consumption is not limited to the 4-20mA currents circulating in the loop.
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