Teledyne hfc-302 ユーザーズマニュアル

  

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The gas stream will increase in temperature due to the heat it gains from the upstream heater. This 
elevated gas stream temperature causes the heat transfer at the downstream heater to gain heat from 

the gas stream. The heat gained from the gas stream forces the downstream bridge control loop to 
apply less power to the downstream heater coil in order to maintain a constant differential 

temperature of 48

o

C.   

The power difference at the RTD’s is a function of 

the mass flow rate and the specific heat of the 
gas.  Since the heat capacity of many gases is 
relatively constant over wide ranges of 

temperature and pressure, the flow meter may be 
calibrated directly in mass units for those gases.  

Changes in gas composition require application of 
a multiplication factor to the nitrogen calibration 

to account for the difference in heat capacity.  

The sensor measures up to 20 sccm full scale flow 

rate at less than 0.75% F.S. error.  The pressure 
drop required for a flow of 20 sccm through the 
sensor is approximately 0.5 inches of H

2

O (125 Pa).   

The stainless steel base has a 1.5" by 1.0” (38.1 mm by 25.4 mm) cross-section and is 3.64"(92.5 mm) 
long.  The length from face seal fitting to face seal fitting is 4.88” (124.0 mm).  The base has an 

internal flow channel that is 0.75"(19.1 mm) diameter.  Metal to metal seals are used between the 
base and endcaps, as well as the base and sensor module.  Gaskets made of nickel 200 are swaged 

between mating face seals machined into the stainless steel parts.  All metal seals are tested at the 
factory and have leak rates of less than 1x10

-9

 std. cc/s.  Because of this corrosion resistant, all metal 

sealed design, the Hastings 300 can measure corrosive gases, which would damage elastomer sealed 

flow meters. 

The flow rate of interest determines the size of the shunt required.  As previously indicated, 9 

separate shunts are required for the range of flow spanning 5 sccm to 10 slpm full scale.  These shunts 
employ a patented method of flow division, which results in a more linear flow meter.  As a result, the 

Hastings 300 flow meter calibration is more stable when changing between measured gases. 

For the 5 sccm, 10 sccm, and 20 sccm flow rates a solid stainless steel shunt is used.  The shunt uses a 

close tolerance fit to block the main flow passage thereby directing all flow through the sensor tube.  
The 50 sccm flow range uses a stainless steel shunt which has been machined flat on an edge.  The gap 
between the main flow passage and the flat machined on the shunt creates an alternate laminar flow 

passage such that the overall gas flow is split precisely between the sensor and the shunt.  By 
increasing the number of flats and the size of the laminar shunt passageway, flow rates up to 200 sccm 

are accommodated. 

For flow rates above 200 sccm, the shunts are made so that an annular flow passage is formed between 

the shunt cylinder and the main flow passage.  A stainless steel plug with an annular spacing of 
0.006"(0.15 mm) accommodates the 500 sccm flow range.  Increased flow rates require larger gap 
dimensions.  Eventually, a maximum annular gap dimension for laminar flow is obtained (~0.020"(0.5 

mm)). This patented shunt technology also includes inboard sensor ports which ensure laminar flow 
without the turbulence associated with end effects.  This unique flow geometry provides an 

exceedingly linear shunt. 

A flow divider for a thermal mass flow transducer usually consists of an inlet plenum, a flow 

restriction, shunt and an outlet plenum. (See Figure 3.3)  Since stability of the flow multiplier is 
desired to ensure a stable instrument, there must be some matching between the linear volumetric 

flow versus pressure drop of the sensor and the shape of the volumetric flow versus pressure drop of 

Fig. 3.2