Hach ‘HQ’ Series Meters: Advanced Water Quality Monitoring: Oxygen/pH/Temperature/Conductivity/Salinity/TDS


The Hach HQ40D Meter with probes for pH and Dissolved

Serious hobbyists and professional aquarists routinely
monitor various chemical and physical parameters within their
aquaria, and for good reason. Substantial investments in
livestock and equipment are often under their charge and
consequences of lost of livestock and displays could range from
serious to cataclysmic. Recently available equipment would be
of use to these folks. Since these instruments are ‘lab
grade’, they would be of potential use to research
scientists as well. Specifically, this article will examine
some relatively new technology found in the ‘HQ’
meter series offered by the Hach Company and will explore
various applications. These meters are offered in different
configurations and offer potential cost savings over
individually purchased instruments. First, we’ll examine
each of the meters within the HQ series, and the parameters for
which they can check. Though advanced hobbyists will be
familiar with the terminology, some readers might not feel
comfortable with terms such as dissolved oxygen and
conductivity. These readers can review the Definitions section
at the end of this article.



Hach (rhymes with Bach) is a well-established firm that has
supplied the water and wastewater industries with
‘cookbook chemistry’ procedures and analytical
instruments for years. Many of their reagents and instruments
are capable of producing results acceptable to the U.S.
Environmental Protection Agency. The Hach HQ series falls into
this category. The EPA approved Method 10360 (luminescent
dissolved oxygen procedure) on July 26, 2006, while the
electrode methods for pH and conductivity have been approved
for years.

You should note that the HQ meters are not compatible with
probes other than the Hach IntelliCAL™ series. Probes
with BNC connectors, for example, will not fit the
meter’s connections.


Figure 1. A macro shot of
the HQ40D’s display. Red lettering explains the meaning
of each. Note that the two DO probes were in different samples
that resulted in differing recorded values.

The HQ40D: Dual Probe
Multi-Parameter Meter (use with available pH, DO and
Conductivity probes)

The HQ40D is the flagship of the meters in the HQ series.
With appropriate probes, it has the capability to measure any
two (or two of any one) of these: Dissolved Oxygen, pH and
Conductivity (including Salinity and Total Dissolved Solids).
The HQ40D also has the ability to store data directly on a
memory stick or to a computer through a USB port when used with
AC power. When running on batteries, the data records are
stored in the meter’s internal memory for later

The HQ40D comes standard with a field kit which includes a
protective glove for the meter, wrist cord, 5 sample bottles
and probe holders (standard probes only) all of which are
housed in a sturdy plastic carrying case.


HQ30D: Single Probe
Multi-Parameter Meter (use with available pH, DO and
Conductivity probes)

This meter is similar to the HQ40D but lacks the ability to
simultaneously monitor two electrodes. If you’re looking
to monitor DO, pH or conductivity independently (with the
purchase of appropriate probes), then this meter could be your
answer. The HQ30D lacks the ‘instant download’ to a
memory stick or computer as seen on the HQ40D.

The HD30D does not include the carrying case, wrist strap,
and other items that are standard with the HQ40.

HQ14D: Dedicated Conductivity

The HQ14D is for use only with the Hach IntelliCAL™
digital conductivity probe. Up to 500 data records are stored
in the meter’s internal memory which can be manually
entered into a spreadsheet.

HQ11D: Dedicated pH Meter

Exactly as described for the HQ14D meter (above) but for use
only with the Hach IntelliCAL™ pH probe.

Power Supply

These Hach meters are powered by 4 ‘AA’
batteries (standard alkaline batteries are supplied with the
meter). Though Hach also recommends use of rechargeable
batteries (nickel metal hydride) and an associated charging
device; neither is offered as an option available through the
company and must be purchased independently. Battery life is
very good. Hach says 200 hours on a set of batteries is normal.
This statement seems to be correct. (Note: Hach does not
recommend mixing alkaline and rechargeable batteries in the


Hach does provide an AC power transformer with each meter,
so simply plugging the meter into an electrical outlet is a
viable option. Use AC power only with a ground fault
interrupter (GFI) outlet as the adapter is not waterproof. The
unit’s power supply does not recharge the internal

Display Languages

Any one of five languages is available for the meter’s
display. These include English, Spanish, German, Italian and

Time and Date

Options on time include a 24-hour (military) format or
AM/PM. Five options are available for the date display. It is
important to note that time and date must be correctly set
before connecting the probe(s). The probes are time-stamped by
the device at first use and are so marked for warranty. In
addition, the meter will advise the user of recommended annual
DO probe maintenance.

