# US 9,437,093 B2 · Differential current measurements to determine ION current in the presence of leakage current

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- Number: US 9,437,093 B2
- Document ID: US9437093B2
- Status: Granted
- Kind: B2
- Application number: 13/633,686
- Filed: 2012-10-02
- Published: 2013-04-11
- Granted: 2016-09-06
- Earliest claimed: 2011-10-06
- Assignees: Microchip Technology Incorporated
- Applicants: Microchip Technology Incorporated
- Inventors: Joseph Julicher; Keith Curtis; Paul N. Katz
- Web page: https://patentgrep.com/patent/US9437093B2

## CPC

- G08B 17/11: using an ionisation chamber for detecting smoke or gas
- G01N 27/66: and measuring current or voltage
- G08B 17/113: Constructional details
- G08B: SIGNALLING SYSTEMS, e.g. PERSONAL CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
- G01N: INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES (measuring or testing apparatus or processes other than immunoassay, involving enzymes or microorganisms C12M, C12Q)

## Abstract

An ion chamber provides a current representative of its characteristics as affected by external conditions, e.g., clean air or smoke. A direct current (DC) voltage is applied to the ion chamber at a first polarity and the resulting current through the ion chamber and parasitic leakage current is measured at the first polarity, then the DC voltage is applied to the ion chamber at a second polarity opposite the first polarity, and the resulting current through the ion chamber and parasitic leakage current is measured at the second polarity. Since substantially no current flows through the ion chamber at the second polarity, the common mode parasitic leakage current contribution may be removed from the total current measurement by subtracting the current measured at the second polarity from the current measured at the first polarity, resulting in just the current through the ion chamber.

## Claims

### Claim 1

A method for determining ion current in an ion chamber, said method comprising the steps of:
coupling first and second electrodes of an ion chamber to a voltage at a first polarity;
determining a first current between the first and the second electrodes of the ion chamber caused by the voltage at the first polarity;
coupling the first and the second electrodes of the ion chamber to the voltage at a second polarity;
determining a second current between the first and the second electrodes of the ion chamber caused by the voltage at the second polarity; and
determining a difference between the first and second currents, wherein the difference is the ion current through the ion chamber.

### Claim 2 (depends on 1)

The method according to claim 1, wherein the second polarity is opposite the first polarity.

### Claim 3 (depends on 1)

The method according to claim 1, wherein the voltage is a direct current (DC) voltage.

### Claim 4 (depends on 1)

The method according to claim 1, wherein the ion chamber comprises a radiation source for ionizing gas molecules therein.

### Claim 5 (depends on 1)

The method according to claim 1, wherein the ion chamber is used to detect smoke particles.

### Claim 6 (depends on 1)

The method according to claim 1, wherein the step of determining the difference between the first and second currents comprises the steps of:
storing the first and second current measurements in a memory; and
subtracting one current measurement from the other current measurement.

### Claim 7 (depends on 1)

The method according to claim 1, wherein the voltage is a plurality of voltage pulses.

### Claim 8 (depends on 1)

The method according to claim 1, wherein
the step of coupling the first and second electrodes to a voltage at a first polarity comprises:
setting a capacitor to a starting voltage;
coupling the first electrode of the ion chamber to a supply voltage;
coupling the second electrode of the ion chamber to the capacitor; and
charging the capacitor for a first period of time;
the step of determining a first current comprises:
converting a first charge voltage on the capacitor to a first digital representation thereof; and
storing the first digital representation in a memory;
the step of of coupling the first and second electrodes to the voltage at a second polarity comprises:
setting the capacitor to the starting voltage;
coupling the second electrode of the ion chamber to the supply voltage;
coupling the first electrode of the ion chamber to the capacitor; and
charging the capacitor for a second period of time;
and wherein the step of determining a second current comprises:
converting a second charge voltage on the capacitor to a second digital representation thereof; and
storing the second digital representation in the memory.

### Claim 9 (depends on 8)

The method according to claim 8, wherein the starting voltage is substantially zero (0) volts.

### Claim 10 (depends on 8)

The method according to claim 8, wherein the starting voltage is substantially the supply voltage.

### Claim 11 (depends on 8)

The method according to claim 8, further comprising the step of determining whether the difference is in an alarm condition range.

### Claim 12 (depends on 11)

The method according to claim 11, further comprising the step of actuating an alarm when the difference is in the alarm condition range.

