Get the correct insert! Years ago, Jofra dry block calibrators offered interchangeable inserts, drilled with bores specific for various sensors, 1/4" (0.250") being the most common standard size available in the US.    I find it hard to believe that if Jofra is still in business that standard or custom bored inserts are not still available. The white powder used to pack the RTD or thermocouple element in its sheath is called "mineral insulation", sometimes abbreviated "MI".  Its insulation properties are both electrical and thermal.   Electrical insulation provides an "ungrounded" junction and minimizes chemical pollution of the junction at elevated temperatures.    Thermal insulation is highly undesireable in a thermal sensor but is accepted because a packing is needed and MI does the job.   But the result is thermal lag.    Filling the gap between the insert bore and a smaller diameter sensor sheath with Silica will produce thermal lag between the sensor under test and the indicated dry block calibrator's displayed temperature value, which is the sensed temperature of the insert.   You will not find a "study" on the web quantifying the thermal lag because the solution is to use an insert with correct bore.  I recall that at one time Jofra even sold 'blank' inserts so that a  local machinist could machine a custom bore.

thank you for your reply

i know that the correct bore should be used in every sensor, but sometimes old temperature sensors are not always the same, old plants still uses cm and inch diameter of sensor, and furthermore different stem length, some sensors have only 1 cm, so when put in the well the sensor doesn't touched the bottom of the well, with this i put silica first in the well to fill the gap then the sensor, sometimes this small sensors used to measure up to 400 degC, so a dry block is really needed, and dry block is also used to calibrate temperature switches like bimetallic and gas filled with varrying sizes

Are you using the Jofra's digital temperature readout or a calibrated reference sensor in the same insert as the 'temperature'?

An oil would be a better selection.

>jofra dry block calibrator, can generate up to 600 degC.

Even the heat transfer oils don't reach 600°C:

And very close attention has to be paid to the oil's flash point:

Typical heat transfer oils, like DowTherm or Syltherm warn about flash point, and its normal useage is in closed piping under pressure:

Dow's documents shows the vapor pressure curve for absolute pressure vs temperature. I'd be very wary because a dry block runs at atmospheric pressure.