From: bhaskar@brtph598.bnr.ca (Shaji Bhaskar)
Subject: Re: Visitherm heater defective???
Organization: Bell Northern Research Inc.
Date: Sun, 26 Jul 1992 01:28:06 GMT


In article <9207251531.AA00565@panther2.panther.mot.com> Ron.Feigen@panther.mot.com (Ron Feigen) writes:

>I just bought a Visitherm heater (100w) for the 30g in my office.
>The problem is that I have to set the Visitherm to 72
>if I want 78 water.

The temperature dial on the Visitherm is unreliable. I have had the
same problem. I use a reliable thermometer to set the heater.

>Another thing I noticed when setting the temp.  When my tank was at
>about 78, if the Visitherm was turned way down and was off, to get it
>to turn on I would have to turn (I did turn it slowly) the temp up to
>about 78.  But to get it to turn off I needed to turn it down to about 72.

Most heaters have a "sticky" relay that turns the heater on and off.
In other words, the temperature at which the heater turns off is
slightly higher than the temperature at which it turns on.  If this
were not the case, the heater would be constantly turning on and off,
and the relay would wear out quickly.  The better heaters have good
relays that can cycle more before they break down, so they can afford
to have less sticky relays and consequently better control over the
temperature.

>OK, I know they say the temp scale is accurate, and I hoped it was,
>even though I wasn't sure how close they would get.  But 6 is off by a
>lot.  Mostly the 6 delta between off and on temps concerns me.

I believe your experiment to determine the accuracy with which the
heater maintains the temperature has at least one flaw.  You cannot
conclude from your experiment that the heater would only have turned
off when the water temperature reached 72 degrees, because the water
temperature (and that of the heater) never actually fell below 78.
The bimetallic strip that controls the relay would be at a different
position at 78 degrees than it would be if the temperature actually
fell to 72.

Even your other experiment - the one in which you set the heater to
just turn on at 78 and observed that the temperature then rose to
above 80 could be wrong, because it is awfully hard to be sure that
you did not turn the thermostat dial beyond the point at which the
heater turned on.

To really get an idea of how accurate the thermostat in your heater
is, set the heater to maintain a temperature about 10 degrees above
the ambient (so that you are sure the thing will turn both on and off).
Then measure the temperature at which the heater turns on, and the
temperature at which it turns off.  Let the heater cycle a couple of
times before you make the measurements.

>
>Does anybody else own a Visitherm?  Does yours work the same?  Does mine sound
>seriously defective.  I mail ordered mine so I can probably exchange it but it
>will be a hassle.
>

I do wish, however, that manufacturers would provide more detailed
specifications of their equipment.  A rough calculation based on how
long my heater remains on typically, its power, and the amount of
water in my tank tells me that the Visitherm in my tank maintains the
temperature in my tank constant to within 1 degree F.

Anybody still with me? :-)

-Shaji.


-- 
Shaji Bhaskar     BNR, Research Triangle Park, NC 27709     (bhaskar@bnr.ca)