Fluke 9112A 6 Operation

6 Operation

The Model 9112A is basically a temperature controlled furnace utilizing a full

PID micro-processor based temperaturecontroller with a Type K thermocouple

temperaturesensor. The temperature controller sends a time proportional signal

to the solid state relay which regulates the current to the heater.The heater

powercan be switched to HIGH or LOW power positions. The object of the

temperature control is the equilibration block with test wells containing the ref-

erence probe and the sensors to be calibrated inside.The b lock providesa ther-

mal mass which tends to stabilize the temperature and reduce the gradients

between the testw ells. The user settable “over-temperature cut-out” can open

the heater circuit with a relay if the safe temperature for the test probe or for

the furnace is exceeded.The enclosure is designed to limit the heat seen by the

variouscomponents of the furnace as well as the user. The control section is in

a separate cabinet belowthe furnace heat preventing damage or accuracy er-

rors. The furnace part of the cabinet contains ventilation holes as well as two

fans controlled by the thermostat. This cooling capability prevents the surface

of the enclosure from getting dangerously hot. In the eventthat the fans should

fail, aseco nd thermostat is installed in the cabinet which shuts down the fur-

nace heaters if the cabinet exceedsa safe temperature.

Operatingthe Model 9112A is straight forward once you have grasped all the

importantprinciples.

When the unit isturned on, the cutoutreset button must be pushed before the

unitwill heat.

Temperatureselection is accomplished by using the up and down arrow keys on

the frontof the temperaturecontroller. The lower display indicates the new

temperature setting while the upper display shows the actual temperature.

When scanning from one temperatureto another, notice that the temperature

controller seems to be ahead of the equilibration block temperature. This differ-

ence is because the temperature control sensor is near the outside of the block

and ittakes some time for the heat to conduct into the center. Depend on an ex-

ternal temperature monitor to establish when the equilibration block has

reached the desired temperature and achievedstability.

The actualtemperature indication made by the temperature controller is not in-

tended to be a calibration reference, butto merely provide a general indication

of the furnace temperature. NIST traceable standard thermometers are available

and should be used inmaking comparison measurements. For less stringent

measurements you may makea calibration of the controller and control probe

at particular temperature points and use that with reasonable accuracy for a

time.

6 Operation

Overview