Sample and Operator

Traceability of ‘who and where’ is important in
parameter measurements involving chain-of-custody or other
critical procedures. To this end, Hach includes various
programmable options for sample and operator identification.
(See Figure 1).


Operating Conditions: Temperature
and Humidity

True to their nature of being field units, Hach HQ meters
are capable of operating in harsh temperatures (0-60°C, or
32-140°F) and humidity (90%, non-condensing). Although the
meter itself can withstand hot temperatures, the range of the
dissolved oxygen sensor limits practical analyses to a slightly
lower temperature (50ºC, or 122ºF).

Cord Length and Probe

All probes are available in two models – standard and
rugged. The standard models are made of plastic, are smaller in
size and have smaller diameter cords. Cords are available in 1
and 3 meter lengths. The rugged probes are robust, made of
stainless steel and plastic (see Figure 2) and are available
with lengthier cords. These are larger than the standard probes
and more suited for field work. ‘Rugged’ probes can
be custom built with non-standard cord lengths.

Unless the application is really severe, the standard (and
lower priced) probes should be suitable for work in aquaria and
‘wet’ laboratories.


Figure 2.
Probes for the HQ meter series. From the left, a
‘rugged’ DO probe, a standard DO probe and a
standard pH electrode.

Water Resistance

It is important to note that the probes are resistant to
water intrusion for a period of 24 hours when fully submerged
(the sensing portion of the probe can be immersed
indefinitely). The probes are, of course, made to test liquids
but have differing water resistance once fully immersed. The
‘standard’ gel-filled pH and DO probes are
fully submersible in water up to a depth of 3 meters for
24 hours; ‘rugged’ probes are built to withstand
pressures of 30 meters for 24 hours. The refillable
‘standard’ pH probe is limited to immersion by the
refill hole on the side of it (an effective depth of about 10

According to Hach, the meter itself is submersible to a
depth of 1 meter for 30 minutes, however, the battery
compartment is not waterproof can take on water after 15
seconds at a depth of 0.64 meter (~2 feet). Obviously Hach has
attempted to make its HQ meters survivable to accidental spills
in wet environments, and intentional submersion is not

Password Protections and Access

Access to instrument use and stored information can be
protected with a password, if desired. Passwords can be up to
12 characters long (including A-Z, 1-9 and spaces).

Data Log

Data logging is a valuable function of the HQ series. Hach
sells its products short when it advertises that
‘only’ 500 ‘data records’ are stored in
the meter’s internal memory. They should state that 500
‘data sets‘ are retained, meaning that for
each, say, dissolved oxygen reading, these parameters are
stored: Dissolved oxygen, temperature, oxygen saturation in
percent, and time and date of the measurement. The HQ40d meter
can record measurements simultaneously from two sensors (pH,
DO, Conductivity) and each reading from each probe counts as
one data set.

Hach Software

It appears that Hach rushed their software to market. If
you’re really interested in working with Comma Separated
Values (.csv) files then you won’t be disappointed. The
software for the HQ40d allows the user to import .csv files
into Microsoft Excel™. At the time of this writing, it is
not compatible with other programs.

The software suffers from several deficiencies. For
instance, data logged from two similar sensors (2 DO, 2 pH,
etc.) are intermingled in a single column. Worse, the user has
to write a formula to get a time and date on the spreadsheet! I
must admit my experience with .csv files is limited and I might
find an easier way to manipulate the data once I’m more
experienced… but do I really want to learn an outdated

Come on, Hach, you’re an international company, you
can do better! Get some decent software out with graphing
capabilities (look, for example, at Spectrum Technologies
software for their data logger – that’s great
software, but it isn’t compatible with Hach’s
equipment). I checked with Hach’s technical support
department and discussed the shortfalls of their software. The
Hach representative said new software was in the alpha-testing
stage and promised prompt answers to my questions after a
pending consultation with their manager – and I’m
still waiting 2 weeks later for confirmation that some improved
software is on the way. I also sent an email to Hach’s
Technical Support site with a copy of this review with no
response what so ever, even though they claim all questions
will have a reply within one working day. Hach Technical
Support – is anyone home?

