From: clarke@hpfcmgw.FC.HP.COM (Clarke Echols)
Newsgroups: comp.sys.hp
Subject: Re: Should HP 720 monitors be left on when not in use?
Date: 4 Nov 92 21:38:43 GMT
Organization: HP Fort Collins, CO

In comp.sys.hp, tyao@uxh.cso.uiuc.edu (Tim Yao) writes:

>   My question:
>   Do frequent on/off power cycles to the HP 720 monitors adversely affect
>   them or neighboring monitors?  When the monitor of a workstation is 
>   powered on, a deep humming noise is heard and the display on an adjacent 
>   monitor (if it is on) wavers/flickers.  Should the monitors be turned off
>   when not in use?  Are there specific tips on running xdm on HP 720's which 
>   are undocumented?  

This is not official HP information, but I'll give you some overall
considerations:

    o On/off cycles do not usually affect neighboring equipment if it is
      not susceptible to line transients of small magnitude.  Some stress
      to internal components occurs, mainly cycling of power to the CRT
      filament, much like turning a light bulb on and off -- may or may not
      shorten picture tube life depending on whether there are any weak
      spots in the filament wire.

    o The deep hum is a electromagnet coil around the screen to demagnetize
      the screen so you don't get color distortion caused by residual
      magnetism in the metal screen mask and other ferromagnetic parts if
      the display has been moved or subjected to a change in external 
      magnetic field.

    o The flicker/waver on another monitor nearby is caused by a drop in
      line voltage, probably due to voltage drop in the house wiring due
      to increased line current.  This can be reduced by using larger
      capacity electrical wiring on the site or putting fewer devices on
      a given circuit breaker.

    o Turning off displays when not in use lowers air conditioning (cooling)
      costs and power consumption, thus preserving resources.  If the
      building requires supplemental heat (during cold weather), the
      reduced heat from the displays may have to be replaced by building
      heating systems, but if natural gas is used for heat, it is likely
      cheaper than the electrical power that produced heat from the display.
      This savings has to be balanced against the cost of replacing the
      power switch if it fails.  Failure of power switches is related to
      the number of times power is cycled and the life expectancy of the
      switch at the current level being switched.

In general, the policy here is:  If you're gone over the weekend or on
vacation, turn off the display.  Overnight is also recommended, but don't
turn it on and off during the day.  You can also turn off the computer in
some situations, but in other cases it is necessary that the computer
remain "up" in a networked environment.  There are also costs associated
with shutdown and booting a system due to the value of the operator's
time (payroll plus overhead costs).

In general, if the equipment is well-designed, turning the display off
when not being used is appropriate, especially if you don't have a screen-
saver feature in use.  However, as power consumption of newer designs
reduces, the point may come where there is relatively little to be gained
from shutting off the power.

Clarke "used to be a hardware designer" Echols