### Claim 13 (depends on 8)

The method according to claim 8, wherein the voltage is a pulse generator comprising a plurality of output voltage pulses.

### Claim 14 (depends on 8)

The method according to claim 8, further comprising the step of charging a guard ring around the ion chamber to a voltage on the capacitor.

### Claim 15 (depends on 1)

The method according to claim 1, wherein
the step of coupling the first and second electrodes to a voltage at a first polarity comprises:
setting a capacitor to a starting voltage;
coupling the first electrode of an ion chamber to a pulse source;
coupling the second electrode of the ion chamber to the capacitor; and
charging the capacitor with a plurality of pulses from the pulse source for a first period of time;
the step of determining a first current comprises:
counting a first number of the plurality of pulses required to charge the capacitor to a second voltage; and
storing the first number in a memory;
the step of coupling the first and second electrodes to the voltage at a second polarity comprises:
setting the capacitor to the supply voltage;
coupling the second electrode of the ion chamber to the pulse source;
coupling the first electrode of the ion chamber to the capacitor;
charging the capacitor with the plurality of pulses from the pulse source for a second period of time;
the step of determining a second current comprises:
counting a second number of the plurality of pulses required to charge the capacitor to the second voltage; and
storing the second number in a memory.

### Claim 16 (depends on 15)

The method according to claim 15, wherein the starting voltage is substantially zero (0) volts.

### Claim 17 (depends on 15)

The method according to claim 15, wherein the starting voltage is substantially the supply voltage.

### Claim 18 (depends on 15)

The method according to claim 15, further comprising the step of determining whether the difference is in an alarm condition range.

### Claim 19 (depends on 18)

The method according to claim 18, further comprising the step of actuating an alarm when the difference is in the alarm condition range.

### Claim 20 (depends on 15)

The method according to claim 15, further comprising the step of charging a guard ring around the ion chamber to a voltage on the capacitor.

### Claim 21 (depends on 15)

The method according to claim 15, wherein
to the plurality of pulses are formed by a plurality of voltage transitions having time periods and voltage amplitudes from substantially zero volts to substantially a voltage.

### Claim 22 (depends on 21)

The method according to claim 21, further comprising the step of determining whether the number of transitions represents an alarm condition range.

### Claim 23 (depends on 21)

The method according to claim 21, further comprising the steps of:
measuring a charge voltage on the capacitor; and
adjusting the time periods for the plurality of voltage transitions when at substantially zero volts and at substantially the voltage, wherein:
when the charge voltage on the capacitor is less than one-half the voltage, the time periods of the plurality of voltage transitions when at the zero volt amplitude are greater than the time periods of the plurality of voltage transitions when at the voltage amplitude;
when the charge voltage on the capacitor is at substantially one-half the voltage, the time periods of the zero volt and voltage amplitudes of the plurality of voltage transitions are substantially the same; and
when the charge voltage on the capacitor is greater than one-half the voltage, the time periods of the plurality of voltage transitions when at the zero volt amplitude are less than the time periods of the plurality of voltage transitions when at the voltage amplitude.

### Claim 24 (depends on 21)

The method according to claim 21, further comprising the step of charging a guard ring around the ion chamber to substantially the charge voltage on the capacitor.

### Claim 25

A method for determining ion current in an ion chamber, said method comprising the steps of:
setting a capacitor to a starting voltage;
coupling an ion chamber and a capacitor in series to a supply voltage at a first polarity;
charging the capacitor for a first time period;
coupling the ion chamber and the capacitor in series to the supply voltage at a second polarity;
discharging the capacitor for a second time period; and
measuring a charge voltage on the capacitor after a detection time period.

### Claim 26 (depends on 25)

The method according to claim 25, wherein the starting voltage is substantially zero (0) volts.

### Claim 27 (depends on 25)

The method according to claim 25, wherein the starting voltage is substantially the supply voltage.

### Claim 28 (depends on 25)

The method according to claim 25, further comprising the step of determining whether the measured charge voltage is in an alarm condition range.

### Claim 29 (depends on 28)

The method according to claim 28, further comprising the step of actuating an alarm when the measured charge voltage is in the alarm condition range.

### Claim 30 (depends on 25)

The method according to claim 25, wherein the supply voltage is a pulse generator having an output comprising a plurality of voltage pulses.

### Claim 31 (depends on 25)

The method according to claim 25, further comprising the step of charging a guard ring around the ion chamber to substantially the charge voltage on the capacitor.