In the meantime, I’ll struggle with the
software’s existing version or manually enter the data
points into Excel.

The software is the major drawback in the Hach HQ series,
but the ability to log data is a powerful resource (See Figure
3). So powerful that it makes me want to disregard sub-standard
software and poor after-the-sale service.


Figure 3. Actual data gathered by
a HQ40D meter equipped with a dissolved oxygen probe. Though it
takes some effort with the existing software to chart data,
valuable information can be obtained with these meters’
data recorders.

Millivolt Readings

Hach advertises that a probe with a range to measure redox
(at least those commonly seen in nature and aquaria) will be
available in late 2006. The millivolt readings currently seen
on my HQ40d when using the pH probe seem limited to Nerst
Equation redox potentials, and is probably restricted by the
range of the meter.

Temperature, Pressure and Salinity

Temperature is an important consideration when measuring pH,
conductivity and dissolved oxygen and each of these Hach probes
has an internal thermometer which will correct the measurements
accordingly. Those meters capable of measuring dissolved oxygen
have an internal pressure sensor and changes in barometric
pressure are automatically compensated. The Hach literature is
confusing in that it advertises automatic correction for
salinity (0-70‰) when measuring dissolved oxygen. In
reality, corrections are made only after salinity values are
manually entered into the meter. This is easy enough to do but
be aware that some data entry is required before automatic
adjustments are made.

Temperature can be reported in either ºC or ºF.
Pressure is reported by default in the SI unit hecto Pascal
(hPa), or alternatively inches of mercury (InHg), milliBar
(mBar), or millimeters of mercury (mmHg).

Averaging intervals can be manually adjusted from
predetermined periods ranging from 30 seconds to 5 minutes.
Default for averaging is ‘Off.’

Backlight, Contrast and

Backlighting of the meter’s display is a useful
function when operating in dark or light-sensitive
environments. Options include backlighting for various periods
ranging from 10 seconds to 10 minutes, plus a
‘Never’ option. This flexibility can go a long way
in conserving battery life where electrical power is not
available. Factory default is set for 1 minute. Another
energy-conservation measure is Auto-Shutoff, which is
programmable in 7 increments ranging from 1 minute to 2 hours,
plus the ‘Never’ option.

Screen contrast is set by default at ‘5’, but is
user adjustable from 1 to 10.


Hach reportedly stands behind their products and offers
limited warranties on its products (other than Customer Service
questions, I’ve never had any reliability issues with any
piece of Hach equipment I’ve owned). The LDO probe
is said to come with a 3 year warranty, 1 year warranties on
the conductivity and refillable pH sensors, and 6 months on the
gel-filled pH electrode.

General Impressions and

As usual, the quality of Hach’s hardware is
first-class and I expect many years of service from the HQ40.
As with any piece of equipment, it has its strong and weak

The DO sensor has some major advantages over the older
polarographic method. Hach’s luminescent DO probe is
ready to use once the meter is switched on (unlike older probes
that require 15-30 minutes ‘warm-up’ time and must
have ‘good’ water flow around and over the
membrane). Hach’s DO probe can function in direct
sunlight, a major advantage over the Ocean Optics fiber optic
dissolved oxygen probe. The IntelliCAL™ DO probe requires
an annual rebuild (it’s easy to do with the $80 rebuild
kit). Those familiar with the older DO probes (such as the
Yellow Springs Instruments and others) may recall the
frustrations sometimes encountered in getting a bubble-free and
wrinkle-free membrane on the sensor. The Hach procedure is
quick and easy.

Although the probes are factory calibrated, I prefer to
check the probes against known standards. The IntelliCAL™
pH probe does not come with calibration standards (these must
be purchased separately even though of the still photographs on
Hach’s website clearly shows packets of calibration
solutions along with the meter and probes, suggesting that they
are included). The liquid-filled pH probe is not completely
submersible. This is an inherent ‘problem’ with all
refillable probes since the vent hole must be left open to
allow flow of the electrolyte through the junction and across
the glass membrane.

The battery hatch can be a little tricky to remove and
replace, especially when the meter is housed in its protective
glove. It would be nice if the battery compartment were
completely waterproof.

Disregard Hach’s claim of ordering a
‘package’ (meter/probe combinations) to
save‘ money. Order the meter and probe(s)
individually (as opposed to the packaged combinations) and you
will truly save five bucks or so.

