Understanding a Loop Powered LCD Digital Panel Meter


There are a number of variants in digital panel meters is more are less and understatement. It however can be stated that there can be a generalized understanding of digital panel meters. There are number of display types and input types of digital panel meters, for the purposes of the this article we are going to look at a very simple panel meter that can be used in many process control applications, and thus has one of the highest rates of utility.
The simple LCD digital panel meter loop powered 4-20mA process meter is a good starting example to have a closer look at digital panel meter’s specifications.
How Display Digits are specified – Displays are specified first by the number of digits. Commonly you will see 3 ½ digits and or 4 ½ digits. The ½ digit is used for 1 or not one and also to provide a negative symbol or not this is referred to as “polarity”. This allows for a maximum displayable value for a 3 ½ digit display of 1999 or -1999 and for a 4 ½ digit display you have a maximum value of 19999 and -19999.
How Display Types are specified – In general you have displays that of type LCD or LED, however whether it is a LCD or LED type display, the displays are commonly 7 segment digits. Seven segment digits are used to create numbers from 0 to 9 for each digit. The dimensions of the 7 segment digits or the digit size can vary from very small to very large. Common sizes include 0.35”, 0.45”, 0.56”, 0.6”, 0.8” 1” and 1.5”
How Display Colors are specified – For LCD displays you have with our without backlight. A LCD without backlight, the digits are generally black in color on a non light positive background. With backlight you have color options for the digits that are commonly green, amber, red and blue. It is also possible to have the digits be negative (black) and have the background be backlight with the common colors of green, amber, red and blue.
How Display Units and Decimals are specified – On LCD displays the units or “Annunciators” are provided, these provide important feedback defining what is being measured. These units are ⁰F, ⁰C, PSI, %, A, V and others. These units are usually selectable in the field.
How Inputs are specified – The most common process control input is 4-20mA DC. A 4-20mA input can specify line impedance; this is usually as a result of the length of wire used in a 4-20mA twisted pair run. A specification will be listed as nominal impedance in Ohms at the max protocol current for example 300Ω nominal @20mA
How Performance is specified –
• Accuracy – This is defined as the percent error rate at full scale (experimental value – expected value) / expected value. The accuracy also specifies the least significant digit as a count. The count value would be representing for example “count +2” this would imply that beyond the 2 digits from the decimal point accuracy is not stable.
• Conversion Rate – This defines the number of conversations that the A/D in the meter, can do and display per second. Typical conversation rates may be something like 5, 3 or 1 conversations per second.
• Normal Mode Rejection – This sometime is abbreviated as NMRR, this is the ability of a panel meter to reject AC normal mode noise or signal. The value is listed in –dB @ power line frequencies; for example -30dB @60 Hz implies -30dB attenuation of a 60 Hz signal.
• Adjustments – This specifies a zero setting and a gain setting capability, this can be sometimes done with coarse and fine adjustments or just with coarse adjustments.
• Temperature Coefficient – This value is specified as ppm or parts per million per ⁰C. An example will be +/-100 ppm per ⁰C, implying that accuracy will be affected 0.0001 per degree change.
How Environmental performance is specified – Operating frequencies are the range of temperatures which a panel meter can function and maintain is stated accuracy. The range is usually a commercial, Industrial or military temperature range or a value similar to these ranges. An example of a commercial panel meter’s operating temperature is 0 to 50⁰C.
How Power Supply Requirements are specified – For a loop power device an expected range is from 8 to 32VDC. Different voltages are available; see your manufactures’ documents for more information.


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