Specifications

molbox™ RFM™ OPERATION AND MAINTENANCE MANUAL

Depending on the placement of the molbloc-S in relation to the DUT and other hardware, and

the availability and capacity of a vacuum pump that may be used, the molbloc-S downstream

pressure will vary in different applications. Calculating Re for different molbloc-S sizes and

flow rates, and estimating the choking ratio (maximum BPR limit) as a function of Re is

somewhat complex, so Table 5 is offered to give the minimum flow that can be achieved with

each molbloc-S size, without exceeding molbox BPR limits, when the molbloc-S downstream

pressure is known: In actual operation, molbox RFM calculates the Re and BPR ratio and

provides an indication of whether the BPR is adequate for measurements to be made.

3.1.3 FLOW READY/NOT READY INDICATION

The character to the left of the measured flow on the MAIN run screen provides a flow

Ready/Not Ready indication. This indication is intended to provide the user with a clear and

objective indication of when a stable flow has been achieved.

Ready <*> is indicated when the current stability (rate of change) of flow is less than the

stability limit. The user can set the stability limit (see Section 3.5.5). The Ready/Not Ready

indication is often used when comparing molbox RFM and a test device to help determine

when steady state flow conditions are present so that a valid comparison reading can be made.

3.1.3.1 molbloc-L OPERATION

In molbloc-L operation, the Ready/Not Ready indication also helps guard against

using molblocs above their valid range by monitoring the Reynolds number of the

flow. If the Reynolds number of the current flow exceeds 1 300, the Ready (<*>)

indicator flashes. The current Reynolds number value can be viewed using

[P&T] (see Section

3.4.5). If molblocs are used within the pressure and flow

range limits for the flowing gas and the pressure dependent calibration type (see

Section 1.2.4.1.2), a Reynolds number of 1 200 will never be exceeded

(1 300 is

used as the warning limit to allow for individual molbloc differences).

Ready/Not Ready character indications are:

<*> Flow Ready (stable).

<*> (Flashing): Reynolds number > 1 300.

<↓> Flow Not Ready (unstable and decreasing).

<↑> Flow Not Ready (unstable increasing).

3.1.3.2 molbloc-S OPERATION

In molbloc-S operation, the Ready/Not Ready indication is also used to warn the

user when the BPR (back pressure ratio) is too high to ensure critical flow (see

Sections 3.1.2). When the BPR is beyond the choking limit, molbloc-S flow

measurements may not be valid and the Ready indicator becomes <P>. The

Ready/Not Ready indicators based on flow stability are also used in molbloc-S

operation, but the <P> indicator takes priority over other indicators.

Ready/Not Ready character indications are:

<*> Flow Ready (stable).

<↓> Flow Not Ready (unstable and decreasing).

<↑> Flow Not Ready (unstable increasing).

<P> Flow Not Ready (BPR high / sub-critical flow)