In conclusion, I like the flexibility of these multi-meters
and their potential associated cost savings (when compared to
individually purchased meters). The data-logging functions have
been very useful to me. In spite of the mediocre software, this
meter and probes have allowed me to determine… Well,
I’m getting just a little ahead of myself. We’ll
discuss the findings in a future article.


Dissolved Oxygen

Dissolved oxygen (DO) is critical for the successful
maintenance of oxygen-loving (aerobic) organisms. By the same
token, control of DO is also critical in the support of
anaerobic and facultative bacteria, such as those found in
denitrators or phosphorus ‘luxury-uptake’ reactors.
Since solutions containing high levels of dissolved solids are
inherently less able to contain dissolved oxygen (with all a
number of other factors being the same), DO levels, as a rule,
will generally be lower in brackish or saltwater environments
than in freshwater. Temperature plays a part – as
temperature increases, the oxygen-holding capacity of water
decreases. Biological loading also plays an important part in
maintenance of proper DO levels. One fish in a large aquarium
will exert a modest oxygen demand while many fish in a small
aquarium might require more oxygen than is able to naturally
diffuse into the water, thus creating a condition where
artificial methods are required to supply the amount of oxygen
to sustain life. In an aquarium, this takes on more importance
at night (exactly when we are least likely to check dissolved
oxygen) or when a power outage occurs and disrupts our
artificial means of oxygen transfer.

Standard Methods, 20th Edition (1998), lists five
acceptable procedures for the determination of DO, including
the azide method (“Winkler”) and the Membrane
Electrode method. When this volume is revised, it presently
appears as if at least one more method will be added –
luminescence. ASTM has already approved luminescent
determinations of dissolved oxygen. The luminescent method
involves measuring the fluorescence of a reactive substance
after irradiation with an excitation source. These elements are
contained within Hach’s DO – ruthenium is the
reactant, and a blue LED’s pulsed light is the

DO concentrations within an aquarium should ideally be as
close to saturation as possible. It is possible for aquaria,
ponds, etc. to exhibit levels above saturation due to oxygen
generation by algal growths. However, DOs of about 7 mg/l are
reasonable for saltwater aquaria, with saturation for
freshwater higher at about 9 mg/l (depending upon temperature,
barometric pressure and other factors).


We can all probably recite the definition of pH we learned
in junior high physical science class: It is a logarithmic
scale indicating hydrogen ion concentration which, in turn, is
indicative of the acidic or basic characteristics of the
liquid. pH is one of the fundamental testing procedures, as
many biological functions are pH-dependent. In the same vein,
pH is temperature-dependent to some degree.


Conductivity is the ability of water to carry an electrical
current. ‘Pure’ water conducts very little
electrical current. On the other hand, seawater – which
is full of dissolved minerals and metals – conducts
electricity very well. Generally, conductivity is used to
monitor the performance of reverse osmosis membranes or
deionization units. It is also useful for determination of
dissolved solids in freshwater (a quick and ‘dirty’
determination of water hardness in lieu of testing for calcium
and magnesium) and salinity of saltwater if appropriate
correction factors are applied. Generally, organic compounds
found in water contribute relatively little to conductivity
– it is, again, generally inorganic compounds that are


Temperature needs no introduction or definition, other than
reef aquarium should be maintained within a range of about 22
to 27° C (72 to 81° F).

  Advanced Aquarist, Advanced Aquarist
Dana Riddle

 Dana Riddle

  (120 articles)

I have been an aquarist since 1964 and a reef hobbyist since the mid-1980’s. I am the owner of a small laboratory (Riddle Aquatic Laboratories) that specializes in investigation of interactions between light and water motion & photosynthetic organisms (especially corals). The results of this research, resulting in almost 250 articles, have been published in Advanced Aquarist Online, Aquarium Frontiers, Koralle, Freshwater and Marine Aquarium, The Breeders’ Registry, Aquarium Fish, Marine Fish Monthly and others. My first article was published in a 1984 SeaScope and relayed my experiences with a refugium – an idea that would catch fire about a decade later. I have had the honor of making over 60 presentations to various groups, including national conferences such as the Marine Aquarium Conference of North America (MACNA) International Marine Aquarium Conference (IMAC), PetsFestival (Italy), regional conferences, and local clubs. I received the Marine Aquarium Society of North America (MASNA) Aquarist of the Year Award in 2011 at the MACNA conference in Des Moines.

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