10 TEGAM WAY • GENEVA, OHIO 44041
440-466-
6100 • FAX 440
-466-
4-34
Section IV
–
Operating Instructions
Heater Circuit Operation
The 1830A heater circuit operates by applying a heating voltage to the mount,
until the mount temperature bridge stabilizes. The heater circuit will then reduce the
voltage to a regulating voltage to maintain the mount temperature.
The figure shows how heater voltage and current change over time, along with the
DC substituted power change reading of a mount warming up. The sharp step down in
voltage and current indicates that the input body temperature has entered the
regulation zone, and that the regulator has transitioned from warming as quickly as
possible to controlling temperature. The flattening of the DC substituted power at the
same time shows that the zero reading of the thermistor mount changes much more
slowly once the heater is in regulation.
The model 1830A, after firmware version 1.1.36a, tries to detect the step down in
voltage to raise the ‘Htr OK’ indicator. This was changed from earlier versions that
raised ‘Htr OK’ at fixed current levels, and as a result sometimes did not ever indicate
‘
Htr OK
’
. Dete
ction is by means of identifying the “rail voltage” during start
-up, and
then finding a step down of a user-selectable amount that is defaulted to be 0.6 Volts.
If the 1830A front panel heater status indicator does not change to ‘Htr OK’ after a
2 hour warm up, the selected step size may be too large. A very small number of
thermistor mounts may enter regulation with a voltage that is less than 0.6 V below the
“rail”
. If this rare condition occurs, first find the voltage and current of the heater when
that mount is cool. Then observe the voltage for approximately one hour. If the mount
will regulate, the voltage will drop by some amount. If you see a drop into regulation,
note the size of the drop and the voltage at which the mount is regulating. The access
to view the heater voltage is SETUP->Maintenance->Heater Circuit->Heater Volt Read.
You can calculate a new step size from
change voltage = ( rail voltage
–
regulating voltage
–
0.2 )
The access to change the step size is, SETUP->Maintenance->Heater Circuit-
>Heater Change Volt.
Please note that the ‘Htr OK’ notification only signals that the heater has changed
to its regulation voltage. It is not an indication that the mount is ready to make
measurements. The zero reading does continue to change by microwatts for some time
after the heater starts regulating, as a result of the distribution of heat in the input body
continuing to even out. TEGAM recommends a two-hour warm-up with heater and
thermistor beads connected, if starting at laboratory temperature. If starting from
storage temperature, TEGAM recommends a 24-hour soak at laboratory temperature
before beginning warm-up. After the heater is regulating, the BEST indication that the
mount is ready for accurate use is that the zero reading remains stable within a few
microwatts for several minutes.
